The main marine bioconstruction in the Mediterranean Sea is localized in the euphotic zone (layer close to the surface that receives enough light for photosynthesis to occur) and is well known under the name of Coralligenous, which is typically considered to be the climax biocoenosis (fully developed systems of interdependent lifeforms) of the circalittoral (nearshore) zone. Coralligenous reefs are widely distributed and consist of thick carbonate concretions mainly built by Red Calcareous Algae, with the variable contributions of Sessile Invertebrate calcium carbonate depositors (e.g., Scleractinians, Serpulids, Bryozoans). The large amount of different habitats associated with such bioconstructions support the highest values of biodiversity in the Mediterranean Sea. However, with increasing depth and as a result of light attenuation, benthic Sessile Invertebrates progressively replace Algal concretions, becoming the most important habitat builders. The biogenic role of Animal bioconstructors has been repeatedly studied in Mediterranean deep-water habitats, where the predominant colonial Scleractinians (Corals) build large three-dimensional carbonate structures referred to as Cold-Water Corals and provide substrate and habitat for a multitude of other organisms. Conversely, little attention has been given to the Mediterranean mesophotic environment that, receiving less than 3% of the surface irradiance, represents the transitional zone between euphotic and dark environments.
In a paper published in the journal Scientific Reports on 14 April 2020,
Frine Cardone, Giuseppe Corriero, Caterina Longo, and Maria Mercurio of
the Dipartimento di Biologia at the Università degli Studi di Bari Aldo Moro, and the Consorzio Nazionale Interuniversitario per le Scienze del Mare, Senem Onen Tarantini, also of the Dipartimento di Biologia at the
Università degli Studi di Bari Aldo Moro, Maria Flavia Gravina of the
Dipartimento di Biologia at the Università di Roma 'Tor Vergata', and
the Consorzio Nazionale Interuniversitario per le Scienze del Mare,
Stefania Lisco and Massimo Moretti of the Dipartimento di Scienze della Terra e Geoambientali at the Università degli Studi di Bari Aldo Moro
and the Consorzio Nazionale Interuniversitario per le Scienze del Mare,
Francesco De Giosa of Environmental Surveys S.r.l., Adriana Giangrande,
also of the Consorzio Nazionale Interuniversitario per le Scienze del
Mare, and of the Dipartimento di Scienze e Tecnologie Biologiche ed
Ambientali at the Università del Salento, Carlotta Nonnis Marzano, again
of the Dipartimento di Biologia at the Università degli Studi di Bari
Aldo
Moro, and the Consorzio Nazionale Interuniversitario per le Scienze del
Mare, and Cataldo Pierri, once again of the Dipartimento di Biologia at
the Università degli Studi di Bari Aldo Moro, and of the Istituto di Ricerca sugli Ecosistemi Terrestri, present the results of a study which
aimed to improve knowledge on mesophotic bioconstructions in the
central Mediterranean Sea using a multiscale approach coupling marine
biology and geology methods.
Bioconstructions of the mesophotic habitat, well known in tropical waters have only recently been investigated in the Mediterranean area. An important contribution to these carbonate structures seems to be provided by the Bivalve Neopycnodonte cochlear, which makes mass aggregations on the sea bottom on both soft and hard substrates, supporting the development of a rich benthic fauna. In addition, the role of zoobenthic taxa as builders in Mediterranean mesophotic environments has been emphasised recently in a description of an outstanding carbonate bioconstruction built mainly by Scleractinians along the southern Italian Adriatic coast.
Mediterranean mesophotic bioconstructions can represent notable biodiversity hotspots and include species of considerable economic and ecological importance. Moreover, as a result of their vulnerability, habitats associated with mesophotic bioconstructions are protected by international agreements. Data in the literature, however, are mainly based on Remotely Operated Vehicles observations and mostly focus on a few conspicuous megabenthic species. Thus, the main morphological features of these bioconstructions remain widely unknown, as well as their biological diversity, both in terms of structuring builder species and the associated fauna.
