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. 2025 Jul 29;59(31):16368–16378. doi: 10.1021/acs.est.5c03049

Technology Solutions for Overcoming the Coral Recruitment Bottleneck

Shannon L Dixon 1,*, Hendrikje Jorissen 2, Ann Marie Hulver 1, Jacob Welter 1, Robert J Toonen 2, R3D Consortium 3, Joshua S Madin 2, Andréa G Grottoli 1,*
PMCID: PMC12356192  PMID: 40728888

Abstract

Recruitment is a vital demographic process that replenishes populations and increases genetic variation, making it fundamental to ecological resilience. On coral reefs, rates of coral recruitment and post-recruitment survivorship are naturally low, resulting in a bottleneck to population growth and restoration efforts. Therefore, we investigated the potential that two technologies, specialized settlement modules and the Underwater Zooplankton Enhancement Light Array (UZELA), improve early life success. We hypothesized that greater complexity of specialized settlement modules increases larval settlement rates in tanks and the combination of settlement modules with UZELA increases subsequent recruit survivorship and growth on the reef. We found that complex settlement modules enhanced recruit settlement, survivorship, and growth relative to the lowest complexity module. The addition of UZELA to the settlement modules further doubled survivorship and quadrupled growth. Our experiment demonstrates a proof-of-concept that combining these novel technologies offers promise for reducing the time for corals to complete their most vulnerable phase of life, accelerating the timeline for establishing genetically diverse coral populations, and offering a possible step-function change in coral recruitment success that could help overcome the recruit survivorship bottleneck in coral restoration.

Keywords: coral recruits, climate change, coral reefs, recruit settlement, heterotrophy

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Introduction

Coral reefs are facing intensifying global and local stressors from anthropogenic impacts. , Some models predict that if the current trends continue, 90% or more of live coral may disappear by midcentury. , However, large-scale, long-term experiments suggest coral reefs will persist but with lower coral cover and diversity. − Regardless of the scenario, the future of coral reefs depends on their capacity to recover from disturbances and cope with chronic stressors. Larval recruitment and subsequent survival of locally adapted coral recruits are critical for the persistence of corals this century. , Coral sexual reproduction promotes reef connectivity and increases genetic variation and reef resilience. − Unfortunately, stressors that impact adult corals tend to interfere with reproduction as well as the early life-stages of their progeny. ,− Globally, coral recruitment has declined over the last three decades with statistically detectable decline beginning in the late 1990s. , This has limited the ability for coral populations to restore themselves and has amplified the cycle of reef decline. The future of reefs now depends not only on mitigation of climate change and local stressors, but also on active restoration efforts. −

Active restoration of coral reefs currently focuses on transplantation of reared or fragmented coral colonies to degraded reefs. Emerging technological interventions to optimize adult coral growth and health are now being developed as strategies to enhance restoration efficacy. − Unfortunately, technological interventions for coral juveniles struggle to overcome the naturally very high larval post-settlement and early juvenile phase mortality rates due to competition, predation, and other environmental stressors. ,− Mortality is lowered with artificial structure technologies that incorporate microhabitat features and provide refugia for coral recruit settlement. − Microhabitat features and certain geometric designs can increase larval settlement by up to 80-fold , and mitigate recruit predation losses, − ,− ,− showing promise for enhancing coral recruitment and juvenile survival, and boosting restoration effort success. However, this strategy may be further enhanced by combining these structures with other noninvasive technological interventions that take advantage of natural coral physiology.

