Potential implications of rising sea level on American Horseshoe Crab (Limulus polyphemus) spawning beaches in two Florida counties

Danielle Contrada; Claire Crowley-McIntyre; Berlynna Heres

doi:10.1371/journal.pone.0333812

. 2025 Nov 5;20(11):e0333812. doi: 10.1371/journal.pone.0333812

Potential implications of rising sea level on American Horseshoe Crab (Limulus polyphemus) spawning beaches in two Florida counties

Danielle Contrada ^1,^#, Claire Crowley-McIntyre ^1,^#, Berlynna Heres ^1,^*,^#

Editor: Phuping Sucharitakul²

PMCID: PMC12588468 PMID: 41191584

Abstract

Coastlines support a diversity of wildlife and are used as spawning sites for many species. Though, as sea levels rise, many coastlines will be inundated, affecting species that live or nest in these habitats. The American Horseshoe Crab (Limulus polyphemus) requires specific environmental conditions for optimal egg development in coastal habitats. This study used reports of horseshoe crab sightings in two Florida counties to identify characteristics associated with their spawning habitats. A 0.6-meter-wide shoreline polygon was created in ArcGIS Pro and each habitat type within the polygon was identified. Using sea-level rise scenarios for 2040, 2070, and 2100, the percentage of habitat loss within these spawning polygons was predicted. In Pinellas County, spawning was predominantly associated with sheltered tidal flats and mangroves. In Brevard County, spawning was most often associated with fine- to medium-grain sand beaches and mangroves. These results demonstrated that each sea-level rise scenario projects a change in habitat area within the shoreline polygons, most notably in the 2040 projection. By 2100 in Pinellas County, 96.3% of mangroves and 87.4% of sheltered tidal flats within the shoreline polygons were predicted to be lost or changed to a different habitat type. In Brevard County by 2100, 98% of fine- to medium-grain sand beaches and 94.8% of mangroves within the shoreline polygons were predicted to be lost or changed to a different habitat type.

Introduction

Over the last century, climate change has caused the global mean sea level to rise at an unprecedented rate. Along the contiguous United States, relative sea level rose approximately 28 centimeters (cm) from 1920–2020, with a notable acceleration after 1970 [1]. Along the Florida coast, sea level is rising 2.5 cm every 11–14 years [2]. Rising sea level poses a threat to coastal communities and can drastically change existing coastal ecosystems. These direct threats include, damage to infrastructure, permanent loss of land and life, ecological regime shifts, and decreased water quality [1]. Indirectly, these communities face worsening of impacts of storm surge, high tides, and wave action, both chronic and acute in nature [1]. To combat this threat, various protective measures, such as sea walls, surge barriers, and nature-based defenses such as wetlands, have been implemented [1]. The installation of such structures can negatively impact species, such as the American Horseshoe Crab (Limulus polyphemus), hereafter horseshoe crab, as their spawning habitats along the shoreline are compressed between the rising sea and infrastructure [3–5]. Bulkheads and seawalls can obstruct access to intertidal spawning beaches, exacerbate shoreline erosion, and prevent natural beach migration [6]. Riprap, a type of rock armor, can also trap and strand species, leading to increased mortality [7]. Additionally, building anthropogenic structures to protect coastal communities can interfere with natural geological processes that are crucial for preserving habitats such as spawning beaches [8]. The amalgamation of these factors can lead to negative consequences for species such as horseshoe crabs that require specific environmental conditions for reproductive success [2].

The horseshoe crab resides along the Atlantic and Gulf of Mexico coastlines from Louisiana to Maine [2,9], and along the Yucatan Peninsula of Mexico [2]. Although horseshoe crabs are known to tolerate a wide variety of environmental conditions, the rate at which water quality and habitat availability changes occur may weaken their long-term viability [10]. The conservation status of the horseshoe crab varies geographically. Some populations are at risk due to limited and fragmented habitats (Gulf Coast of Maine), while others are considered stable or rising (Mid-Atlantic and Southeast) [2]. There is evidence of mixed trends along Florida’s coastline, but the causes of localized population decline in Florida are poorly understood [2]. Across regions, the availability of spawning locations that offer optimal conditions for egg development is a critical factor that can affect the conservation status and long-term viability of horseshoe crab populations.

Horseshoe crabs mature at nine or ten years old and typically begin spawning along sandy shorelines when the tides are highest during the spring and fall [2]. The horseshoe crabs gather in sometimes large congregations, the female crabs dig a small hole in the high tide line of the shore to lay her eggs while an attached male fertilizes them externally [2]. These eggs develop over the course of one month to six weeks, in which time the habitat must remain consistently inundated with water but not fully covered, to prevent the eggs from drying out or becoming anoxic due to extended inundation [10,11]. Although horseshoe crabs will spawn in suboptimal areas if suitable locations are unavailable, they typically choose spawning sites that provide optimal conditions for embryonic development [11]. Generally, the properties of shoreline sediments influence horseshoe crab egg viability and development by affecting moisture retention and the rate of water movement through the beach [12]. Studies have revealed that the grain size of Delaware beaches, which have a high concentration of spawning horseshoe crabs, varies between 0.5 to 2.0 millimeters (mm) in diameter, with a median grain size of 0.7 mm [13]. Beaches in South Carolina and Florida with spawning horseshoe crabs have reported mean grain sizes of 0.2–0.4 mm [14] and 0.3 mm [12], respectively. Wave action is also a factor in optimal development because beaches with high-energy wave action may exhume buried eggs, exposing them to suboptimal moisture and oxygen gradients [15]. Preferred areas are characterized by lower wave energy, which reduces the risk of exposing or washing away horseshoe crab eggs. Just above the mean high-tide line is considered the most suitable place for spawning since sediments at higher elevations are susceptible to desiccation [12]. In comparison, sediments in lower elevations have lower oxygen levels that can impair egg development [12].

The slope of the beach is a determining factor that affects the conditions for the development of eggs. Horseshoe crabs exhibit a preference to spawn on beaches that are sloped 2°–8° to ensure ideal conditions for embryonic development [12,16]. In New Jersey, spawning beaches have recorded slopes in the range of 3°– 7° seaward [17], while in Delaware and along the Gulf Coast of Florida, beach slopes ranged from 2°– 5° [12,16]. Generally, steeper-sloped beaches have larger grain sediment, such as pebbles and boulders, which are unsuitable for egg development [17]. Alternatively, sand and gravel substrates with adequate amounts of aeration and moisture are considered ideal for spawning success [12,15].

The timing of horseshoe crab spawning varies depending on the location. However, it is typically triggered by warmer water temperatures in the spring, which stimulate adult horseshoe crabs to migrate from deep to shallow waters [18]. Florida Horseshoe crabs are unique because they have two distinct spawning periods: February to May and August to November. This is unlike the northern populations from Delaware to Maine, which have one short spawning period, typically in May and June [19].

The timing of spawning also plays an important ecological role, especially in the northern United States, where studies have indicated a relationship between horseshoe crab spawning events and the Red Knot (Calidris canutus rufa), a migratory shorebird. The Red Knot relies on horseshoe crab eggs in Delaware Bay to fuel their migration from tropical wintering grounds to Arctic Canada [20]. Several species of crustaceans and fish have been directly observed feeding on horseshoe crab eggs and larvae, and found in the stomach contents of fish in the tidal creeks of New Jersey [2]. Furthermore, while horseshoe crabs feed, their plowing action helps aerate the substrate, which promotes species diversity, richness, and abundance [21].