The focus of Cardone et al.'s study was on the characterization of the mesophotic habitat dominated by the Bivalve Oyster Neopycnodonte cochlear along the southern Italian coast (northern Ionian Sea). The fine structure of these carbonate bioconstructions was for the first time investigated by describing their morphological framework and characterizing the structuring taxa associated with bioconstruction at two different sites. Cardone et al.'s general goal was to highlight the role of Neopycnodonte cochlear as a peculiar ecosystem engineer in the mesophotic environment, contributing to a better understanding of the ecological role of mesophotic bioconstructions and enhancing the possible future application of effective management and conservation tools.
The Neopycnodonte bioconstructions were studied in two different areas of the southern Apulian coast, Otranto and Santa Maria di Leuca. In the Otranto area, Neopycnodonte bioconstructions were discontinuously detected along 600 m of the coastline within a bathymetric range of 45–64 m, reaching a total length of 200 m. In the Santa Maria di Leuca area, a carbonate formation built by the Bivalve almost uniformly covered the northern and eastern sides of the cliff for a total length of approximately 450 m in the bathymetric range of 45–70 m.
Locations of the two study areas: Otranto (OT) and Santa Maria di Leuca (SML). They occur off the southern Italian coast and are identified by red circles. Map created with ESRI ARCMAP 10.2. Cardone et al. (2020).
The two study areas, Otranto and Santa Maria di Leuca, showed similar geomorphological features. The seafloor geometry was dominated by the presence of a slope that connected a large coastal flat area to deeper sectors (depth range: 39–64 m in Otranto and 27–70 m in Santa Maria di Leuca). This slope ran parallel to the coastline in the Otranto area (north north west-south south east), while in the Santa Maria di Leuca area, it ran along an east south east-west north west direction, transversally to the coastline. In both cases, the slope was locally steep and showed irregular morphology.
Geophysical survey of the Otranto area and localization of dive points. (a) Digital elevation model of the seafloor derived by the multibeam survey. (b), (c) Areas 1 and 2 are characterised by the largest slope gradient. (d), (e) Details of the raw data from the Side-Scan Sonar survey in areas 1 and 2. Note the presence of hard substrate along the slope. (f) Classification of the seafloor and localisation of dive points (black circles). Cardone et al. (2020).
Data from the Side-Scan Sonar survey allowed a more detailed assessment of the nature of the seafloor. In the Otranto area, the acoustic signal was monotonous in the shallower (39 m) and deeper sectors (64 m), likely in relation to the presence of soft bottom. Rocky substrate randomly occurred in only the slope sectors, forming localised submerged 'headlands' with an irregular slope and a nearly flat top.
Geophysical survey of the Santa Maria di Leuca area and localization of dive points. (a) Digital elevation model of the seafloor derived by the multibeam survey. (b) Details of the east south east-west north west-oriented slope. Note that the slope seems to be very irregular along the portion with a higher gradient. (c) Raw data from the Side-Scan Sonar survey. (d) Classification of the seafloor and localisation of dive points (white circles); the bioconstructions can be easily mapped, while shallower and deeper sectors seem to have a similar, even though unclear, acoustic signal. Cardone et al. (2020).
In the Santa Maria di Leuca area, the flat and shallow sectors (25–27 m) had a rocky substrate. The mosaicked acoustic signal suggested the presence of coralligenous bioconstructions or concretions, while close to the top of the slope (30 m), the substrate showed acoustic features that can be interpreted as small bioconstructions with irregular morphology. The slope was stiff and continuous, showing a complex acoustic signal likely related to the presence of large-scale bioconstructions (comparable with the irregular slope of the multibeam dataset. Deeper areas (70 m) seemed to have a fine-grained sediment substrate.