The Underwater Zooplankton Enhancement Light Array (UZELA) is a programmable underwater light that capitalizes on the phototactic behavior of zooplankton , and locally concentrates natural zooplankton by 7-fold in Hawai’i. Corals are symbiotic organisms that obtain nutrition two ways: 1) heterotrophically via the ingestion of zooplankton, dissolved organic matter, and particulate organic matter that they capture with their tentacles, and 2) photosynthetically via the translocation of photosynthetically derived sugars from their endosymbiotic algae. , Zooplankton feeding provides up to 40% of daily metabolic requirements in healthy adult corals and over 100% in some bleached corals. − Zooplankton is also a critical source of nutrition for tissue and lipid synthesis in healthy and stressed corals. − Corals provided enhanced feeding opportunities can also have higher growth and survivorship. − Since feeding rates in adult corals increase as zooplankton concentrations increase, ,, it is likely that the same would be true for coral recruits and could be a mechanism for increasing recruit growth and survivorship. In coral recruits, the onset of zooplanktivory occurs as early as 1 day post-settlement − and is vital for early recruit survivorship to replenish energy reserves expended for settlement. , In tanks, enhanced zooplankton concentrations enhance recruit survivorship and growth. − In the field, UZELA is the only available technology known to concentrate natural zooplankton and increase adult coral feeding by up to 50-fold in Hawaiian corals. We suspect UZELA would benefit coral recruits in the field as well.

Here, we 3D printed artificial settlement modules that provide microrefugia for coral recruit settlement. We evaluated the effect of increasing settlement module complexity (flat surface, smooth dome, and spiral dome shape) on Pocillopora acuta coral recruit settlement in tanks. We then added UZELA to the settlement modules, placed them on the reef, and quantified the coral recruits’ survivorship and growth after six months. We hypothesized that the recruits on settlement modules with the highest surface complexity would have greater initial settlement and once coupled with UZLEA-enhanced zooplankton concentrations, would have the highest survivorship and growth after six months on the reef. With global settlement declining and high post-settlement mortality in recruits, developing technological interventions to assist recruit survivorship and growth is one possible strategy for facilitating reef restoration and persistence.

Methods

Settlement Module and UZELA Technologies

This study includes two provisionally patented technologies: 3D-printed ceramic larval settlement modules and UZELA (Underwater Zooplankton Enhancement Light Array) (Figures a and b) and a provisional patent has been filed for their combined use. Settlement modules were designed using Blender v3.4 software and consisted of a cylindrical base (9.5 cm diameter by 6.5 cm high) topped with a flat surface, smooth dome, or spiral dome cap (0, 7, 7.5 cm tall, respectively) (Figure b). The resulting meshes were exported as.stl files, converted to a 3D print path using the slicing software Simplify3D, and printed with the Delta Wasp 40100 3D clay printer (Wasp, Italy). The clay (midfire pottery clay, Soldate 60, Aardvark Clay and Supplies, United States) was prepared using a pugmill (Pugmill/Mixer NVS-07, Nidec, Japan) and the modules were printed using a 4 mm diameter nozzle, creating a microstructure of horizontal layers on the smooth dome and spiral dome settlement modules. Each module surface was pierced with nine holes of 6 mm diameter, air-dried in an airconditioned environment for 5 days, and then fired in a kiln (Jupiter Sectional Kiln, L&L Kilns, United States) using cone 8 (with a 2 h preheating program). The second technology, UZELA, is an autonomous, battery-operated, programmable, submersible underwater light that locally concentrates natural zooplankton by at least 4-fold and enhances coral feeding rates by up to 50-fold , (Figure a). Briefly, UZELA includes a clear PVC housing with a light emitting diode (LED) that emits white light at an average of 700 umol photons m–2 s–1 through the acrylic lens when tested in the lab in air. The UZELAs were programmed to turn on for 1 h after sunset daily, which is consistent with the timing and duration of previous studies on coral zooplankton feeding , and capitalizes on the peak zooplankton migration period. The onset of the experiment coincided with the beginning of peak recruitment in many Hawaiian species and ran for six months, which was the lifespan of the UZELA batteries. An empty-UZELA housing (control housing) or a UZELA was placed under the cylindrical component of each settlement module, allowing light from UZELA to shine upward through all settlement module holes and into the water column at night (Figure c).