In addition to their ecological roles, horseshoe crabs have been economically important for many years. From the mid-1800s until the 1960s, they were harvested in Delaware Bay and used primarily for fertilizer [22]. Today, horseshoe crabs are commercially used as bait in the American eel (Anguilla rostrata) and whelk (Busycotypus canaliculatus) fisheries [23,24], educational and aquarium purposes, and biomedical research [25]. In 2021, the coastwide bait landings included 741,684 horseshoe crabs, which were well below the coastwide quota of 1.59 million. Florida has a unique “marine life” fishery where horseshoe crabs are harvested for aquarium trade and educational and scientific purposes. In the biomedical industry, over 600,000 horseshoe crabs are collected each year [26], and according to the Atlantic States Marine Fisheries Commission [23], 718,809 horseshoe crabs were collected in 2021 and their blood used by the medical industry for contaminant testing. Assessing the threat of sea level rise on critical spawning habitat for horseshoe crabs is essential because of these sectors’ economic dependence on robust horseshoe crab populations and the need for mitigation that considers horseshoe crab spawning beach requirements in the design.

This study describes the potential impact of sea-level rise on horseshoe crab spawning habitats in two regions of Florida with the highest recorded sightings. Data from public reports were used to assess what habitats are used by spawning horseshoe crabs in Pinellas County on the Gulf Coast and Brevard County on the Atlantic Coast of Florida. Then, sea level rise scenarios for 2040, 2070, and 2100 were examined to identify and quantify the potential loss or change of spawning habitats due to rising sea levels. While these results are specific to Florida, the methods can be applied to other areas throughout the horseshoe crab’s range, specifically in areas where similar public, citizen science, or fishery-independent surveys are routine.

Methods

Study sites

To assess the potential change of horseshoe crab (L. polyphemus) spawning habitat due to sea level rise in Florida, two highly developed coastal counties, Pinellas and Brevard, were selected for analyses based on their high concentrations of public reported horseshoe crab activity [27]. Pinellas County is located along the Gulf Coast of Florida, with a coastline that spans 946 kilometers. Brevard County is located along the Atlantic coast of Florida, with an estimated 785 kilometers of coastline, including the Indian River Lagoon. Both counties are home to a wide array of habitats, ranging from sandy beaches to mangrove forests. While these sites are geographically separated and the horseshoe crabs are part of genetically distinct populations [25], spawning behaviors were assumed to be similar. The sites were not selected to represent different populations or climate change regimes.

Horseshoe crab spawning data

Horseshoe crab spawning in Florida has been reported by the public through phone, email, and online surveys, including Survey 123 [28] since 2002 [27]. Beginning in 2017, observations of horseshoe crabs reported through online surveys and Survey 123 (S1 Fig) included GPS locations; therefore, in the present study, observations collected through these methods from 2017–2023 were chosen based on their location accuracy. Horseshoe crab sighting reports from January 2017–June 2023 from Pinellas County and sightings from January 2020–June 2023 from Brevard County were included in the analyses due to the high reporting frequency in these counties. Heat maps were created using a Kernel Density Analysis, which calculated the density of sightings in both counties [29]. It is important to note that subtidal spawning was not considered in this study.

Horseshoe crab sighting reports were examined to distinguish which observations reflected a location where horseshoe crabs can physically nest. All data were reviewed by examining coordinate locations using Google Earth version 10.42.0.2 or in-person accounts. Based on previously studied behavior, a ranking system was established to evaluate the possibility of horseshoe crabs being able to spawn at each reported location and eliminating those areas with a hardened or armored shoreline, which has been shown to prevent active spawning [12,21] (Table 1). Habitats ranked “zero”, which classified areas where spawning was possible, were used for further analyses.

Table 1. Description of ranking system used to evaluate each publicly reported horseshoe crab spawning location.

Rank	Description
0	Spawning physically possible in this location
1	Location not possible for spawning
2	Cannot make sufficient conclusion about spawning site

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Spawning habitat characterization

ArcGIS Pro version 3.1 (Esri, Redlands, CA, USA) was used to spatially identify critical horseshoe crab spawning locations and their vulnerability to rising sea levels. Data were projected in the coordinate system for Florida using NAD 1983 (2011) and Florida GDL Albers (meters) [30]. The starting dates were selected to coincide with each county’s newest Environmental Sensitivity Index (ESI) publications (2016, 2020) [31]. The ESI is a spatial dataset that characterizes the marine and coastal environments according to their sensitivity to oil spills. ESI layers, which are collections of geographic data, were chosen to identify coastal habitat designations in Florida that correlate with horseshoe crab spawning sites. ESI Shorelines are classified by map developers through a combination of overflights, aerial photography, remotely sensed data, ground truthing, and existing maps and data spanning the previous 10 years [31]. Although habitats such as riprap and man-made structures are not considered typical horseshoe crab spawning sites, they have been included in this analysis. Where the ESI designated the shoreline of a spawning location as riprap, it was in proximity or adjacent to sandy beach patches where spawning is likely to occur. These sandy patches are smaller than the larger areas of riprap or man-made structures and were therefore classified based on the prominent habitat type in that location. By including habitats such as riprap that are adjacent to areas where horseshoe crabs can spawn, it can be assumed that the decline of such habitats because of rising sea levels may also decrease sand accumulation in these small sandy beach patches.

The ESI was used to characterize the spawning habitat types of horseshoe crabs and to examine potential habitat changes due to sea level rise in Pinellas County and Brevard County, Florida. Although the ESI was created for oil spill response, it is also a robust characterization of coastal habitats that provides a baseline for calculating horseshoe crab spawning habitats. The ESI includes sensitivity rankings, landward and seaward shore type designations, and a generalized ESI type [31]. Shoreline types, particularly seaward designations, were used for these analyses. The newest ESI versions for Florida’s “Northwest Peninsula” (2016) and “East Coast” (2020) were projected in ArcGIS Pro and clipped to the Pinellas County and Brevard County extents [31]. The outer edges of the ESI, which include the Atlantic and Gulf Coast facing beaches were removed from coastline measurements as spawning does not occur in these areas due to wave action.

To further elucidate the characteristics of spawning habitats used by horseshoe crabs in Florida, as compared to other areas within the horseshoe crabs range, the Digital Elevation Model for each county was used to determine the slope of these habitats. Slope calculations were established using the “Slope” tool in the spatial analyst toolbox. Subsequently, the “Add Surface Information” tool was used to calculate the median slope of the locations where spawning horseshoe crabs were present. Man-made structures and hardened structures were removed from slope analysis because the slopes reflected a vertical structure and skewed slope medians to exceed 90°.

Sea-level rise scenarios

In the present study, sea-level rise scenarios were based on the most recent “Intermediate-Low” scenario by the National Oceanic and Atmospheric Administration (NOAA) predictions [1] to determine a conservative estimate of habitat changes in areas most likely used by spawning horseshoe crabs. This scenario predicts the global mean sea level to rise by 0.5 meters by 2100 and includes estimates of vertical land motion and probabilities for extreme water levels along the U.S. coastline.

Horseshoe crab spawning locations

To establish a polygon of each shoreline type (henceforth, shoreline polygon) and identify a conservative estimate of the shoreline polygon that could be impacted by a 0.5 m sea-level rise, a 0.6 m buffer toward land was created to represent the area of optimal spawning habitat surrounding the high tide line. The ranked horseshoe crab reports were projected in ArcGIS Pro (Esri, Redlands, CA, USA), and a spatial join was created between the horseshoe crab and ESI layers to characterize the seaward shoreline type closest to each sighting. If the seaward designation was uncharacterized, the landward shoreline type was substituted.