In both study areas, the bioconstructions developed in thick pinnacles or globose formations, protruding perpendicularly with respect to the cliff for approximately 50 cm at Otranto and for more than 1.5 m at Santa Maria di Leuca and often interconnected with one another to form a framework of high structural complexity. The pinnacles were organized in successive terraces proceeding from the top to the bottom of the bioconstruction. According to the analysis of the video images, sampled material and resin slices, the pinnacles and their basal layer were mainly formed by the massive, multilayered aggregation of shells of Neopycnodonte cochlear. In both study areas, Neopycnodonte cochlear occurred in 100% of the analysed images, with average covering values of 84 and 82% in Otranto and Santa Maria di Leuca, respectively. At both sites, most of the bioconstructions were composed of dead specimens of Neopycnodonte cochlear. Live specimens were present in scattered clusters of a few individuals (6–20) grafted onto the superficial layer of the bioconstruction. Specifically, the large-scale analysis of the resin slices showed how the general framework of the bioconstruction was always shell supported and derived from the complex superposition of new valves on the preceding ones. In general, Neopycnodonte cochlear specimens seemed to be irregularly arranged with respect to each other, with individuals settling on the surface of older shells and sharing few points of contact with adjacent valves. Locally, some shells were arranged parallel to each other, thus increasing the contact surface between adjacent valves. The random orientation of the shells and the presence of point-like contacts determined the formation of an overall porous structure. At the same time, the contacts between larger surfaces favored the stability of the bioconstruction.
Video recorded on a vertical cliff off Otranto. In this video it is possible to observethe globose formations edified by Neopycnodonte cochlear, protruding perpendicularly fromthe cliff. The bioconstruction is plentifully populated by Red Coral, whose colonies aggregate especially below the biogenic formations built by Neopycnodonte. Large encrustationsof the Demosponge Dendroxea lenis (grey) and abundant plaques of the Bryozoans Schizomavella spp. are also visible. Cardone et al. (2020).
Among the other taxa structuring the bioconstructions, there were Cnidarians, Serpulids and Bryozoans, all contributing to the deposition of calcium carbonate, and Poriferans, helping to bind shells together or eroding carbonate by boring species. Indeed, boring Sponges were often recorded living into the carbonate structures, and colonial Scleractinians and Serpulids were found within the concretion. In particular, among the secondary structuring taxa, the Scleractinians Cladopsammia rolandi, Leptopsammia pruvoti, and, to a lesser extent, Caryophyllia (Caryophyllia) inornata and Hoplangia durotrix, were the most frequent structuring species, strongly contributed to the bioconstructions. It showed a patchy distribution, with aggregates of several specimens concentrated below the pinnacles of the structure.
Video recorded on a vertical cliff off Santa Maria di Leuca. In this video it is possible to observe the thick pinnacles edified by Neopycnodonte cochlear, protruding perpendicularly from the cliff. The structural complexity of the bioconstructions and their marked thickness are easily recognizable.The framework is populatedby large colonies of Paramuricea clavata. Red Coral shows a patchy distribution mainly thriving below the pinnacles, where it is also easy to observe the Demosponge Aplysina cavernicola. Cardone et al. (2020).
The outer portions of the bioconstructions as well as the reef interstices were extensively encrusted by Serpulid tubes. In particular, most Spirorbid Polychaetes, especially Protolaeospira (Protolaeospira) striata, Pileolaria militaris, and Vinearia koehleri, colonized the bare parts of the substrate, such as the external edges of the shell of living Neopycnodonte specimens and their smooth inner parts, corresponding to the pioneering role that these organisms played in the community colonisation pattern. Other Spirorbid species exhibited their particular adaptation to the cryptic and dark crevices of the bioconstruction according to their small dimensions and often-wrapped tubes.
The Neopycnodonte bioconstructions. Underwater images of the bioconstructions: (a) globose formations at Otranto and (b) thick pinnacles at Santa Maria di Leuca protruding perpendicularly with respect to the cliff and interconnected with one another to form a framework of high structural complexity. (c) Detail of the Neopycnodonte bioconstruction formed by the massive, multilayered aggregation of shells. (d), (e) Scleractinian facies. (e) Detail showing the main structuring taxa: Cladopsammia rolandi/Leptopsammia pruvoti complex (yellow Corallites) and Hoplangia durotrix (light brown Corallites). (f) Corallium rubrum facies. (g) Large portion of the substrate covered by the encrusting Sponge Dendroxea lenis (grey). (h) Paramuricea clavata facies characterised by large colonies. (i, j) Detail of Neopycnodonte bioconstruction heavily infested by the boring Sponge Siphonodictyon infestum (arrows) in a fresh sample (i) and in a sample embedded in resin (j). Cardone et al. (2020).
