Megabenthic assemblages from South Adriatic marine mesophotic environments

Abstract

Mesophotic habitats in the Mediterranean Sea host rich and diverse benthic assemblages, dominated by invertebrates alongside sciaphilous algae. Recent findings suggest that certain mesophotic bioconstructions built by invertebrates, while classified under the coralligenous definition, differ significantly in their taxonomic composition. This study investigates diversity patterns in megabenthic assemblages associated with algal and invertebrate bioconstructions along the Apulian coast, using α- and β-diversity metrics derived from an image analysis approach. Across 360 analyzed frames, 81 taxa were identified, revealing distinct coverage patterns that separate mesophotic algal assemblages from both coral and oyster bioconstructions. Morphological seabed features and primary bioconstructors played a key role in shaping the associated communities. These findings highlight the substantial differences between upper and deeper megabenthic assemblages and emphasize the ecological importance of mesophotic bioconstructions as biodiversity hotspots, underscoring their role in supporting Mediterranean marine ecosystems and the need for targeted conservation strategies.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-15386-x.

Introduction

Marine mesophotic ecosystems are functionally defined as zones where irradiance levels remain sufficient to sustain net positive photosynthesis¹. Their lower boundary typically corresponds to the depth at which only 1% of surface photosynthetically active radiation (PAR) is available². In terms of bathymetric range, the most widely accepted definition of the mesophotic zone refers to benthic habitats distributed between depths of approximately 30 and 150 m, occurring under dim conditions in tropical and temperate zones^3–6. However, as environmental factors such as water clarity and solar radiation play a crucial role in determining the limits of the mesophotic zone, and these vary considerably within and between locations, this interval might not necessarily represent the depth at which mesophotic conditions occur. Therefore, they should be defined more based on shifts in the taxonomic composition of assemblages associated with regional environmental factors, rather than on fixed bathymetric limits^7,8.

Interest in mesophotic habitats has increased in recent decades, paving the way for studies focused on the biodiversity of their communities. These habitats indeed harbor rich benthic assemblages dominated by sciaphilous algae and invertebrates, including sponges, cnidarians, bryozoans, and polychaetes⁹. Mesophotic habitats have historically been less explored due to the technical and logistical challenges associated with greater depths, resulting in large areas remaining mostly unknown, particularly in their deeper portions¹⁰. In recent years, however, the development of technical diving (e.g., rebreathers) and remotely operated vehicles (ROVs) has facilitated their exploration².

In the Mediterranean Sea, benthic assemblages at mesophotic depths typically consist of a mix of calcareous red algae and a wide variety of invertebrates, usually referred to as coralligenous communities¹¹. Here, calcareous algae and, to a lesser extent the skeletal remains of invertebrates, contribute to building hard substrates of biogenic origin, which support the most diversified benthic communities of the Mediterranean. Recently, however, some mesophotic assemblages that do not meet the criteria of coralligenous sensu stricto (due to the lack of the algal component) have been discovered along the Apulian coast^12–14. In these habitats, invertebrates with calcareous skeletons, such as the bivalve Neopycnodonte cochlear and the scleractinians Phyllangia americana mouchezii and Polycyathus muellerae, serve as the primary engineers, providing substrate for a rich and diversified associated benthic fauna, often characterized by the development of invertebrate filter-feeder facies^15,16.

Although the benthic assemblages associated with invertebrate bioconstructions are often included within the broad and heterogeneous coralligenous definition, recent studies¹⁷ have revealed significant differences in their taxonomic composition, both in the organisms responsible for constructing the structures, defined as ecosystem engineers¹⁸ due to their ability to modify the environment by creating complex three-dimensional habitats¹⁹ and in the benthic communities they support. The observed differences suggest that the diversity and abundance of the associated fauna are influenced by the interplay between the nature of the primary substrate, which initially shapes the colonizing fauna, and the identity of these engineer species, which increases structural complexity, along with other environmental factors, as supported by other studies^20,21. To enhance our understanding of the taxonomic composition of mesophotic bioconstructions in the Mediterranean, the analysis of substrate cover by benthic species serves as an essential methodological complement. It allows the quantification of spatial distribution and dominance patterns within benthic communities, providing insights into the ecological structure and functional roles of the species involved^22,23.