Digital elevation model data

The Digital Elevation Model (DEM) [32] was used in conjunction with the intermediate-low risk scenario [33] to create sea level rise predictions. The DEM is lidar based and the resolution is approximately 3 meters. To visualize these scenarios, the “Raster Calculator” tool was used to create an expression for each timestep (2040, 2070, 2100). The timesteps identified cells in the raster considered “underwater” in a specified year. The thirty year interval was selected to observe an immediate or gradual change using established projections [1]. Next, the “Reclassify” tool was used to remove cells that were above sea level based on their elevation. Finally, the “Raster to Polygon” tool converted the raster layers for each timestep into polygons for further analyses.

The “Erase” tool was used in conjunction with the shoreline polygon and sea level rise polygon layers to determine the percentage of habitat within the shoreline polygon that would be lost or transition to a different habitat type. The percentage was calculated for each habitat polygon in years 2040, 2070, and 2100.

Results

Horseshoe crab spawning data and habitat characterization

Pinellas County.

Horseshoe crab sightings in Pinellas County varied by location and year. From 2017–2023, there were a total of 250 horseshoe crab spawning sightings (Fig 1). In 2017, there were 17 Horseshoe crab spawning reports, followed by 34 in 2018, 56 in 2019, 53 in 2020, 15 in 2021, 27 in 2022, and 48 in 2023, which included reports through June (Fig 1).

Fig 1 — Darker colors represent a higher concentration of reports.

Based on a Kernel Density analysis, there were three main clusters of points around Honeymoon Island, St. Petersburg, and Fort De Soto Park (Fig 2). Peak Horseshoe crab sightings were found during March (n = 45) and April (n = 46).

The habitats associated with spawning horseshoe crabs in Pinellas County varied, with the highest proportion in sheltered tidal flats (41.2), followed by mangroves (24), exposed tidal flats (16.8), course-grain sand beaches (6.4), fine- to medium-grain sand beaches (5.6), and mixed sand and gravel beaches (2.4) (Fig 3). When considering the landward habitat association, 23% of sightings were adjacent to man-made structures. In Pinellas County, the median slope of all spawning habitats calculated together was 0.9° (minimum = 0°; maximum = 19.8°). When isolating the beach habitats (fine to medium-grain sand, course-grained sand, and mixed sand and gravel beaches), the median slope is 2° (minimum = 0°; maximum = 9.7°).

Brevard County

Brevard County had 278 horseshoe crab spawning sightings from 2020–2023, which varied by location and year (Fig 1). In 2020, there were 112 horseshoe crab spawning reports, followed by 27 in 2021, 77 in 2022, and 62 in 2023, which includes reports through June 2023 (Fig 1). In general, spawning reports peaked in February (n = 83) and March (n = 77). Based on a Kernel Density analysis, there were two main clusters of points near Titusville, and Port Canaveral (Fig 4).

There were six habitats with the highest proportion of spawning horseshoe crab sightings in Brevard County (Fig 5): fine to medium-grain sand beaches (29.1), mangroves (17.6), scrub-shrub wetlands (16.6), vegetated low banks (11.5), riprap (7.6), and sheltered riprap (5). When considering the landward habitat association, 17% of sightings were adjacent to man-made structures. In Brevard County, the median slope of all spawning habitats calculated together was 0° (minimum = 0°; maximum = 19.1°). When isolating the beach habitats (fine- to medium-grain sand, course-grain sand, and mixed sand and gravel beaches), the median slope is 2.7° (minimum = 0°; maximum = 19.1°).

Sea–level rise scenarios

Sea level rise projections for 2040 showed a significant loss or change in habitat types for the shoreline polygons created in both Pinellas and Brevard Counties. The total area of Pinellas County shoreline polygons for 2016 was 819,763.16 m², and the total area of Brevard County shoreline polygons for 2020 was 838,797.60 m², based on the assumed 0.6 m buffer of the landward designation (Fig 6 and Fig 7). Mangrove habitats in Pinellas County had the most significant percent loss within the polygon area by 2040 (90%). For Brevard County, all beach habitats including mixed sand and gravel, course-grained sand, and fine-medium grained sand had the most significant percent loss by 2040 (95%, 92%, 91.9%, respectively). Both counties show a slight decrease in the rate of habitat loss between 2040 and 2100.

The largest loss in Pinellas County shoreline habitats associated with horseshoe crab spawning occurs from 2016 to 2100 (Table 2). Within the shoreline polygons, the habitat losing the most area was mangroves (96.3%), followed by sheltered tidal flats (87.4%), exposed tidal flats (75%), course-grain sand beaches (63.2%), and fine- to medium-grain sand beaches (56.6%).

Table 2. Potential loss of existing habitat based on the shoreline polygons associated with horseshoe crab spawning in Pinellas County from 2016–2100 due to sea level rise.

ESI Habitat Type	2016–2100 Percent Area Loss Within Shoreline Polygons
Scrub-Shrub Wetlands	24.2%
Fresh Water Marshes	39.7%
Sheltered Man-Made Development	47.2%
Sheltered Riprap	50.7%
Exposed Solid Man-Made Development	52.3%
Swamps	54.3%
Sheltered Vegetated Low Banks	55.7%
Fine-Medium Grain Sand Beaches	56.6%
Course-Grained Sand Beaches	63.2%
Exposed Riprap	66.0%
Exposed Tidal Flats	75.0%
Mixed Sand and Gravel Beaches	79.9%
Sheltered Tidal Flats	87.4%
Salt and Brackish Water Marshes	94.9%
Mangroves	96.3%

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The largest loss in Brevard County shoreline habitats associated with horseshoe crab spawning occurred from 2020 to 2100 (Table 3). Within the shoreline polygons, the habitat that lost the most area is fine- to medium-grain sand beaches (98%), followed by mangroves (94.8%), scrub-shrub wetlands (94.3%), riprap (84.4%), and sheltered vegetated low bank (71.2%).

Table 3. Potential loss of existing habitat based on the shoreline polygons associated with horseshoe crab spawning in Brevard County from 2020–2100 due to sea level rise.

ESI Habitat Designation	2020–2100 Percentage Area Loss Within Shoreline Polygons
Sheltered Man-Made Development	60.3%
Exposed Solid Man-Made Development	61.7%
Salt and Brackish Water Marshes	63.4%
Sheltered Vegetated Low Banks	71.2%
Riprap	84.4%
Sheltered Riprap	87.9%
Exposed Tidal Flats	92.7%
Swamps	94.1%
Sheltered Tidal Flats	94.2%
Scarps and Steep Slope	94.3%
Scrub-Shrub Wetlands	94.3%
Mangroves	94.8%
Course-Grained Sand Beaches	95.5%
Mixed Sand and Gravel Beaches	97.8%
Fine–Medium Grain Sand Beaches	98.0%

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Discussion

This study determined that the habitats where horseshoe crabs were frequently observed in Florida’s Pinellas and Brevard Counties were predicted to change under three sea-level rise scenarios. The continued rise in sea level directly threatens these spawning habitats and the future ecological and economic contributions of horseshoe crabs. This study characterized which habitat types and features were associated with spawning horseshoe crabs in Pinellas and Brevard County, Florida, and quantified the potential change of those habitats under conservative estimates of sea level rise from present day to 2100. These methods, combined with horseshoe crab spawning data, can be applied throughout the horseshoe crab range and can be used in consideration with shoreline armoring and restoration efforts.