The Bryozoans Schizomavella spp. and Schizoporella spp., particularly well represented in the Otranto study area, formed thin crusts on the reef surface that contributed to the compactness of the structure. Sponges were mainly represented by encrusting species covering large portions of substrate. Among them, Dendroxea lenis occurred frequently in both study areas, with covering values that reached 42% of the substrate of the bioconstruction. In contrast, massive and erect forms were less represented in both study areas, with the exception of small specimens of Axinella, which were mainly concentrated in the sub-horizontal portions of the substrate. Among the Boring Sponges, Siphonodictyon infestum played an important role as a bioeroder of the bioconstruction.
Large sample of the bioconstruction after collection. Neopycnodonte shells mainly appear irregularly arranged, forming a framework of high structural complexity. Cardone et al. (2020).
In addition, the reef ’s crevices were also inhabited by Hiatella spp. Molluscs living as nestlers or as borers, enlarging the reef ’s holes. Additionally, the soft-bottom Bivalve Kellia suborbicularis preferred the sediment trapped in the crevices and the spaces among the Neopycnodonte shells. Finally, Red Coralline Algae were only sporadically detectable during the analysis of both images and biological samples.
Details of the large-scale slices of the bioconstruction impregnated with epoxy resin. (a) Shells sharing single points (yellow circles) or large surfaces (yellow line). (b) Parallel Neopycnodonte shells that grow together, sharing large surfaces (yellow line). (c) Locally, the bioconstruction shows a remarkable porosity. (d) Traces of Sponge perforations. (e) Scleractinian corallites on a Mollusc shell. (f) Serpulids and Bryozoans encrusting the surface of Neopycnodonte shells. Cardone et al. (2020).
Regarding large epibenthic taxa, dense populations of the Gorgonians Paramuricea clavata and Eunicella cavolini characterised the seascapes of both the habitats at both sites, representing the main three dimensional habitat makers, although at Otranto Gorgonians were limited to a few areas of the bioconstruction.
The mapping of the area occupied by different taxa in the large-scale slices of the bioconstruction showed the following average percentage values: Neopycnodonte shells: 73.8%, Scleractinians: 13.6%, Serpulids: 8.8%, Bryozoans: 2.3%, encrusting Algae: 1.3%. The analysis of images showed that the bioconstructions were characterized by marked porosity (73.2%) that was due to spaces within and between individuals and small-scale porosity related to bioerosion. The boring Sponges occurred in all sampled material at both sites, where they heavily bioeroded carbonate structures, showing a clear decreasing gradient of perforation from the oldest to the youngest parts of the bioconstruction.
Analysis of the biological samples revealed the presence of a total of 165 structuring taxa (153 identified to the species level), 110 of which were detected at Otranto and 136 at Santa Maria di Leuca, and 81 (49%) were shared by both sites. Overall, the phylum Porifera had the greatest species richness (65 taxa), followed by Annelida (38 taxa) and Bryozoa (34 taxa), while Algae were present to a lesser extent (5 taxa). The patterns of species of the different taxa showed total overlap between the two study sites for Algae, Cnidarians and Bivalves, with Santa Maria di Leuca hosting all the taxa recorded at Otranto plus some exclusive ones. In contrast, Sponges, Annelids and Bryozoans diverged in terms of species composition.