In the present research, we studied the diversity patterns in megabenthic assemblages associated with mesophotic algae (hereafter MAB), and invertebrate (oyster and coral, hereafter MOB and MCB, respectively) bioconstructions across spatial geographic scales along the Apulian coast. Nomenclature follows what was proposed by Corriero et al.¹⁷. To describe key patterns of substrate coverage, we estimated the percentage cover of the main megabenthic assemblages associated with the bioconstructions through an image analysis approach. This visual approach allowed us to detect overlap and turnover in communities among the different bioconstructions by assessing α- and β-diversity patterns. Quantifying taxonomic changes along a geographic gradient and between shallow and deep habitats is a major prerequisite for testing ecological hypotheses and implementing conservation strategies^24,25.

Methods

Study areas

The research was carried out in six areas along the Apulian coast (Southern Italy), where preliminary surveys^{12–14,16,17} indicated the occurrence of both algae and invertebrate mesophotic bioconstructions. The sites, from north to south, are defined as follows: the Tremiti Islands (TRM), Monopoli (MON), Capitolo (CAP), San Foca (SFC), Otranto (OTR) and Santa Maria di Leuca (SML) (Fig. 1).

Fig. 1.

Study area. Geographic location of the study sites along the Southeastern coast of Italy. TRM= Tremiti, MON= Monopoli, CAP= Capitolo, SFC= San Foca, OTR= Otranto, SML= Santa Maria di Leuca. The map was created using QGIS (version 3.34.12; QGIS Development Team, 2024. QGIS Geographic Information System. Open Source Geospatial Foundation Project. https://qgis.org).

Sampling method

According to Corriero et al.¹² when both types of bioconstructions coexist at the same site, animal-based structures are typically found at greater depths compared to algal ones, which tend to be in shallower areas. In this way, two distinct bathymetric ranges were explored at each site: (i) within 35 m and (ii) deeper than 45 m. These ranges correspond to the general distribution observed across all selected sites, where algal bioconstructions are predominantly found between 25 and 35 m, while invertebrate outcrops occur between 45 and 65 m. At the six selected sites, algal-based bioconstructions (MAB) are consistently present, whereas the two types of invertebrate-based bioconstructions are mutually exclusive. Specifically, Mesophotic Oyster Bioconstructions (MOB) were found at four sites (TRM, CAP, OTR, SML), while Mesophotic Coral Bioconstructions (MCB) were observed at two sites (MON, SFC). Table 1 summarises the bathymetric distribution of bioconstructions at each site studied.

Table 1.

List of sampling sites with their corresponding depths. TRM = Tremiti, MON = Monopoli, CAP = Capitolo, SFC = San Foca, OTR = Otranto, SML = Santa Maria Di Leuca. MAB = Mesophotic algae bioconstructions, MOB = mesophotic oysters bioconstructions, MCB = mesophotic coral bioconstructions.

Type	Site	Depth
MAB	TRM	25–30 m
	MON	25–35 m
	CAP	25–35 m
	SFC	20–35 m
	OTR	25–35 m
	SML	25–27 m
MCB	MON	50–55 m
	SFC	45–53 m
MOB	TRM	40–55 m
	CAP	53–64 m
	OTR	45–64 m
	SML	45–65 m

A Remotely Operated Vehicle (ROV), the Mariscope FO III, was employed to capture images at both depths across all sites. The ROV was outfitted with high-definition video cameras and two laser beams positioned 10 cm apart, which were projected onto the substrate to gather quantitative data, such as coverage estimations. At each site, 30 images were captured for each bioconstruction type, yielding a total of 60 images per site. To assess the spatial distribution of the bioconstructions, 180 images were analyzed for MAB, 120 for MOB, and 60 for MCB. Images were selected based on criteria such as the proximity of the camera to the substrate (approximately 10 cm), the inclination of the substrate (sub-vertical, never horizontal), and the clear visibility of the two laser beams.

Images were then analyzed to estimate the percentage cover of the main megabenthic taxa associated with the bioconstructions. Coverage values were calculated by measuring the substrate portion covered by each species. All megabenthic organisms were identified to the lowest taxonomic level possible. PhotoQuad software²⁶ was used to determine the coverage percentage of each benthic taxon present. Due to their similar external morphology, some species were grouped into operational taxonomic units (OTUs), such as “Keratosa” (KE), which includes Ircinia variabilis, Sarcotragus spinosulus, and Sarcotragus foetidus, and “encrusting orange sponges” (EOS), which includes Crambe crambe and Spirastrella cunctatrix. The use of OTUs for benthic invertebrates is a widely accepted method for determining distribution patterns²².