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Experimental design. (a) Side view of UZELA. (b) Rack setup showing flat surface (bottom left and top right), smooth dome (bottom right) and spiral dome (top left) settlement modules. Control housings or UZELAs were under the settlement modules. (c) Holes in the settlement modules allowed light from UZELA to shine through at night and attract zooplankton. (d) Arrangement of racks in situ with control housing and UZELA treatments in alternating racks. Gray racks were attached to PVC frames and weighed down with cinder blocks. Photos by (a) AM Hulver and (b–d) AG Grottoli.

Experimental Design

Sixteen of each flat surface, smooth dome, and spiral dome settlement modules were equally divided into two flow-through seawater tanks for preconditioning on 18 May 2023 at the Hawai’i Institute of Marine Biology. Twelve Pocillopora acuta colonies were collected at 1.5 ± 0.5 m depth between 15–16 May 2023 from the Point Reef in Ka̅ne’ohe Bay (21°26′02.0″N, 157°47′12.0″W) and placed in holding tanks covered by 73% shade cloth to mimic light levels at collection depth. On 30 May 2023, the P. acuta colonies were equally divided between the two tanks with the preconditioned settlement modules (Supplementary Table 1). The colonies naturally spawned and new recruits settled onto the settlement modules over 9 days in the tanks. On 8 June 2023, all adult colonies were removed from the tanks and the number of recruits on each settlement module surface were counted using a UV light (SOLA NightSea) under a yellow NightSea mask filter. When excited with UV light, living coral fluoresces green, allowing for the accurate counting of the total number of recruits on each settlement module. Ten settlement modules of each type, with similar numbers of recruits within module type, were selected for the next phase of the experiment and randomly assigned to one of two feeding treatments: the control housing or UZELA-enhanced zooplankton treatment (UZELA). The control housing served as a control for any UZELA housing effects.

Settlement modules, control housings, and UZELAs were secured to eight 60 cm × 60 cm perforated PVC racks that were each attached to separate 60 cm × 60 cm x 30 cm PVC frames (Figures b and d). Four racks were assigned to the control housing treatment and four to the UZELA treatment. Control housings and UZELAs were placed in predrilled holes within each rack and secured with zip-ties. Settlement modules were placed over the control housings and UZELAs and secured with mini bungee cords. All three settlement module types were represented at least once on each rack (Figure b & Figure ). Six of the eight racks consisted of four settlement modules, while the remaining two racks had three settlement modules (Figure ). Loaded racks were deployed on the Point Reef at the Hawai’i Institute of Marine Biology in Ka̅ne’ohe Bay (21°26′02.0″N, 157°47′12.0″W) at 1.5 m below mean tide and 19 m from shore on 13 June 2023 in a line parallel to the shore, alternating control housing and UZELA racks, and anchored with cement blocks (Figure d & Figure ). The midpoint of each rack was approximately 2 m apart. The distance between racks was sufficient to ensure that UZELA rack lights did not affect zooplankton concentrations on the control racks. Every week, the UZELA light lenses were cleaned by hand and the holes on the settlement modules were kept open by cleaning them with pipe cleaner brushes. The UZELA lights turned on from 20:00 to 21:00 each night for the duration of the six-month experiment (Figure c, Supplementary Table 1).

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Schematic of the experimental design setup in situ on the Point Reef (21°26′02.0″N, 157°47′12.0″W) at the Hawai’i Institute of Marine Biology. Experimental racks were 1.5 m below mean tide and 19 m from shore. Each rack included a mixture of flat surface (green), smooth dome (yellow), and spiral dome (red) settlement modules that were placed either over blank-UZELA control housings (light blue inner circle) or UZELAs (black inner circle). Midpoints of racks were approximately 2 m apart. The temperature loggers (blue square) and one light logger (gray triangle) were attached to racks with zip ties. Photo by A.M.H.