Horseshoe crabs were predominantly sighted near sheltered tidal flats in Pinellas County and fine- to medium-grain sand beaches in Brevard County. These results are consistent with previous research that found juvenile horseshoe crabs occupy tidal flats during early life and that spawning horseshoe crabs favor sandy beach areas protected from wave energy, mainly between tidal flats and the extreme high tide water line [2,12]. The spawning beach habitats found in Pinellas and Brevard Counties were observed to have median slopes of 2° and 2.7°, respectively. The median slope of all habitat types combined was close to 0° because habitats such as mangroves, where a large proportion of horseshoe crabs were found in proximity, had a slope of 0°, which lowered the overall median. Previous studies have shown that horseshoe crabs prefer beaches with slopes ranging between 2°– 8° [12,16], where adequate aeration and moisture are favorable for embryonic development [12,15]. Since horseshoe crabs can become disoriented in flat areas after spawning, they will also rely on the slope of the beach rather than their vision to orient themselves and travel downslope [34].

Horseshoe crab sightings were most frequent around Fort De Soto Park, Honeymoon Island, and St. Petersburg in Pinellas County, in March and April. These spawning reports aligned with the timing of peak spawning in the nearby Florida counties of Pasco, Hernando, and Manatee [35]. Peak horseshoe crab sightings occurred in February and March in Brevard County, likely because of increasing water temperature, which triggers adults to move from deeper water to their spawning sites [36]. In this study, the habitats associated with horseshoe crab spawning, such as tidal flats, mangroves, and fine- to medium-grain sand beaches, were those most affected by the sea-level rise scenarios.

While both counties had similar quantities of horseshoe crab sightings, the habitats that horseshoe crabs were sighted spawning were not consistent. This may indicate that horseshoe crab populations, which have been proven to be genetically distinct [37], have different habitat preferences [38]. Alternatively, shoreline development and armoring may vary between counties, and thus horseshoe crabs may be settling for suboptimal spawning habitats. In particular, Pinellas County has highly developed shorelines. This study determined that 23% of spawning horseshoe crab sightings in Pinellas County and 17% of spawning horseshoe crab sightings in Brevard County were closest to armored or hardened shorelines in their landward spawning designations. As sea levels rise, the sand patches near these structures that horseshoe crabs use for spawning will be inundated, and no longer available for spawning. Horseshoe crabs have adapted to changing environmental conditions over time and space, however, their ability to adapt may not be enough to overcome the challenges posed by armored structures such as bulkheads and seawalls [2]. These structures are designed to prevent shoreline erosion, but they can interfere with and prohibit the natural spawning habits of horseshoe crabs [16]. Living shorelines are often used as a natural alternative to hard-armoring techniques in areas where the shoreline is eroding [39]. This method uses natural materials such as plants, stones, oysters, and sand fills to protect, restore, or enhance the natural shoreline habitat [40]. By designing and implementing robust, long-term adaptation strategies, coastal communities and critical shoreline spawning habitats can be protected.

Limitations

There are several limitations to the current study that should be considered in the interpretation of results. The nearest habitat to horseshoe crab sightings was based primarily on seaward designations. In cases where the seaward designation was absent, landward shore type was applied. As a result, in some instances the nearest habitat associated with horseshoe crabs was atypical and needed further review. If the ESI designation was not a habitat typically associated with horseshoe crab spawning, such as man-made structures or riprap, the site was reviewed in Google Earth. In some cases, small patches of sand adjacent to the structures were present, which may have been the actual spawning site but could not be identified by the ESI designations. Therefore, it is likely that man-made structures were classified as spawning habitats. Future studies may benefit from ground-truthing atypical spawning habitats. Since the sightings data is reliant on public reports, there may be a bias of sightings associated with more publicly accessible locations. Finally, this study did not analyze potential habitat gain or examine potential habitat changes. These percentages may not indicate absolute loss because habitat changes are likely to occur. However, it demonstrated the potential loss of a specific spawning habitat that was used by horseshoe crabs. Identifying the spawning habitats that may not suffer complete loss to sea level rise can narrow the focus in predicting whether they will remain the same or transition to another habitat type. It is also important to consider the habitat changes that may occur on the periphery of frequently used spawning habitats, as this could either hinder or enhance spawning success. For example, if a spawning site becomes unfavorable, an alternative habitat may become more suitable, enabling horseshoe crabs to spawn in that area instead [38].

Habitat changes

The natural movement of beaches over time can be restricted by hard structures adjacent to the coastline [41]. Developed coastal ecosystems are less capable of dynamically responding to sea level rise compared to natural areas, where beaches could adjust by changing landward to maintain equilibrium [42]. Several studies have begun investigating the consequences of rising sea levels on different habitats and their likelihood of changing to other habitat types. One such study conducted by Geselbracht et al. [43] focused on six estuaries along the Gulf Coast of Florida. The study employed a Sea Level Affecting Marshes Model (SLAMM) to predict the possible changes in habitat until 2100 and considered three scenarios with varying degrees of sea level rise (0.7 m, 1 m, and 2 m). According to their findings, if sea levels rise by 1-meter, tidal flats will likely lose the most spatial extent of all coastal habitats in the model. In contrast, mangrove forests are expected to increase their spatial extent in the first two sea-level rise scenarios (0.7 m and 1 m). However, if sea levels rise by 2 meters, mangroves will lose their total area to open water. This demonstrates a threshold beyond which they will be unable to cope with the rising sea levels. Therefore, further research is needed to determine accretion rates for various habitat types and changes that may occur in specific areas where horseshoe crabs spawn. If changes occur, identifying habitats that promote or deter successful egg development will aid in protecting shorelines and preserving horseshoe crab populations. Several studies have demonstrated that horseshoe crabs are not limited to sandy beaches and have been observed to spawn in alternative habitats such as peat beds, muddy sediments, and fringing salt marshes [38]. Although these habitats were previously thought to be unsuitable for embryonic development [16], recent studies have shown that clutches of horseshoe crab eggs laid in muddy salt marsh habitats contain similar developmental stages as clutches laid on sandy beaches [11,38].

Conclusion

Horseshoe crabs have adapted to changing environmental conditions over millions of years [44]. However, it is important to consider the different rates of change at which populations will be affected before assuming they will continue to persist [44]. The potential consequences of rising sea levels can be far reaching and can affect many species, especially those with life history strategies such as horseshoe crabs. This paper proposes a practical approach to using available data on species presence and characterizing shoreline habitats that are critical and likely to be impacted by sea level rise. Managers can use these projections to identify spawning habitats that are likely to change over the next 100 years. This information can help in making well-informed decisions regarding the best locations of living shorelines and beach replenishment programs. Such initiatives can support and mitigate the effect of rising sea levels, safeguard coastal communities, secure the survival of keystone species, and promote a healthy and sustainable ecosystem.

Supporting information

S1 Fig. Screenshot of horseshoe crab sighting questionnaire.

(PDF)

pone.0333812.s001.pdf^{(288.7KB, pdf)}

S1 Data. HSC data from Survey123 website (2009 - 2023) Edited.

(XLSX)

pone.0333812.s002.xlsx^{(253.7KB, xlsx)}

Acknowledgments

This manuscript was made possible by the support and collaboration of H. Jane Brockmann, Casey Butler, Ryan Gandy, Katharine Becker, and Kara Radabaugh. We are grateful for the exchange of ideas within our community. Thank you to the Citizen Scientists of Florida Horseshoe Crab Watch and the public for reporting spawning horseshoe crabs.

Data Availability

The raw survey data is available as supplemental material. The remaining data is open source and the origins are described in the methods section of the manuscript.

Funding Statement

The author(s) received no specific funding for this work.