Statistical analysis

The number of taxa in each study area was calculated as a measure of alpha (α)-diversity²⁷. Differences in α-diversity between the bioconstructions were assessed using the Kruskal-Wallis test. Additionally, mean species richness and percent coverage of megabenthic taxa were calculated and graphed over time. A non-parametric Wilcoxon test was used to evaluate significant differences in the total coverage percentage at each site.

Two data matrices were created to analyze the quantitative coverage data of benthic species detected. One matrix included the ecosystem engineers (e.g., N. cochlear and P. mouchezii/P. muellerae), allowing for the assessment of their physical presence in the communities, while the other excluded these engineer species to avoid self-correlation between mesophotic bioconstructions at different sites. This approach ensured that the analyses focused solely on the fauna associated with the mesophotic bioconstructions.

Canonical Analysis of Principal Coordinates (CAP) based on Bray-Curtis distances, using square-root transformed data, was employed to visualize changes in the benthic assemblages. Taxa contributing most to the similarities among sample groups were identified through Similarity Percentage Analysis (SIMPER), with a 90% cutoff criterion for species coverage. Differences between algal, coral, and oyster mesophotic assemblages, both within and among sites, were analyzed using Permutational Multivariate Analysis of Variance (PERMANOVA) based on Bray-Curtis dissimilarities. The two-way PERMANOVA included the factors Type of bioconstruction (3 levels: MAB, MOB, MCB) and Site (6 levels: TRM, MON, CAP, SFC, OTR, SML). Models were run with 9999 unrestricted permutations of the raw data.

Species turnover (β-diversity) was assessed using Bray-Curtis similarity, based on quantitative coverage data, and visualized through box plots representing the distance of each sample from the group centroid. This method allowed us to evaluate the extent to which the bioconstructions differed from each other in terms of species composition.

CAP, SIMPER and PERMANOVA analyses were conducted using PRIMER v6 + PERMANOVA²⁸, while β-diversity analysis was performed in R software (version 4.2.3)²⁹.

RESULTS

The analysis of 360 frames revealed the coverage patterns of 81 megabenthic taxa across the study sites (see Supplementary Material, Table S1). Alpha (α)-diversity, measured as the richness of species associated with each bioconstruction, did not differ significantly (H = 0.41, p = 0.51) between the two animal-based bioconstructions (MOB and MCB). However, it was notably higher in MAB, which had an average of 72 taxa, compared to the 47 and 43 taxa for MOB and MCB, respectively.

The general trend showed that at MAB, green, brown and red algae, cnidarians, and bryozoans dominated in terms of coverage. At MCB, the highest coverages were observed for poriferans, and cnidarians. At MOB, mollusks, poriferans, and cnidarians were the dominant groups (Fig. 2). The internal taxonomic composition of each mesophotic bioconstruction type is shown in Fig. 3.

Fig. 2.

Relative abundance (%) of major taxonomic groups observed in each bioconstruction type: MAB = mesophotic algal bioconstructions, MOB = mesophotic oyster bioconstructions, MCB = mesophotic coral bioconstructions.

Fig. 3.

Percentage cover of megabenthic taxa. Distribution of coverage values (%) grouped into phyla in the studied mesophotic assemblages. MAB = mesophotic algal bioconstructions, MCB = mesophotic coral bioconstructions, MOB = mesophotic oyster bioconstructions.

Overall, both algal- and invertebrate-based bioconstructions exhibited high coverage values, ranging from 80% to 131%, with significant differences between sites (Wilcoxon test = p < 0.05) (Fig. 4). Coverage values were generally higher in algal bioconstructions. Among the invertebrate bioconstructions, SFC and TRM showed slightly higher coverage values.

Fig. 4.

Percent coverage (mean ± SD) of megabenthic communities in MAB (orange), MOB (green) and MCB (violet) bioconstructions across sites. The last column (“Total”) represents the overall mean percent cover per habitat type (MAB, orange; invertebrate bioconstructions, dark green). Numbers above bars indicate the number of taxa observed per site.