Recruits were counted in situ on 1 September 2023 and 4 December 2023 to capture the midpoint and end point of the study. In situ counts were conducted at night using a yellow NightSea mask filter over a UV light (SOLA NightSea). Only recruits on the surface of the settlement modules were counted. Recruits on the supporting cylinder base and in the holes were excluded as they were not part of the module structure being investigated. On both 5 and 6 December 2023, four racks were removed from the reef, placed in outdoor flow-through seawater tanks, and photographed either at night or during the day under an opaque cover using an underwater UV light (SOLA NightSea) and an Olympus Tough camera fitted with a yellow NightSea mask filter. A ruler was included in each photo for scale (Supplementary Figure 1). The number of surviving recruits on the surface of the module and the planar area of each recruit were determined from the photographs. Percent recruit survivorship in September and December were calculated from in situ counts relative to starting recruit counts in June for each settlement module. Recruit planar area was quantified using image analysis software ImageJ, by drawing an ellipse around each recruit and using the measure area tool embedded in the software.

Environmental monitoring and zooplankton concentrations

Temperature and downwelling photosynthetically active radiation (PAR) were recorded every 15 min with two HOBO Water Temperature Pro v2 Data Loggers and one HOBO Pendant Light 64K Data Logger (Onset HOBO, Bourne, MA, USA 02532), respectively, from 13 June 2023 to 4 December 2023. Daily average temperature was calculated, and only daylight PAR was used for average daily downwelling light intensity. Seawater temperature averaged 27.2 ± 0.05 °C for the first five months of the study, then dropped to 24.8 ± 0.08 °C in December (Supplementary Figure 2, Supplementary Table 1). Daytime light intensity averaged 2174.275 ± 143 μmol photons m–2 s–1 in June and gradually decreased to 722.942 ± 55 umol photons m–2 s–1 by October (Supplementary Figure 2, Supplementary Table 1). Both the changes in temperature and light are expected changes in response to the seasonal transition from summer to winter in Hawai’i. Zooplankton tows were conducted directly above a control housing rack and a UZELA rack using an 8.8 cm diameter sieve with 50 μm mesh on 13 June, 3 December, and 5 December 2023. All plankton tows were separated by size class (55–150 μm, 150–400 μm, 400–800 μm, and >800 μm), preserved in 10% formalin, and the number of zooplankton per functional group were identified under a dissecting microscope and their concentrations standardized to zooplankton m–3.

Statistical analyses

A one-way ANOVA was used to evaluate the effect of settlement module type on recruit settlement numbers in the tanks. A Kruskal–Wallis one-way analysis of variance and Dunn’s posthoc tests with Benjamini-Hochberg correction for multiple comparisons was used to test the effect of settlement module type on initial recruit density (counts standardized to settlement module surface area), since normality could not be achieved. A three-way ANOVA with added random effect of rack number was run to evaluate the effect of settlement module type, UZELA, and time on recruit survivorship where all factors were fixed and fully crossed. As experimental racks were not found to be significantly different from each other (Supplementary Table 2), we concluded that racks represented natural variation in environmental conditions. Therefore, rack number was removed from the models, and three-way ANOVAs were run (settlement module, UZELA, and time) and used in interpretation of data. A two-way ANOVA was used to evaluate the effects of UZELA and zooplankton size on zooplankton concentrations. Posthoc Tukey tests were used to evaluate differences within effects for all ANOVAs. All data were tested for normality and log-transformed if needed prior to analysis to achieve normality. The effect of UZELA on zooplankton functional group diversity was evaluated with a Shannon–Wiener index of diversity. To test the effect of settlement module type and UZELA on recruit planar area, we conducted nonparametric Kruskal–Wallis one-way analysis of variance and Dunn’s posthoc tests with Benjamini-Hochberg correction for multiple comparisons, as normality could not be achieved with the data. Since all recruits died on the flat settlement modules, this analysis was conducted on the smooth and spiral dome settlement module data only. All parametric and nonparametric statistics were carried out in RStudio with significant differences reported when p ≤ 0.05.

Results

Settlement

Immediately after settlement in the tanks, the initial recruit counts were at least three times higher on the smooth and spiral dome settlement modules than on the flat surface settlement modules (Figure a, Supplementary Table 3A). However, once standardized to surface area, recruit density was similar on the spiral dome and flat surface settlement modules and two times higher on the smooth dome settlement module (Figure b, Supplementary Table 3B).