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PLoS One. doi: 10.1371/journal.pone.0333812.r001

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13 Dec 2024

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Reviewers' comments:

Reviewer's Responses to Questions

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1. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #1: No

Reviewer #2: Partly

**********

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Reviewer #1: I Don't Know

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**********

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The PLOS Data policy

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**********

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**********

Reviewer #1: This manuscript by Contrada et al. seeks to assess “the potential impact of sea-level rise on horseshoe crab nesting habitats”. The authors do this by combining sightings of horseshoe crab spawning with GIS characterization of how shorelines differ across sea level rise scenarios, based on current habitat characterizations. The findings may be relevant if presented in the proper contexts and if the authors restrict their discussion to the results of their analyses. The methods of the paper are also unclear, but I think they can be improved with greater organization and a clearer workflow of the process. My specific comments below are meant to be helpful in providing constructive feedback to improve this manuscript.

• L27: There are no data on living shorelines presented in this paper to support this conclusion. Please remove.

• L40: Add “hereafter ‘horseshoe crab’” so the reader knows that you will be using that name to refer to this species.

• L51: Which changes are the authors referring to when they mention “these changes”? Please be specific about which changes you are referencing.

• L90: Only the rufa subspecies relies on horseshoe crab eggs for their migration.

• L103: Please clarify what the symbol “±” is meant to represent. Presumably this is a measure of error, but is it the standard error of the mean, or some other measure?

• L105: Please provide a link or another citation so readers know how to locate this information.

• L120-123: The font in this section is different than surrounding text.

• L142-143: This sentence indicates that the present study used only data from 2017-2023, but the sentence before includes reference to a figure with data from 2002-2023. Please clarify the use of these two datasets in the present study (i.e., how and why the data from 2002-2023 are used, and how and why the data from 2017-2023 are used.

• L143-144: Please provide specifics on the type and timing of imagery used to develop to ESI layers used in your analysis. This is important context for understanding when your habitat characterizations took place as compared to when your horseshoe crab sighting information was collected.

• 143-155: Maybe create a separate section for habitat designation (e.g., ‘Habitat Characterization’, or something similar) where you can describe the ESI? Right now this information is included in the section of ‘horseshoe crab nesting data’.

• L170-180: This paragraph would also fit better in a ‘Habitat Characterization’ section.

• L191-194: Please include specific information on which analyses were used.

• L189-196: This section currently comes after the sentence stating that “habitats ranked “zero”, which classified areas where nesting was possible, were used for further analysis”. So were the horseshoe crab reports discussed here (L189-196) all of the horseshoe crab reports, or just the ones ranked as zero? I think this is referring to all the horseshoe crab reports, but the way this is currently written I am unsure. Please clarify your workflow.

• L230: Kernal density analysis is not discussed in your methods. Please include information in your methods and workflow regarding what this analysis is and how it was conducted.

• L248: Kernal density is again mentioned here but not in the Methods, please address.

• L238-241, L255-259, L280-282: Why are figure captions located in the middle of the text?

• L308: Please clarify why is meant by ‘economic success of horseshoe crabs’.

• L310: Changing ‘shift’ to ‘loss’ would be more accurate. The results quantified the potential loss of habitats. A ‘shift’ in habitat would suggest that you quantified how habitat changed from one habitat to another.

• L348-360: The results provide no information on horseshoe crab eggs or their use by predators such as shorebirds. This section of the discussion is not relevant, but L337-339: Redundant with introduction.

• L373: The authors here acknowledge that they did not “examine potential habitat shifts”, an important point that I am glad they acknowledge here. I implore the authors to review their own language throughout the rest of the manuscript to be clear that they are not suggesting this elsewhere in the manuscript.

• L405-427: Living shorelines are not in the results of the manuscript. They are not tested experimentally nor do they appear to be listed as one of the habitat types in the analyses. The discussion of living shorelines thus serves purely a review of published studies, none of which appear to be specifically related to horseshoe crab habitat. As such, this section is not relevant to the results of the study and should be removed from the manuscript.

• L429: You use the term ‘horseshoe crab’ throughout the manuscript to refer to L. polyphemus, but in this instance you are using ‘horseshoe crab’ to refer to a broader taxonomic group of horseshoe crabs over millions of years. This should be clarified. Maybe you could change this to say ‘Xiphosurid horseshoe crabs have adapted to changing environmental conditions over millions of years’?

• L434-435: This statement is inconsistent with the limitations described by the authors. Please re-frame this statement to acknowledge the uncertainties around how these findings might be used by managers.

• L441: Is there acknowledgement of the funds that supported personnel to conduct this work or to collect horseshoe crab data?

• L452: Publication is missing

• L518: Is this a publication or a website? More information is needed.

• L615-617: This is not peer-reviewed. Please cite peer-reviewed study. Maybe this one?

https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/fee.2738

• Figures are blurry and hard to read.

• Figure 5: Please add a x-axis with values

• Figures 6 & 7. These are difficult to view for color-blind readers

Reviewer #2: Synopsis

In this manuscript, the authors characterized the possible change in nesting habitat at where American horseshoe crab were observed nesting in FL. Using crowd-sourced, community science data, the authors generated a dataset of HSC observations, assigned landcover classifications where HSC were observed, and quantified loss of landcover classifications at three future timesteps. The authors found habitat where HSC were observed in 2017 and 2020 was predicted to be lost due to anticipated sea level rise. The authors propose that living shoreLmay be a more suitable climate adaptation than hardened structures, such as sea walls, as living shoreLare also beneficial for spawning HSC. I see value to this work and highly encourage the authors to pursue revision of this work.

In the manuscript’s current form, there is inconsistent and unclear use of terminology that makes methodology unclear, and additional clarification is necessary to fully comprehend the study findings. Below, I’ve attempted to provide thorough feedback in the spirit of facilitating revision.

General feedback

-Consider writing in first person (e.g., we studied, we collected data, we analyzed, we found) throughout.

-Consistency in terminology: Throughout the manuscript, there’s inconsistent use of terminology that makes it difficult to follow the authors’ message. Some terms, like landward and seaward, should be clarified on their first use. There’s inconsistent formatting of name of horseshoe crab-consider dropping American after first use. Please consider using anthropogenic instead of ‘man-made’ throughout. There’s discussion on habitat shift, though what a ‘shift’ is was not defined, and there’s contradictory statements in the manuscript as to whether evaluating shifts occurred (see L337-374 vs L116-118, 126-129, 170-172, 308-311; I do not believe ‘shifts’ were measured, as this, to me, suggests an area of landcover classification x transforming to landcover classification y, though the methods are unclear). Please consider using change instead of loss when quantifying the area of landcover classifications predicted to be affected by sea level rise.

-Introductory content: I found it unusual to read a horseshoe crab paper that does not use the word ‘spawning’ to describe horseshoe crab breeding behavior. I recommend using the word ‘spawning,’ when referring to active breeding behaviors, and ‘nests,’ to refer to the locations where HSC eggs are deposited.

-Methods and habitat selection/preference: This manuscript presents a characterization of the areas where horseshoe crabs were reported. There is no use vs availability analysis, which is an analysis frequently used to explore organismal habitat selection (and preference). Without a use vs availability analysis or a similar statistical analysis, the use of ‘habitat selection’ or ‘preference’ is inappropriate.

-Community science: There’s a lot of excellent value from crowd-sourcing data collection, though there can be some caveats to consider. As written, I’m unclear on the potential caveats and more information is needed to understand the data that was submitted by community members. I’ve outlined some points for consideration in the manuscript.