For MAB, the highest number of megabenthic species was observed at TRM (36 species), while the lowest was at SML (26 species). In contrast, for invertebrate bioconstructions, MOB ranged from 21 species (CAP) to 30 species (SML), and MCB ranged from 30 species (SFC) to 32 species (MON) (Fig. 4).

The Canonical Analysis of Principal Coordinates (CAP) based on the quantitative data of coverage values (Fig. 5) revealed that mesophotic algae assemblages were separated from both coral and oyster assemblages, based on their coverage patterns (permutations 999, p = 0.001). These results were consistent across both types of matrices: those that did not contain the ecosystem engineer species (Fig. 5a) and those that did (Fig. 5b).

Fig. 6.

Box plot showing the distance to the centroid as a proxy for beta diversity in each type of bioconstruction (a), within algal bioconstructions MAB (b) and invertebrate bioconstructions (MOB + MCB) (c). Black bold lines indicate the median values, grey boxes indicate the first and third quartiles and dashed lines indicate the range between minimum and maximum values.

Fig. 5.

Canonical Analysis of Principal Coordinates (CAP) plot of sites by species percentage coverage, calculated with Bray-Curtis dissimilarity matrix. (a) without engineer species; (b) with engineer species N. cochlear and P. americana mouchezii/P. muellerae overlaid on the plot as vectors.

The PERMANOVA analysis (Table 2) revealed significant differences in megabenthic assemblages based on the factors Site and Type of bioconstruction. Moreover, significant differences were observed in the pair-wise comparison of bioconstruction types, particularly between algal and invertebrate assemblages, as well as between coral and oyster habitats. A PERMDISP test indicated significant differences in dispersion among groups (p = 0.001), suggesting that observed dissimilarities may also reflect heterogeneity in group variability. However, the CAP ordination (Fig. 5) showed a clear separation among bioconstruction types, and the SIMPER analysis (Table 3) consistently identified specific taxa contributing to assemblage differences.

Table 2.

Permutational multivariate analysis of variance (PERMANOVA) performed on the dataset. Bray–Curtis resemblance matrix based on percentage of coverage. Permutation n = 9999.

PERMANOVA table of results
Main test
Source	SS	Pseudo-F	P (perm)	P(MC)	Unique perms
Type	2.4946E5	123.3	0.001	0.001	9917
Site	1.7598E5	34.7	0.001	0.001	9856
Ty x Si	1.2544E5	31	0.001	0.001	9876
Pair-Wise test
Groups		t	P (perm)	P(MC)
MCB, MOB		9.559	0.001	0.001
MCB, MAB		12.002	0.001	0.001
MOB, MAB		13.711	0.001	0.001

Table 3.

Results of SIMPER analysis. Average coverage percentage of megabenthic taxa contributing most of the Bray-Curtis similarity among bioconstructions. ECR: encrusting calcified rhodophytes.

MCB			MOB			MAB
Average similarity: 44.29			Average similarity: 57.68			Average similarity: 55.36
Species	Av.Abund	Contrib%	Species	Av.Abund	Contrib%	Species	Av.Abund	Contrb%
P.axinellae	3.82	42.24	OTU KE	3.26	16.61	Hydrozoa	3.17	23.63
Hydrozoa	2.72	25.31	C.rubrum	1.91	10.84	ECR	2.62	14.21
OTU KE	1.49	8.4	S. evansi	1.23	8.81	D. dichotoma	1.53	9.36
A.cavernicola	1.72	7.18	Hydrozoa	2.02	8.17	S.cervicornis	1.23	7.27
S.mamillata	1.14	5.45	S.mamillata	1.51	7.79	Algal turf	1.16	6.61
ECR	1.22	5.32	P.clavata	1.52	7.69	P.fascialis	1.01	5.46
			Dysidea	1.8	6.79	C. cylindracea	0.96	4.75
			P.axinellae	1.47	6.63	Dudresnayasp.	0.71	4.49
			Leptop/cladopsammia	1.23	6.38	OTU KE	0.91	3.4
			Clavelina sp.	1.11	4.03	D. polypoides	0.52	2.45
			A.cavernicola	0.8	3.63	M.truncata	0.49	2.19
			ECR	0.98	3.56	A.cannabina	0.38	2.06
MCB & MAB			MOB & MAB			MCB & MOB
Average dissimilarity = 80.22			Average dissimilarity = 84.85			Average dissimilarity = 71.89