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(a) Average number of recruits (±1SE) and (b) average recruit density on the flat surface, smooth dome, and spiral dome settlement modules in the tanks in June (N = 16 settlement modules per module type). Significant differences among settlement module types are indicated with upper case letters. Statistical details in Supplementary Table 3A and 3B.

Survivorship

After three months on the reef without UZELA, there was 0%, 6%, and 11% survivorship compared to 13%, 12%, and 23% survivorship with UZELA on the flat surface, smooth dome, and spiral dome settlement modules, respectively (Figure , Supplementary Table 4).

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Average percent survivorship of recruits (±1SE) after three and six months in situ. Percent survivorship of recruits on the reef with control housings (white bars) or UZELA (gray bars) in (a) September and (b) December, relative to their starting values in June (N = 5 settlement modules per treatment). Survivorship was higher with UZELA than with controls. Uppercase letters indicate significant differences between settlement module type. Statistical details in Supplementary Table 4.

The 2-fold higher survivorship with UZELA corresponded to the 14-fold higher zooplankton concentration and higher diversity of zooplankton over the UZELA racks compared to the control racks (Figure , Supplementary Tables 5 and 6).

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Comparison of zooplankton concentrations between treatments. Average zooplankton concentrations (±1SE) over control housing racks (white) and UZLEA racks (gray) sampled on three separate nights (N = 3 per size fraction). Zooplankton concentrations were significantly higher with UZELA than with the control housing. Size classes that share upper case letters do not differ from each other (Supplementary Table 5 and 6).

After six months on the reef, the surviving coral recruits were visible and appeared healthy, all racks were present, the settlement modules had accumulated small amounts of algae, and the UZELA lights had lasted for the duration of the study. While survivorship decreased from September to December, and none survived on the flat surface modules, the pattern of 2-fold higher survivorship with UZELA on the smooth and spiral dome settlement modules persisted (Figure , Supplementary Table 4).

Growth

After six months on the reef, recruit planar area did not differ between smooth and spiral dome settlement modules in the control housing treatment (Figure white bars, Supplementary Table 7). However, with UZELA, planar area quadrupled on smooth dome settlement modules and doubled on spiral dome settlement modules (Figure gray bars, Supplementary Table 7). Growth was not measurable on the flat surface settlement modules as none of these recruits survived.

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Average planar area of recruits (±1SE) on the flat surface, smooth dome, and spiral dome settlement modules with control housing (white bars) or UZELA (gray bars). Sample size is indicated at the base of each bar. Significant differences of recruit planar area between control housing and UZELA within a settlement module type are indicated with *. The upper-case A and B letters indicate that the planar area of recruits was higher on the smooth UZELA compared to the spiral UZELA settlement modules (i.e., comparison of gray bars). There were no survivors on the flat modules at the end of the study. Statistical details in Supplementary Table 7.

All raw data used in this study are given in Supplementary Tables 8 and 9.

Discussion

Smooth dome and spiral dome settlement modules had greater recruit settlement and survivorship than completely flat settlement modules (Figures & 4), and UZELA further enhanced both survivorship and growth of coral recruits (Figures & 6). These findings support our hypothesis that complex settlement modules increase recruit settlement and that the combination of complex settlement modules and UZELA-enhanced local zooplankton concentrations increase recruit survivorship and growth post-transplantation. Here, we discuss the nuances in recruit responses of these results and explore their implications to the persistence of corals in the future.