--Consider adding a paragraph later in the Introduction that explains the HSC community science program in FL, then get into the details of this program in the Methods. It would be helpful to describe the value of using community science data in evaluating species distributions. The end of the Introduction would likewise be an excellent place to mention the value of the community-derived HSC database.

--What is meant by ‘horseshoe crab activity’ (L134-135)? Does this mean that when community members saw a spawning horseshoe crab (as suggested by header on line 133), they reported, or was the data portal intended for any observation of a crab? If observations were of any crab, how was nesting distinguished from a dying or deceased crab on the beach, or a crab that may be embedded in the substrate between tides?

--Were observations only of marked crabs (e.g., https://www.fws.gov/project/horseshoe-crab-cooperative-tagging-program) or did observations also include unmarked crabs? If the dataset included unmarked crabs, how did the authors control for the potential of repeat observations?

--Were there efforts to control for sampling effort (frequency of reporting) which could vary based on popularity of the beach with recreationists/community members, or in areas that may be more popular with nature-minded individuals that would be more inclined to use a wildlife reporting app?

--What training resources were available for the public? For example, the data include fields for HSC sex, which may not be readily apparent to community members. Please consider explaining signage, visuals of the app as an appendix, and other resources that community members could use when submitting a report.

-Use of citations: Some sentences in the manuscript appear to be missing citations (e.g., L69-70). Numerous review articles are cited in lieu of the primary sources; please consider citing primary articles. Some citations appear to be used incorrectly (e.g., reference [21] in L92-94). Please consider a careful review of references to ensure all are used appropriately.

Specific feedback

-L1-2: The use of populations in the title suggests an evaluation of demographic impacts, which were not part of the study objectives. Not evaluating demography is not a detractor, though perhaps a recast to reflect that this manuscript evaluates projected habitat changes for spawning HSC would be more appropriate. Consider including full study organism name (American horseshoe crab) and Latin binomial epithet.

-L12: Sentence starts with a value-laden word. Consider replacing ‘unfortunately’ with ‘Though.’

-L13 and throughout: Lowercase H and C in horseshoe crab. After first mentions of American horseshoe crab in Abstract and Introduction, consider consistently using ‘horseshoe crab.’

-L17 and throughout: Consider replacing ‘habitat type’ from ESI products with ‘landcover classification,’ re: Krausman, P. R. and M. L. Morrison. 2016. Another plea for standard terminology. The Journal of Wildlife Management 80(7):1143-1144. doi: 10.1002/jwmg.21121

-L32-41: This paragraph about climate change indicates that sea level rise will affect coastal ecosystems, but does not provide detail on what changes sea level rise is predicted to cause. Describing anticipated consequences from sea level rise will be important for understanding the potential implications for HSC.

-L36-38: Consider removing ‘in response to the threat of rising sea levels,’ as the starting clause of the sentence already indicates this.

-L42 vs 148: rip rap vs riprap.

-L49-50: HSC also breed along the Yucatan Peninsula of Mexico, though the Mexican population is genetically distinct (García-Enríquez, J. M. et al. 2023. Genetic study of the American horseshoe crab throughout its Mexican distribution. Conservation and management implications. Biodiversity and Conservation 32:489-507. doi: 10.1007/s10531-022-02508-4).

-L58-59: The background information on HSC breeding ecology is scattered in the Introduction. It would be helpful to describe HSC life history, spawning behavior, and development in a single, cohesive paragraph, and this location would be an appropriate location.

-L59-61: Consider a recast for this sentence. Many organisms will breed/reproduce in suboptimal areas if preferred habitat is unavailable (Fretwell, S. D. 1972. Populations in a seasonal environment. Princeton University Press.) and there’s a lot of work in understanding population density, age, and other factors in habitat preference and habitat selection. Perhaps recast to focus on habitat features that facilitate HSC spawning and embryonic development. Consider moving a paragraph from the Discussion (habitat shifts paragraph on L382-404) into the Intro, which feels more appropriate when considering beneficial habitat features for HSC.

-L65-67: Consider adding “respectively” after 0.3 mm [12] to clarify measures correspond to each state.

-L90: Consider Red Knot Calidris canutus rufa, the subspecies that migrates from South America to Arctic Canada and relies on HSC eggs in DE bay.

-L92-94: There’s limited evidence to suggest HSC are a major dietary item of sea turtles since the 1980s. The reference used here [21] does not support HSC are a dietary item of sea turtles.

-L96-112: While indicating HSC importance is valuable, this paragraph contains a lot of information on the economic value of HSC, which feels tangential to the study goals. Consider shortening this section to succinctly highlight the economic value of HSC, and incorporate threats to HSC.

-L99: Consider Latin binomial epithets for American eel and whelk.

-L103: Please clarify measure after average-is this SD, SE?

-L105: Citation should be numeric.

-L109: Remove space between ‘sea-’ and ‘level’

-L115: Study does not truly evaluate habitats, as this suggests a statistical comparison. Instead, the manuscript presents a characterization of the habitats and simulates a projected change at three distinct times. Please consider recasting to clarify this is a descriptive study.

-L116-118: Why were these years chosen? Please consider including a rationale for these years.

-L119: Introduction seems to end abruptly. Please consider including statements on the importance of this work for FL, and how these results could extend elsewhere along the Gulf and Atlantic coasts.

-Methods line 120: Consider placing study site description before statement on ArcGIS-i.e., reorganize Methods with L125-132, L133-168, then include L121-123 between L169-170.

-L121: Add manufacturer information (e.g., Esri, Redlands, CA, USA).

-L126: Remove Latin epithet for horseshoe crabs.

-L130: Replace text citation with numeric.

-L126-132: Please consider adding additional detail on study sites, as this will be useful for others in both contextualizing findings and understanding how results here may extend elsewhere. Consider adding approximate coordinates for locations (can be centered in county) for readers. Consider describing the usage of shorelines-how much of the area is protected (e.g., wildlife refuges), what’s the ratio of protected to unprotected areas (e.g., space center in Brevard county). Additionally, this study includes areas on the Gulf and Atlantic coasts, which may have very different temperatures, currents, and habitats. The authors may consider framing this study as an Atlantic coast/Gulf coast comparison, which may expand the interest of readers outside FL.

--Relatedly in this section, please consider a separate map that shows study areas, and limit locations on the map to only include names. The information displayed in Figure 1 is a result and would be more suitable in the Results section.

--Consider explaining how sea level rise is expected to affect the barrier islands in the study areas. Are barrier islands migrating, changing?

-L146-147: A brief explanation of how coastal habitat designations were made in ESI would be useful. E.g., were these designations the result of supervised classifications in ArcGIS, were they

-L134-135, Figure 1: Shows a result, would be more appropriate to reference in the Results section and not in Methods.

-L134-135: Who were results reported to (i.e. which agency)? How were people able to complete the form (e.g., was there signage, or did people have to know of the reporting system) and how did they know how to fill out fields in the form? Understanding the resources and training available to respondents is necessary for understanding the validity of behavioral observations that include fields for HSC sex. Are these of marked HSC (e.g., https://www.fws.gov/project/horseshoe-crab-cooperative-tagging-program)? Are reports of HSC standardized to control for ‘sampling effort,’-e.g., I would a priori predict more reports of HSC in areas with more people. If HSC are unmarked, how might the possibility of repeated observations of the same individual in areas with high human traffic skew interpretation of results?

-L138: How was it determined if an observation was ‘robust’?

-L141: Include a reference for kernel density analysis.

-L142: How was subtidal nesting ruled out from the data? From a quick look at the data, I did not see a field explicitly for spawning/nesting. How was it determined if an observation was below the water line? This is especially important, as tideLcan change rapidly within a year, and vary drastically between years.