The SIMPER analysis, using a 90% cutoff for structural biodiversity, identified the taxa that contributed most to the observed differences in coverage between the studied assemblages. All pairwise comparisons revealed dissimilarities greater than 70% (Table 3). The lowest similarity (44.29%) was found within MCB bioconstructions, with only six taxa—Parazoanthus axinellae, Hydrozoa ind., the OTU KE, Aplysina cavernicola, Schizomavella mamillata, and encrusting calcified rhodophytes (ECR)—accounting for this similarity. In contrast, MOB exhibited the highest similarity (57.68%), with 12 taxa contributing more evenly to this similarity, with the OTU KE and Corallium rubrum as the major contributors. Finally, MAB showed an average similarity of 55.36%, with hydrozoans and ECR as the primary contributors, along with Dictyota dichotoma, Smittina cervicornis, algal turf, and Pentapora fascialis associated with these bioconstructions.

Regarding β-diversity, MCB exhibited the lowest values, indicating lower β-diversity, while MAB and MOB showed higher distances, reflecting greater compositional differences (Fig. 6a). These differences were statistically significant (p < 0.001) for both MAB and MOB types.

At the site level, in algal bioconstructions (MAB) (Fig. 6b), the northernmost sites exhibited moderate internal diversity, with some consistency in species composition. Among these, CAP had the lowest values of diversity. In contrast, SFC showed the highest internal variability, with a higher median and wider range. The southern sites appeared to have less internal diversity. A similar trend was observed in invertebrate bioconstructions (MIB) (Fig. 6c), where CAP showed lower β-diversity compared to SFC and SML.

Using per cent coverage as a proxy for abundance, notable variability was observed across the different communities in terms of both taxonomic composition and three-dimensional structure. In invertebrate bioconstructions, some sites were characterized by dense, multi-layered assemblages of encrusting and erect organisms, creating complex three-dimensional structures (Fig. 7). These structures were primarily formed by branching gorgonians (e.g., Paramuricea clavata) and large sponges (e.g., Petrosia ficiformis), which provided habitat complexity and microhabitats not only for small sessile and vagile invertebrates but also for larger mobile fauna, including fish and crustaceans, which use these habitats for shelter, foraging, and reproduction. In contrast, many algal bioconstructions were dominated by encrusting species, resulting in reduced vertical complexity. These assemblages typically featured flat, encrusting algae such as Peyssonnelia spp. and Padina pavonica, alongside smaller colonies of bryozoans, notably Myriapora truncata and Pentapora fascialis.

Fig. 7.

Different aspects/images of the studied assemblages. MAB = mesophotic algal bioconstructions, MIB = Mesophotic invertebrate bioconstructions. All images are our own material, extracted from underwater video footage collected during field surveys.

Discussion

According to the literature, mesophotic bioconstructions formed by filter-feeding invertebrates, particularly scleractinians and bivalve mollusks, represent a distinct type of benthic biocenosis along the Apulian coast¹⁷. To date, although biogenic formations mainly dominated by bryozoans, sponges or octocorals have been widely documented across the Mediterranean Sea^30–32 the specific spatial organization of these unique animal-based bioconstructions, built instead by oysters and hexacorals, has been mainly reported in mesophotic habitats of the southern Adriatic Sea^12–17. Their presence could be linked to the peculiar characteristics of this region, where the intermediate and deep currents from the northern Adriatic Sea, rich in nutrients, flow southward, providing nourishment to the reef. With increasing depth and decreasing light penetration, these invertebrate bioconstructions progressively replace the calcareous bioconstructions formed by encrusting coralline red algae, such as Lithophyllum spp. and Mesophyllum spp., which dominate the coralligenous assemblages, one of the most widespread biocenoses in the Mediterranean circalittoral zone. The occurrence of mesophotic algal bioconstructions has been documented at various locations across the Mediterranean, in both the eastern and western basins, at depths comparable to those of the animal bioconstructions described in this study^16,33.