A total of 5 species of the class Florideophyceae were identified. Three of them are non-geniculated encrusting forms: Titanoderma pustulatum and Lithophyllum stictiforme belonging to the order Corallinales, and Mesophyllum alternans belonging to the order Hapalidiales. Jania sp. and Amphiroa sp. are geniculated species belonging to the order Corallinales. Titanoderma pustulatum and Mesophyllum alternans are the only species that were detected in both study areas. Coralline species showed a patchy pattern in both study areas, where they were represented by small thalli, with a maximum surface covering of a few square centimetres. Encrusting species were attached to tiny rocks, Scleractinians and Mollusc shells. Geniculated species were attached to encrusting Coralline species or other Rhodophyta species.
Porifera encompassed 65 taxa, with 61 identified to the species level, as follows: 4 species of Homoscleromorpha (1 order, 2 families, 4 genera) and 61 of Demospongiae (3 subclasses, 13 orders, 30 families, 41 genera). The order Dictyoceratida was most represented, with 4 families, 6 genera and 11 species. Poecilosclerida was represented by 4 families, 5 genera and 8 species. Finally, Axinellida,
Haplosclerida, and Tetractinellida were other representative orders. Most of the sponge taxa recorded show an Atlanto-Mediterranean distribution, and 12 taxa are currently reported only in the Mediterranean Sea.
The number of sponge taxa found at the study sites was 43 at Otranto and 45 at Santa Maria di Leuca, 23 of which were shared, with a high number of exclusive species (20 and 22, respectively).
In general, encrusting forms prevailed at both sites, and the few massive and erect species, when present, were of small size. Among the encrusting forms, Dendroxea lenis showed the highest frequency, Axinella verrucosa was the most frequent sponge among erect species, and massive species were only sporadically detected. Seven species were boring sponges, but only one of them, Siphonodictyon infestum, was widely represented in samples from both sites; this species is able to produce large boring chambers in the shell walls of Neopycnodonte cochlear.
Fourteen anthozoan taxa belonging to the orders Alcyonacea (3 families and 4 genera), Scleractinia (4 families, 9 genera) and Zoantharia (1 family, 1 genus) were detected in both study areas. Most of the species recorded have an Atlanto-Mediterranean distribution, with the exception of Eunicella cavolini and Parazoanthus axinellae, whose current known distribution is limited to the Mediterranean.
The two study areas showed high similarity in terms of composition of the Anthozoa. Indeed, with the exception of Alcyonium coralloides, detected at Santa Maria di Leuca only, all the species were found at both sites.
The order Scleractinia was the most represented in terms of species richness and frequency of occurrence. Scleractinians colonized the shells of Neopycnodonte cochlear or settled inside and between the valves of the empty shells throughout the bioconstruction. Hoplangia durotrix and Caryophyllia (Caryophyllia) inornata were particularly abundant, scattered throughout the framework. Cladopsammia rolandi and Leptopsammia pruvoti formed large facies mainly in shaded regions of the bioconstruction. Finally, Caryophyllia (Caryophyllia) inornata, Monomyces pygmaea and Stenocyathus vermiformis were more rarely recorded.
Among the Alcyonaceans, Paramuricea clavata showed the highest density, with large colonies reaching 50 cm in fan diameter.
The living mollusc fauna sampled at Otranto and Santa Maria di Leuca was represented by 9 species belonging to the classes Gastropoda (5) and Bivalvia (4). Among the Gastropoda, 4 species belonged to the order Littorinimorpha and 1 to the order Lepetellida. With regard to Bivalves, Neopycnodonte cochlear belonged to the order Ostreida and to the family Gryphaeidae, and the other 3 species belonged to the orders Galeommatida (1 species) and Adapedonta (2 species). All the Mollusc species have an Atlanto-Mediterranean distribution, with the exception of the Gastropods Alvania carinata and Sandalia triticea, which have a distribution limited to the eastern Mediterranean basin. The two study areas shared 55% (2 Gastropoda and 3 Bivalvia) of the Mollusc fauna, while the remaining 45% (4 species) was exclusively found at Santa Maria di Leuca. In general, the mollusc fauna was characterized by low abundance values, with the exception of Neopycnodonte cochlear, the main builder of the bioconstructions. Living specimens of Neopycnodonte cochlear formed clusters of a few individuals scattered on the upper part of the calcareous framework, while most of the bioconstruction was composed of dead specimens. The abundance of living specimens of Neopycnodonte cochlear calculated for 300 ml of the bioconstruction varied from 1.7 to 9.1 at Otranto and Santa Maria di Leuca, respectively. In particular, the living specimens detected in the Otranto samples were mainly represented by juveniles (1.3%) with the length of the main valve measuring approximately 1 cm.