-L143-149: Additional information describing the ESI dataset is necessary. How did NOAA designate landcover classifications? Please consider including a table that lists the landcover classifications (habitat types) and a description of that classification.

-L160-161: Were in-person or spatial assessments of HSC observations at high tide or low tide? Were assessments made at the same year as the reported observation? How did the authors account for potential changing shorelines?

-L165-166: It’s unclear what kinds of landcover classifications were considered, and so it remains unclear what habitats were considered likely, unlikely, or unknown nesting substrates. Please include a table of landcover classifications, a description of that classification, and sources that support assignments in the ranking system.

-L178-180: It was surprising to see HSC do not nest on FL’s Atlantic and Gulf coast facing beaches; elsewhere on the North Atlantic, HSC nesting can occur on Atlantic beaches. A citation to help justify the exclusion of these beaches in this study would be useful.

-L182, 206: There are two subheaders for ‘Sea-Level Rise Scenarios,’ in the Methods section.

-L186-188: The Atlantic Coast and Gulf Coast are anticipated to experience differing extents of sea level rise. Were the study areas evaluated under the same scenario, or were scenarios spatially explicit?

-L190-193: The explanation behind the choice to use 0.6 m wide polygons suggests a possible misinterpretation of sea level rise. If I understand, text appears to suggest that a 0.6 m strip is a conservative boundary that encompasses shoreline habitats that will be affected by a 0.5 m sea level rise. However, this would imply that a 0.5 m sea level rise will result in a waterline shift of 0.5 horizontal m across the beach, when in practicality, the waterline should shift 0.5 m in elevation. To understand the land area affected by the elevational change of the waterline, analyzing the DEM is necessary to determine that line.

-L199-204: I’m unclear what additional information is gained from the inclusion of nesting habitat slope. Was there one DEM available to quantify slope, and if so, how did the authors consider the potential for annual variation in beach slope and can slope be reasonably extrapolated to 2040, 2070, and 2100 to inform estimates of habitat change due to sea level rise? Were slope estimates averaged across all habitat types in an area? It’s also impossible to glean HSC selection of slope in this study without a use vs availability analysis.

-L207-209: Consider moving reference to the digital elevation model to line 199, which is the first reference of the DEM in the Methods. Greater description of the DEM would be useful.

-L210-212: It’s unclear if the timesteps reflect a 2D area of change (‘subtracting’ the area anticipated to be lost to sea level rise from the coast at time step t) vs a 3D area of change that considers the volume of water and the beach slope. Additional clarification is necessary to understand how pixels were considered ‘underwater’ at each of the three timesteps.

-L214: Remove extra apostrophe between quotes of “Erase”

-L217-218: Consider omitting how percent is calculated.

-~L222: Consider providing some general summary statistics about the study areas-lengths of shorelines, annual areas of landcover classifications. Consider also including summary statistics on community science reporting, which could be used to explain Figure 1 (vs in the Methods). On average, how many observations are made per year (± SD)? In Fig 1, how many years of data are shown? Would results change if data were presented as mean annual crab sightings? The Introduction notes two distinct spawning periods for FL HSC (Feb-May, Aug-Nov; L84-85), which is very unique. Please consider describing temporal variability in HSC nests observations.

-L225, 243: Separating results into counties makes this paper very FL-specific, and difficult to glean generalities that extend beyond FL. Consider recasting to focus on Gulf coast vs Atlantic coast, and characterize the habitats available on each coast.

-L233-236, 250-253: How were these habitats (landcover classifications) delineated? This wasn’t clear in the Methods

-Fig 2, 4: I note that some of these ‘clusters’ overlap with densely populated or popular areas (e.g., St Petersburg). How were data handled to account for sampling effort? Consider combining this figure into one with two panels.

-Fig 3, 5: These show percents, not proportions. Consider combining this figure into one with two panels.

-Table 2, 3: Consider including a starting area of the landcover classification from 2017/2020. As an example, the projected loss of 94.8% of mangroves appears alarming, though may be of minimal ecological consequence if the starting area of mangroves represented an insignificant area in 2020.

-Results: Consider including tables for each county (consider presenting as Atl and Gulf coasts) with numbers of reported crabs in each landcover classification, and/or numbers of crabs per year.

-L304-313: This line buries the lede by restating results. Consider recasting to focus on the most important finding, that habitats where HSC were observed most frequently were predicted to be lost under sea level rise scenarios. Then contextualize this finding.

-L328: Capital S in De Soto Park

-L337-374: This sentence is at odds with statements in the objectives (L116-118), methods (L1226-129, 170-172), and the discussion (L308-311).

-L336-337: The use of habitat preference is not appropriate here, as there was no use vs availability analysis. Further, there was no contextualization of Atlantic coast vs Gulf coast, or ocean vs bay areas, which prevents inference on preference.

-L336-347: How does the area of hardened and developed shoreLin the two study counties compare to other areas along FL’s coast?

-L362-363: Consider removing this sentence.

-L348-360: This section feels out of place, especially considering that temporal variation of results was not presented. Consider removing.

-L361-381: Consider recasting the Discussion, and incorporating this information, as relevant, in Methods and Discussion. While it is critical to transparently acknowledge limitations of any given study, singular dedicated paragraphs can be unintentionally disparaging. Please consider contextualizing results and within that same paragraph, provide the relevant limitations for that finding.

-L443-434: The objectives of this study did not include providing a practical approach for identifying areas with high concentrations of species that rely on shoreline habitats for nesting, but rather investigated the potential impact of sea level rise on HSC nesting habitas (L113-114). The methods used did not appear to prioritize elucidating high concentrations of HSC, as the authors note they chose to focus on two counties (L116). Further, the methods are unclear if the analysis accounted for sampling effort, which would be needed to fully understand species abundances and distributions.

-Data: The data submitted with the manuscript should include metadata that define each of the columns. But concerningly, the data contain sensitive and personally-identifying information. I recommend removing columns U through AI, which contain sensitive information.

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PLoS One. 2025 Nov 5;20(11):e0333812. doi: 10.1371/journal.pone.0333812.r002

Author response to Decision Letter 1

16 Jul 2025

We have attached our response to reviewers. We found the critique extremely helpful and we appreciate their efforts.

Attachment

Submitted filename: PONE-D-24-52275_Review_comments.docx

pone.0333812.s004.docx^{(344.7KB, docx)}

PLoS One. doi: 10.1371/journal.pone.0333812.r003

Decision Letter 1

Phuping Sucharitakul

10 Aug 2025

Dear Dr. Heres,

I think your manuscript is nearly ready for publication. Although one of the reviewers recommended a major revision, I believe only minor revisions are needed. If you make the required changes, I will be happy to accept it for publication.

Please submit your revised manuscript by Sep 24 2025 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org . When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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We look forward to receiving your revised manuscript.

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Phuping Sucharitakul

Academic Editor

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

Reviewer #1: (No Response)

Reviewer #2: All comments have been addressed

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2. Is the manuscript technically sound, and do the data support the conclusions??>

Reviewer #1: No

Reviewer #2: Yes

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Reviewer #2: Yes

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Reviewer #1: The revised manuscript by Contrada et al. describes the potential impacts of sea level rise on coastal habitats used by spawning horseshoe crabs. The authors combine sightings of horseshoe crab spawning with sea level rise scenarios and shoreline characterization. Unfortunately, the authors chose not to incorporate a number of edits suggested by the previous reviews, which continue to make the manuscript difficult to interpret. Most importantly, this is related to inconsistent terminologies. A few specific items are mentioned below.