Overall, mesophotic megabenthic assemblages associated with these bioconstructions exhibited high diversity, encompassing species from several ecological groups, including Porifera, Cnidaria (Anthozoa), Mollusca, and Bryozoa. Among these, demosponges were the most diverse and abundant group of structuring benthic taxa across all study sites, exhibiting higher coverage in invertebrate bioconstructions than in algal ones. In both cases, demosponges played a triple role: as constructors, aggregating carbonate particles; as 3D habitat-formers, through the presence of massive and erect species (e.g., Aplysina cannabina, Petrosia ficiformis, S. foetidus); and as bioeroders (e.g., Cliona schmidtii), maintaining the dynamic equilibrium between growth and erosion processes within the bioconstruction^34–36.

Animal ecosystem engineers, particularly gorgonians, were predominantly found in MOB, highlighting their key role in increasing the structural complexity of these habitats. Hydrozoans were present in both algal and invertebrate bioconstructions, with particularly high abundance at CAP, where they covered almost the entire substrate, forming extensive, well-developed colonies. Among the ecosystem engineer species, those producing carbonate skeletons played a key role. Scleractinians Leptosammia pruvoti and Cladopsammia rolandi were abundant, covering large portions of the substrate even in areas dominated by oysters, along with the octocoral Corallium rubrum. Serpulids were found both attached to shells or bare substrates in small patches and larger masses formed by the fusion of their calcareous tubes, contributing secondarily to the stability of the bioconstructions.

Bryozoans, despite being considered important in mesophotic communities, contributed less to invertebrate bioconstructions than to algal ones. They were observed throughout the entire bathymetric range, with coverage inversely correlated with depth. At shallow depths in MON, scleractinians were replaced by extensive facies of P. fascialis. Most bryozoan species occurred as small colonies, either encrusting (e.g., Schizomavella spp., Schizoporella spp.) or erect (e.g., P. fascialis, M. truncata). In mesophotic bioconstructions, bryozoans primarily functioned as binders, uniting the components of the framework and settling sediment, rather than as primary engineers³⁷.

The algal component was predominantly represented by Rhodophyta, mainly encrusting coralline species. Algal cover decreased significantly at deeper sites, with low substrate coverage. This is unusual since coralligenous algae can thrive in deep waters with very low light levels, with certain species found at depths greater than 250 m³⁸. A likely explanation for the limited distribution of algae in these deeper areas could be sedimentation, a well-known factor limiting the growth of coralline algae^11,33,39,40.

Canonical analysis of taxa coverage distinguished different mesophotic communities, separating those associated with algal bioconstructions from those linked to corals and oysters. Upper mesophotic bioconstructions (25–35 m deep) were mainly composed of thin calcareous laminae, a few centimeters thick, with a compact texture that was locally loosened by cavities and holes. These structural irregularities were likely caused by bioeroders or discontinuities in the superposition of calcareous algal layers due to the presence of skeletal animal clusters^41–43. In contrast, coral and oyster bioconstructions were much thicker (several decimeters), with abundant holes, cavities, and crevices, and a looser or very loose texture^12,14,22. Literature suggests that different biogenic substrates, shaped by the type of dominant bioconstructor, support distinct reef communities by providing settlement areas for mesophotic benthic organisms²¹. The biogenic structure strongly influences the diversity and abundance of associated species²⁰. By building a framework, bioconstructors alter the physical structure of the substrate, thereby modifying habitat complexity and local hydrodynamic, sedimentary, and topographic features. These changes can influence macrofauna recruitment⁴⁴. Additionally, the trophic roles of filter-feeding engineer species (such as oysters and hard corals) likely play a significant role in determining the substrate occupation patterns of the associated benthic communities^17,19.

Our data show that animal-based mesophotic assemblages were distinct from algal-based ones in terms of megabenthic composition and α- and β-diversity. The observed shifts in community structure and composition, indicated by percentage cover values for each taxon, suggest a taxonomic transition. Specifically, there was a decrease in the coverage of photoautotrophic taxa (e.g., macroalgae) and an increase in heterotrophic taxa (e.g., sponges, non-symbiotic scleractinians), driven primarily by depth⁴⁵. Depth is not an ecological factor per se, but rather a proxy for other abiotic factors, such as light availability, water movement, nutrient levels, sedimentation, and temperature, which fluctuate predictably with depth and influence benthic community organization⁴⁶. Since depth is generally negatively correlated with temperature and light, it is often considered the most important variable shaping benthic reef communities^47,48.