Regarding the other Mollusc species, most were recorded inside and between the valves of dead specimens of Neopycnodonte cochlear, although Vermetus triquetrus colonised the exposed surface of the bioconstruction, covering some of the Neopycnodonte valves. The Gastropod Pseudosimnia carnea is locally abundant and was mainly found to be associated with Red Coral colonies, on which it is parasitic.
Thirty-eight species of Serpulidae, with 29 Serpulinae and 9 Spirorbinae, were recorded. Almost all species (36) were collected at Santa Maria di Leuca, and 28 species were found at Otranto, with 26 shared between the two sites. Most species were found with few individuals, while Vermiliopsis infundibulum, Semivermilia crenata, Filogranula annulata, and Semivermilia pomatostegoides, ogether with some spirorbids, such as Protolaeospira (Protolaeospira) striata and Pileolaria militaris, were particularly abundant. Such Polychaetes have a Mediterranean and north-eastern Atlantic distribution, and some of them are cosmopolitan, e.g., Vermiliopsis infundibulum, Serpula vermicularis, Josephella marenzelleri, Pileolaria militaris, Neodexiospira pseudocorrugata, and Janua heterostropha; 6 species are endemic to the Mediterranean, i.e. Placostegus crystallinus, Semivermilia pomatostegoides, Vermiliopsis monodiscus, and Spirobranchus lima, and 3 species, i.e. Serpula cavernicola, Serpula annularis and Nidificaria clavus, only occur in the Mediterranean Sea, Gibraltar area and Canary Islands. The Polychaete species exhibit a high level of adaptive radiation and can be ascribed to different ecological groups; meso-infralittoral shelf species, e.g., Spirobranchus polytrema, Spirobranchus triqueter and Janua heterostropha; characteristic coralligenous species, e.g., Semivermilia crenata and Vinearia koehleri; detrital continental shelf species, e.g. Semivermilia cribrata and Spirorbis (Spirorbis) cuneatus; deep-water and bathyal species, e.g. Vermiliopsis monodiscus, Serpula israelitica, and Filogranula gracilis; and cave species, e.g., Serpula cavernicola and Filogranula annulata.
Thirty-four taxa of Bryozoans were identified: 30 belonging to the class Gymnolaemata, order Ctenostomatida, and 4 belonging to the class Stenolaemata, order Cyclostomatida (the latter were not identified to the species level). The bioconstructions at Santa Maria di Leuca showed a higher species richness (27 taxa) with respect to that at Otranto (19 taxa). Twelve taxa were shared between the two sites, while the exclusive species accounted for 21% at Otranto and 44% at Santa Maria di Leuca. Most of the bryozoan species are distributed in the north-eastern Atlantic Ocean and largely in the Mediterranean Sea, but some of them, e.g., Schizoporella mutabilis, Schizoretepora serratimargo, Rhynchozoon sp., and Pentapora fascialis, are endemic to the Mediterranean.
Many taxa were found living close to one another. Most of them exhibited both thick (11 taxa) and thin (7 taxa) encrusting habitus, others were present in petraliform and celleporiform colonies (3 species), and 7 taxa were found to form erect colonies. The encrusting species of the genus Puellina were the major occupiers of the substrate, together with Schizomavella and Schizoporella spp., which developed sheets that extensively covered the surface of the bioconstruction. Moreover, encrusting Bryozoans grew epibiotically on serpulid tubes and on other bryozoan colonies. Bryozoans of the genera Crassimarginatella and Beania, with petraliform colonies, and those of the genera Rhynchozoon and Turbicellepora, with celleporiform colonies, populated interstices, cavities and crevices of the bioconstruction. The few erect Bryozoans mainly colonized the outer edges of Neopycnodonte shells with both rigid, Myriapora truncata and Crisia sp., and flexible, Bugula gautieri, colonies.