Settlement

Initial settlement of recruits was higher on smooth and spiral dome settlement modules compared to the flat surface settlement modules (Figure a). Our findings are consistent with previous studies showing that coral larvae preferentially settle in crevices and the undersides of substrates, typically avoiding horizontal surfaces, such as the flat settlement modules. − The differences in settlement among the module types could be due to one or more of the following factors: settlement preference in the microstructure layers produced during 3D printing of the domes, surface area differences, hydrodynamic conditions during settlement, the presence of grazers, or some combination of factors. Microstructure has been observed to enhance recruit settlement, which is thought to be driven by similarity in size between larvae and microtopography of rugose surfaces. − The microstructure within the horizontal layers produced during printing of the smooth and spiral dome modules may have provided comparable initial complexity compared to the flat surface settlement modules which lacked microtopography (Supplementary Figure 3). This could explain greater raw counts on the smooth and spiral dome settlement modules, compared to the flat. Second, the smooth dome settlement modules have 51.33% lower surface area than the spiral dome modules but have similar recruit counts, which results in greater recruit density (Figure ). Therefore, differences in settlement module surface area are not likely the driver of the settlement patterns observed in this experiment. Hydrodynamics play an important role in the settlement of coral larvae. , The low flow tank conditions may have facilitated the preferential settlement in the microtopographical grooves formed during 3D printing of the smooth and spiral dome settlement modules. Unfortunately, flow conditions in the tanks were not measured during spawning and settlement but were likely lower than typically observed on reefs. Finally, the coral larvae were exposed to the settlement modules in tanks without grazers, which may have disproportionately benefited the smooth dome module compared to the spiral dome module. In a complementary in situ study with grazers present and higher flow conditions, recruit densities were highest on spiral dome settlement modules. Therefore, a combination of microtopography, hydrodynamic conditions, and the lack of grazers likely played an important role in the recruit settlement patterns in this study.

Survivorship

On the reef, average recruit survivorship without UZELA (i.e., control housings) ranged between 0 and 11% in September and 0–3% in December (Figure white bars). Decreases in recruit survivorship over time are consistent with the life history strategy of spawning corals, which have their highest mortality immediately after settlement. , When coral larvae settle in cryptic undersides of settlement modules, they experience less predation which increases survivorship. , However, our smooth and spiral dome settlement modules had similar survivorship after six months (Figure b, white bars), indicating that the microtopography of the smooth dome modules, rather than the recesses in the spiral dome module, may be sufficient for predation protection and/or that there is no post-transplantation advantage to one dome settlement module type over another for P. acuta recruits. Interestingly, the pattern of survivorship after six months (Figure b) was the same as the initial settlement count pattern (Figure a) suggesting that initial settlement counts may be a predictor of survivorship. The lack of any long-term survivors on the flat surface settlement modules, with or without UZELA, enforces earlier findings that coral post-settlement survivorship is limited on surfaces with low structural complexity ,, (Figure ). A pattern of survivorship of recruits in the holes of the flat surface settlement modules reinforces the importance of microrefugia in artificial seeding devices (Supplementary Figure 4).

With UZELA, survivorship on the reef at least doubled and ranged from 11 to 23% in September and 0–6% in December (Figure gray bars), supporting our hypothesis that UZELA increases recruit survivorship. These findings further suggest that enhanced survivorship was due to the greater feeding opportunities provided by UZELA (Figure ). These findings are consistent with previous studies showing that coral recruits reared in tanks with zooplankton have higher survivorship once transplanted onto reefs. ,, The doubling of survivorship with UZELA demonstrates the importance of zooplankton feeding in the early stages of recruit survival and the potential for UZELA to be used to enhance recruit’s survivorship in situ. Additionally, increased grazing on algae by nocturnal herbivorous invertebrates and fish that may have been attracted to the UZELA light may be advantageous to corals because it would reduce competition for space and possibly contribute to the observed patterns of survivorship. However, this is unlikely since accidental grazing by herbivorous fish is a leading cause of high post-settlement mortality. ,,, Although survivorship at the end of six months is below 10%, the probability of long-term survival is much higher with complex settlement modules coupled with UZELA than without (Figure ).