The large availability of suspended and dissolved organic matter in deeper waters may explain the prevalence of heterotrophic bioconstructions, such as those formed by bivalve mollusks and scleractinians, as opposed to autotrophic bioconstructions based on calcareous algae¹².

While differences between bathymetric levels (and hence between algal- and animal-based bioconstructions) can be explained by light availability, variations between adjacent sites are likely due to a complex set of co-factors, including species distribution patterns, life cycle interactions, and marine currents. The connectivity between megabenthic assemblages along the Adriatic coast is likely influenced by the northwest-to-southeast flowing currents, which transport nutrients and larvae/propagules of sessile organisms, contributing to ecological success^49,50. Recent studies comparing coralligenous communities along the Albanian coast²² show a similar composition of megabenthic communities on both sides of the Adriatic, at least for the mesophotic algal component.

Few studies have explored the differences in megabenthic assemblages along a broad depth gradient in mesophotic environments within the Mediterranean^23,51. Most research focuses on tropical reefs^5,7,52while Mediterranean studies often address depths up to 30 m^22,53–55 or the deepest mesophotic environments^14,51,56. However, a recent comparative analysis¹⁷ identified significant differences in the benthic species patterns associated with these bioconstructions, all traditionally considered part of coralligenous communities^10,57.

Finally, our results highlight the ecological importance of mesophotic bioconstructions as biodiversity hotspots, as they promote species richness and host complex benthic communities. However, despite their ecological value, these ecosystems are exposed to several threats, including bottom-contact fishing, pollution and climate change⁵⁸. They are also often overlooked in current monitoring and protection frameworks, emphasising the need for targeted conservation efforts. In this context, our study provides valuable insights that can inform marine conservation and restoration efforts, particularly in the context of the EU Habitats Directive (92/43/EEC) and the recently adopted EU Nature Restoration Regulation.

Conclusion

The present study reveals significant differences between mesophotic megabenthic assemblages along the Apulian coast. By quantitative (percentage cover) data obtained from photo image analysis, we highlight the marked distinctions between upper mesophotic assemblages, associated with algal bioconstructions, and deeper assemblages, linked to biogenic substrates built by invertebrates, primarily hard corals and oysters. These findings underscore the crucial role of mesophotic bioconstructions in supporting Mediterranean biodiversity.

Despite the recent increase in studies exploring deeper mesophotic layers^10,59 much remains unknown about the dynamics and ecological processes governing these heterogeneous environments. Existing literature^11,43,60,61 suggests that understanding the functional roles of mesophotic assemblages and their connectivity at the Mediterranean scale is essential for improving our understanding of their potential role in resilience processes, such as serving as refuges for many species and enhancing habitat complexity.

Our results emphasize the importance of distinguishing between algal- and animal-based bioconstructions, considering their taxonomic composition, species roles, and the ecological mechanisms that drive their formation. Given our findings, it seems more appropriate to conceptualize the mesophotic bioconstruction system along the Apulian coast of southern Italy as a cohesive ecological framework, rather than isolated, site-specific observations. The processes driving the formation of these communities are not random or solely related to propagule dispersal; instead, they are shaped by specific environmental and ecological patterns that warrant further investigation.

Moreover, while the ecological importance of ecosystem-engineering animals is well established and widely recognized across marine systems^19,31,62 the specific occurrence of animal-based mesophotic bioconstructions could be more widespread than currently recognized. The ecological conditions observed in the central Mediterranean, particularly in the South Adriatic, as persistent turbidity and nutrient-rich waters, may create favourable environments for the development of these structures. These key factors could extend the lower depth limit for photosynthetic processes, allowing filter-feeding organisms to establish significant communities closer to the surface.

The growing interest in these communities is driven by concerns over biodiversity shifts linked to ongoing climate change. These invertebrate-dominated bioconstructions may serve as important biodiversity hotspots. It is critical to explore the potential of mesophotic ecosystems as refugia for shallow-water species and assess their role in future conservation strategies.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Conceptualization, G.C and G.G.; methodology and fieldwork, G.G. and C.P; formal analysis, G.G. and C.P.; resources, M.M., C.N.M., G.B. and J.G.; data curation, C.P. and G.G.; writing—original draft preparation, G.C. and G.G.; writing—review and editing, C.P, G.C., G.B. and G.G.; supervision, C.L. and G.C.; project administration, M.M. All authors have read and agreed to the published version of the manuscript.

Data availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.