During recent years, there has been increasing interest in Mediterranean circalittoral and bathyal communities, mostly due to technological improvements, which have provided increased investigation accessibility to the deepest benthic areas. These explorations have emphasized the high species richness and diversity of the benthic assemblages that thrive in such ecosystems and the notable role of engineer animal species in building three dimensional animal forests. Most such studies have stressed the role of Scleractinians as main reef-building organisms, highlighting the paramount ecological role of such calcifying bioconstructors. At Mediterranean scale, literature data on marine Gastropod bioconstructions have mainly focused on shallow waters, where large Vermetid reefs are known from the Late Miocene and from off Israel. In contrast, very limited information is available about biogenic formations built by bivalves on circalittoral and bathyal seabeds because most of the literature mainly reported distributional data rather than providing a fine-scale description of such formations.
In the bathyal environment, the few existing data concern the unique Coral-Bivalve biotope, where the deep-sea OIyster Neopycnodonte zibrowii is described as a notable builder species. In mesophotic environments, the congeneric Neopycnodonte cochlear was reported to be able to make biogenic formations scattered over both soft and hard substrates or build thick bioconstructions on the walls of submerged karst dolines along the northern Apulian coast. In addition, Neopycnodonte cochlear was one of the secondary bioconstructors in the Coral reef recently described on the northern Apulian coast.
The present study describes at a fine scale and with a multidisciplinary approach the massive bioconstructions built by Neopycnodonte cochlear, including their local distribution, morphological framework and structuring taxa. The bioconstructions recorded off the southern Italian coast (northern Ionian Sea) resulted unnoticed until now despite past investigations carried out in the same geographic area. The novelty of Cardone et al.'s study is the description of large and thick biogenic formations never observed before for this species. At both study sites, the bioconstructions showed a wide extension and appeared as complex frameworks entirely composed of a large number of living and dead specimens of Neopycnodonte cochlear associated with numerous other taxa with accessory structural function, helping to increase habitat heterogeneity.
In comparison with Mediterranean coralligenous reefs, sensu stricto, and the recently described mesophotic Coral Reef, the Neopycnodonte bioconstruction showed peculiar features, since it lacked the major contributions of encrusting coralline algae and scleractinians as reef builders, respectively. The bioconstruction built by Neopycnodonte cochlear was very complex and diversified in the associated community of structuring organisms. It hosted a large number of benthic species attributable to different ecological groups occurring in different microhabitats of the bioconstruction.