Growth

Interestingly, without UZELA, there was no difference in average recruit growth between the smooth and spiral dome settlement modules (Figure white bars), suggesting that the dome component of the settlement module is more critical than the complexity of the dome itself for growth. A consideration when designing settlement modules is providing sufficient light availability for growth. When coral larvae settle in cryptic undersides of settlement modules, they experience less light, which restricts growth. , Although we did not measure light availability within the spirals, there were no differences in growth between the smooth and spiral dome settlement modules without UZELA, suggesting that either the crevices in the spiral dome settlement module did not shade recruits and/or that P. acuta recruits are not susceptible to low light stressors. Additionally, only recruit diameter could be reliably measured from the photos. It is possible that recruits in the crevices of the spirals grew more vertically than horizontally. Therefore, recruit growth may be underestimated in one or both smooth and spiral dome settlement modules, and future work on metrics such as vertical growth, volume or biomass would help to quantify growth in more detail.

With UZELA, average recruit growth quadrupled on the smooth dome settlement modules and more than doubled on the spiral dome settlement modules (Figure gray bars), supporting our hypothesis that UZELA increases recruit growth on the reef. These results are consistent with previous findings showing that enhanced feeding in tanks can increase recruit growth. , The advantage of recruit tank feeding continues post-transplantation with previously fed recruit growth surpassing that of unfed recruits. , This implies that the growth benefits of UZELA may continue even once the device is removed, further enhancing the probability of recruit survival in situ. Larger recruits with faster growth rates have a greater chance of survival due to their ability to outcompete algae and avoid incidental grazing from fish. , Smaller recruits (i.e., < 3 mm diameter) have only a 20% chance of surviving for three months in tanks. Therefore, faster recruit growth with UZELA likely contributed to their enhanced survivorship (Figures & 6). Assuming linear growth rates, diameter growth of recruits without UZELA averaged 0.8 mm month–1, while recruits exposed to UZELA grew 1.2–1.8 mm month–1 (Figure , diameter was calculated from December planar area). Thus, recruits without UZELA would be <3 mm in diameter after three months, while those with UZELA would average 3.6–5.3 mm in diameter and have a higher chance of reaching a critical size needed for long-term survival. Furthermore, some corals become reproductively mature once they reach a specific size threshold, and faster recruit growth could result in corals reaching reproductively viable sizes faster.

Implications

Increasing recruit settlement and resilience is critical as coral settlement, survivorship, and growth are declining globally due to ocean warming and other stressors. − Since feeding on zooplankton enhanced recruit survivorship following heat stress in a tank experiment, it is likely that UZELA-enhanced feeding could increase recruit survivorship in situ following heat stress on reefs. Further research is needed to test this hypothesis, to evaluate the longer-term impact of both settlement module structural complexity and UZELA on coral maturation and reproduction, and to evaluate any possible positive or negative effects these technologies might have on other co-occurring reef organisms. Additionally, UZELA does introduce artificial light at night. Ambient light at night has been documented to disrupt the circadian rhythm and behaviors of coral and other marine animals. , However, in these scenarios, the artificial light at night was typically downwelling or diffusing over a large area for longer than 6 h per night. , UZELA shines upward, illuminates a very small area, and is on for only 1 h per night. Therefore, the potential negative impact of UZELA light on coral biology and other animals is likely minimal, though further testing is needed. Lastly, this study was conducted with one species in a single location to establish proof-of-concept for these combined technologies. Since artificial seeding devices and tank-based enhanced zooplankton concentrations increase survivorship and growth of recruits of several coral species from many geographical areas, ,,, we expect that in situ our complex settlement modules coupled with UZELA would also benefit other coral species, corals at other locations, and potentially other benthic organisms that rely on zooplanktivory in their early life stages. Further research is needed to validate the applicability of the combined technologies to other species and organisms at different locations and to explore the underlying biological and biochemical drivers of the enhanced growth and survivorship.