• L137-141: If I’m reading this correctly, Figure 1 represents information that was compiled and presented for this article and has not previously been published. As such, it would be more appropriate for Figure 1 to be considered a result, and should be presented in the Results section. In this way, the author can then clarify which datasets were used for which portions of this current study.

• Please include justification for how calculating slope advances the stated goal of the study describing the “potential impact of sea-level rise on horseshoe crab spawning”?

• L377: Suggest using ‘anthropogenic’ instead of man-made.

• L380-381: The authors are quoted as saying “Finally, this study did not analyze potential habitat gain or examine potential habitat shifts”. Both reviewers from the previous version of the submitted manuscript pointed out the inconsistency in terminology and requested more consistency. Given the authors own words here, ‘habitat change’ would be an appropriate phrase to use instead of ‘habitat shift’.

• L387-388: The cited study doesn’t seem to support the statement. If I’m reading it correctly, the paper doesn’t address a shift in spawning areas.

• L360-361: I believe this statement overstates the benefits of living shorelines. Please provide a citation for this statement: “One effective solution to protect shorelines from the impact of rising sea levels is to implement “living shorelines”.

Reviewer #2: PONE-D-24-52275R1

Potential implications of rising sea level on American Horseshoe Crab (Limulus polyphemus) spawning beaches in two Florida counties

In this manuscript, the authors characterized the possible change in spawning habitat at where American horseshoe crab in FL. Using crowd-sourced, community science data, the authors generated a dataset of HSC observations, assigned landcover classifications where HSC were observed, and quantified loss of landcover classifications at three future timesteps. The authors found areas of spawning habitat where HSC were observed in 2017 and 2020 was predicted to be lost due to anticipated sea level rise. The authors propose ways their work can inform restoration and management efforts, and how methodology could be used elsewhere.

I appreciate the authors' care and dedication during revision, which has improved their interesting and valuable work. I have very minor areas of feedback:

-Contextualization of sites: I understand the authors' decision to retain reference to FL counties (vs suggestion to reframe as Atlantic coast/Gulf coast). In Methods (~lines 129-132), please consider adding a brief clause indicating that HSCs in these sites are not expected to differ significantly in behavior (as noted in the response to reviewers document, and cite any supporting information that already exists in the Lit Cited section). Doing so will help reinforce that these are analagous sampling points and not necessarily points chosen to represent different populations/climate change regimes.

-Line 129: Unnecessary 'n' in sentence.

-Lines 118, 133, 225, 232, 348, 356: Unusual punctuation (e.g., space before period, two periods, missing period, missing space, missing period). Please consider a careful review to check for other typos.

-Lines 442-444: I believe the complete citation for this should be: Sweet WV, Kopp RE, Weaver CP, Obeysekera J, Horton RM, Thieler ER, Zervas C. 2017. Global and regional sea level rise scenarios for the United States (Technical Report NOS CO-OPS 083). National Oceanic and Atmospheric Administration, Silver Spring, MD. doi: 10.7289/V5/TR-NOS-COOPS-083

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Reviewer #1: No

Reviewer #2: Yes: Christy N Wails

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PLoS One. 2025 Nov 5;20(11):e0333812. doi: 10.1371/journal.pone.0333812.r004

Author response to Decision Letter 2

12 Sep 2025

Our response to reviewers can be found in the response to reviewer document attached. We found the advice very helpful and are grateful for their time and effort.

Attachment

Submitted filename: Comments to the Author_05_09_2025.docx

pone.0333812.s005.docx^{(11.9KB, docx)}

PLoS One. doi: 10.1371/journal.pone.0333812.r005

Decision Letter 2

Phuping Sucharitakul

18 Sep 2025

Potential implications of rising sea level on American Horseshoe Crab (Limulus polyphemus) spawning beaches in two Florida counties

PONE-D-24-52275R2

Dear Dr. Heres,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Reviewers' comments:

PLoS One. doi: 10.1371/journal.pone.0333812.r006

Acceptance letter

Phuping Sucharitakul

PONE-D-24-52275R2

PLOS ONE

Dear Dr. Heres,

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

S1 Fig. Screenshot of horseshoe crab sighting questionnaire.

(PDF)

pone.0333812.s001.pdf^{(288.7KB, pdf)}

S1 Data. HSC data from Survey123 website (2009 - 2023) Edited.

(XLSX)

pone.0333812.s002.xlsx^{(253.7KB, xlsx)}

Attachment

Submitted filename: PONE-D-24-52275_Review_comments.docx

pone.0333812.s004.docx^{(344.7KB, docx)}

Attachment

Submitted filename: Comments to the Author_05_09_2025.docx

pone.0333812.s005.docx^{(11.9KB, docx)}

Data Availability Statement

The raw survey data is available as supplemental material. The remaining data is open source and the origins are described in the methods section of the manuscript.

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PERMALINK

Potential implications of rising sea level on American Horseshoe Crab (Limulus polyphemus) spawning beaches in two Florida counties

Danielle Contrada

Claire Crowley-McIntyre

Berlynna Heres

Roles

Abstract

Introduction

Methods

Study sites

Horseshoe crab spawning data

Table 1. Description of ranking system used to evaluate each publicly reported horseshoe crab spawning location.

Spawning habitat characterization

Sea-level rise scenarios

Horseshoe crab spawning locations

Digital elevation model data

Results

Horseshoe crab spawning data and habitat characterization

Pinellas County.

Fig 1. Cumulative public reports of spawning horseshoe crabs by county in Florida from January 2002– June 2023 (N = 6,752).

Fig 2. Horseshoe crab spawning locations in Pinellas County, Florida.

Fig 3. Proportion of horseshoe crabs by ESI habitat types within the shoreline polygon of Pinellas County, Florida.

Brevard County

Fig 4. Horseshoe crab spawning locations in Brevard County, Florida. Color denotes density of sightings in each location from 2020–2023.

Fig 5. Proportion of horseshoe crabs by ESI habitat types within the shoreline polygon of Brevard County, Florida.

Sea–level rise scenarios

Fig 6. Total area (m2) of each horseshoe crab habitat type projected to be present within the shoreline polygon in Pinellas County, FL in each sea-level rise scenario.

Fig 7. Total area (m2) of each horseshoe crab habitat type projected to be present within the shoreline polygon in Brevard County, FL in each sea-level rise scenario.

Table 2. Potential loss of existing habitat based on the shoreline polygons associated with horseshoe crab spawning in Pinellas County from 2016–2100 due to sea level rise.

Table 3. Potential loss of existing habitat based on the shoreline polygons associated with horseshoe crab spawning in Brevard County from 2020–2100 due to sea level rise.

Discussion

Limitations

Habitat changes

Conclusion

Supporting information

Acknowledgments

Data Availability

Funding Statement

References

Decision Letter 0

Phuping Sucharitakul

Roles

Author response to Decision Letter 1

Decision Letter 1

Phuping Sucharitakul

Roles

Author response to Decision Letter 2

Decision Letter 2

Phuping Sucharitakul

Roles

Acceptance letter

Phuping Sucharitakul

Roles

Associated Data

Supplementary Materials

Data Availability Statement

ACTIONS

PERMALINK

RESOURCES

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Links to NCBI Databases

Fig 6. Total area (m²) of each horseshoe crab habitat type projected to be present within the shoreline polygon in Pinellas County, FL in each sea-level rise scenario.

Fig 7. Total area (m²) of each horseshoe crab habitat type projected to be present within the shoreline polygon in Brevard County, FL in each sea-level rise scenario.