Overall, the main structuring species were represented by invertebrate suspension feeders, suggesting the high trophic availability of the surrounding waters. The calcareous framework resulted from the stratification of different generations of benthic invertebrates, with the highest contribution of Neopycnodonte cochlear. Conversely, Algae were poorly represented both in terms of frequency of occurrence and species richness. Mesophyllum alternans and Titanoderma pustulatum, which compose a large part of coralligenous bioconstructions, although observed in both study areas, showed a patchy distribution and were represented by small-sized thalli. The scarce presence and low diversity of coralline algae, usually well represented in this bathymetric range, might be explained by the high sediment deposition observed in both study areas. Indeed, high sedimentation rates, together with water movement and pH, are usually considered to be the main factors limiting the growth of Coralline Algae. Scleractinians showed a dominant role among secondary structuring taxa, colonising the valves of dead Neopycnodonte cochlear specimens and becoming embedded within the calcareous frame. The Alcyonaceans played a predominant role as 3D habitat makers, in accordance with the literature which describes such arborescent invertebrates as being able to form complex Animal forests. Serpulid Polychaetes as well exhibited a notable role in increasing habitat heterogeneity, with a large number of tubes, mostly represented by species typical of shallow and detrital bottoms, being cemented to the outer portions of the bioconstruction. Species characteristic of deep-water biotopes as well as of cryptic microhabitats and caves preferentially colonized Neopycnodonte valves and the interstices of the structure. Most of the Spirorbids showed a pioneering role, as their tubes settled on bare substrate, such as the external edges of living Neopycnodonte valves and their smooth inner parts. In addition to this colonisation pattern, in accordance with their small dimensions and often-wrapped tubes, Spirorbids particularly adapted themselves to cryptic interstices and dark crevices of the bioconstruction. Within the bioconstruction, it was also noteworthy that the spirorbid-bryozoan interaction was exhibited by encrusting Bryozoans’ extensive cover on most of the Spirorbids’ tubes. Bryozoans settled as epibionts on other organisms and offered their colonies as a suitable surface for subsequent colonisation. Most of them showed unilaminar encrusting growth and were typical of deep-water habitats subjected to low light intensity. In particular, Schizomavella and Schizoporella species mainly played the role of binders, forming sheets that covered large portions of the bioconstruction. Poriferans were dominant in terms of number of taxa. They were mainly represented by encrusting forms, with a scarce contribution of massive and erect specimens. Overall, their role as three dimensional habitat makers appeared to be negligible, while their function as substrate binders was remarkable. On the other hand, their action as substrate eroders appeared to be very important because of the abundance of boring species throughout the bioconstruction. In particular, Siphonodictyon infestum was always present on the shells of dead specimens of Neopycnodonte cochlear, appearing to be increasingly pervasive towards the deeper layers of the bioconstruction.
Overall, the benthic assemblage associated with the Neopycnodonte bioconstruction showed a certain degree of variability between the two study areas, with differences depending on the taxonomic group. The overlap of species was approximately 50% of the total, and Santa Maria di Leuca had a greater number of exclusive species than Otranto. Differences were negligible for cnidarians, molluscs and algae, while they were greater for Annelids, Poriferans and Bryozoans, thus suggesting different ecological conditions between the two sites. In this regard, an important role could be played by the strong currents occurring at Santa Maria di Leuca, where waters of the Ionian Sea and Canale d’Otranto meet, generating water turbulence that also affects the deeper portions of the seabed, thus determining a different food supply in the two areas.
Similar to what has already been noted for coralligenous and other Mediterranean bioconstructions, the Neopycnodonte bioconstruction enhances habitat heterogeneity and promotes biodiversity, thus supplying ecosystem services for human society. For this reason, biogenic structures formed by the Mollusc habitat-forming species Neopycnodonte cochlear and Neopycnodonte zibrowii are already included on the list of Marine Habitat Types for the Selection of Sites in the National Inventories of Natural Sites of Conservation Interest in the Mediterranean Sea. In particular, Neopycnodonte cochlear is included in the section of circalittoral rocky habitats and Neopycnodonte zibrowii in the bathyal rocky habitats section. Furthermore, because of their sensitivity to different anthropic impacts, such bioconstructions are classified as Vulnerable Marine Ecosystems according to the General Fisheries Commission for the Mediterranean.
Cardone et al. recognise that animal-dominated biogenic formations would have larger extensions in the south Adriatic twilight zone, and a larger, similar bioconstruction is currently under investigation in the central Adriatic Sea (unpublished data from the same authors). In addition, we are aware of the need for better knowledge of both the occurrence and extent of such vulnerable habitats and their main biological aspects. These latter include the functional roles and life history traits of the species, to monitor their environmental status, assess possible adverse impacts and establish sustainable management strategies. Finally, the need to improve the knowledge on Mediterranean mesophotic bioconstructions seems to have become even more crucial in the light of the recent finding of remarkable scleractinian bioconstructions in the same bathymetric belt. This highlights the need to better clarify the identity of mesophotic bioconstructions in the Mediterranean basin, until now numbered in the great mosaic of coralligenous formations, even though often structurally different from the coralligenous sensu stricto.
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