Our study is the first to show an in situ combined technology method for enhancing recruit survivorship and growth. The increase in survivorship and growth with UZELA implies that recruits exposed to higher concentrations and diversity of zooplankton feed more, similar to their adult counterparts (Figures & , Supplementary Tables 4, 5, 6, and 7). In the field, our complex settlement modules enhanced survivorship and growth, and that survivorship further doubled, and growth quadrupled with UZELA. Our findings provide proof-of-concept validation needed to investigate pathways for scaling up the implementation of settlement modules coupled with UZELA into active coral restoration programs. UZELA is currently undergoing commercialization with a manufacturing goal of making it cost-effective and user-friendly. The clay domes can be made with a relatively inexpensive 3D printer and clay extruder. The feasibility of deploying these technologies at scale still needs to be investigated and will be dependent on available resources. Thus, enhanced feeding opportunities with UZELA coupled with nonflat settlement dome modules appears to be a viable in situ intervention that could be implemented in restoration practices to maximize recruit survivorship and growth and help to overcome the current recruitment bottleneck on reefs. Although mitigating global and local stressors is key to the long-term survival of coral reefs, these technologies could theoretically be implemented at small scales to enhance restoration success. Overall, the combination of these two novel technologies of complex dome settlement modules and UZELA provides a tool to overcome the recruit survivorship bottleneck to aid in the persistence of corals in the face of climate change.

Supplementary Material

es5c03049_si_001.pdf (825.8KB, pdf)
es5c03049_si_002.xlsx (14.6KB, xlsx)
es5c03049_si_003.xlsx (13.6KB, xlsx)
es5c03049_si_004.xlsx (13.1KB, xlsx)

Acknowledgments

We would like to thank Austin Krupar and Lain Weaver for field and lab assistance. Funding for this project was provided by the University of Hawai’i Foundation (#MA1783) and the National Science Foundation (OCE 1838667) to A.G.G., the Defense Advanced Research Projects Agency (DARPA) to J.S.M., R.J.T., and R3D, and the Coral Restoration Foundation (CRF-2025-002). This material is based upon work supported by the Defense Advanced Research Projects Agency under the Reefense Program, BAA HR001122C0134. The views, opinions, and/or findings expressed are those of the author and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government. The Rapid Resilient Reefs for Coastal Defense (R3D) Consortium is an association of academic and industry partners who are focused on developing and testing nature-based coastal resilience solutions, including project management, site assessments, data management, and permitting support for experimental campaigns to test and prove these technologies. The R3D Consortium Members are listed in Supplementary Table 10. Corals were collected under Amendment 1 to SAP 2023-31 issued by the Hawai’i Division of Aquatic Resources.

The data underlying this study are available in the published article and its Supporting Information.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.5c03049.

  • Additional experimental details, temperature and light profiles, and detailed photographs of the settlement modules, statistical tables, zooplankton data, and figure of recruit survivorship in the holes of settlement modules (PDF)

  • Recruit settlement and survivorship raw data (XLSX)

  • Recruit planar area in December raw data (XLSX)

  • R3D Consortium author list and ORCIDs (XLSX)

‡.

S.L.D. and H.J., and J.S.M. and A.G.G. contributed equally to this work.

A.G.G., J.S.M., R.J.T., and R3D wrote the proposal and secured the funding for the study. A.G.G., J.S.M., S.L.D., H.J., and A.M.H. designed the study. A.G.G. conceptualized and patented UZELA. S.L.D., H.J., J.S.M., and A.G.G. built the two technologies tested in the study. S.L.D., H.J., A.M.H., J.W., J.S.M., and A.G.G. participated in fieldwork and data collection. SD carried out data analysis, produced the figures, and drafted the initial manuscript. All authors contributed to the interpretation of results and revisions of the manuscript. All authors have read and agreed to the published version of the manuscript. S.L.D. and H.J. are co-first authors. A.G.G. and J.S.M. are co-senior authors.

The authors declare no competing financial interest.

Published as part of Environmental Science & Technology special issue “Ocean Health”.

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Associated Data

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

Supplementary Materials

es5c03049_si_001.pdf (825.8KB, pdf)
es5c03049_si_002.xlsx (14.6KB, xlsx)
es5c03049_si_003.xlsx (13.6KB, xlsx)
es5c03049_si_004.xlsx (13.1KB, xlsx)

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

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