United Nations recognises three conservation projects as new World Restoration Flagships.

The United Nations has recognised three conservation projects as World Restoration Flagships, according to a press release issued by the United Nations Environment Program on 4 December 2025. World Restoration Flagships are projects intended to have large-scale and long-term impacts, held up as examples which embody the 10 Restoration Principles of the UN Decade on Ecosystem Restoration (2021–2030). World Restoration Flagships already cover an area of over 10 million km², an area larger than China.

The first new project recognised is the Shellfish Reef Building Program in Australia. This project, a partnership between the Nature Conservancy and the Australian Government, aims to restore reefs of Oysters and Mussels which were once found around much of the southern coast of Australia but which have been greatly depleted by over-harvesting, sedimentation and pollution.

Globefish amongst Mussels and restored shellfish reef in Dromana, Port Phillip Bay. Jarrod Boord/Streamline Media in Reef Builder (2024).

Since its inception in 2020, the Reef Builder Program has worked with local communities to restore reefs at thirteen locations along the southern coastline of Australia; it aims to restore 30% of Australia's original shellfish reefs by 2030. The project has generated over 425 jobs, and about US$10 million in income for over 50 small and medium sized businesses, as well as helping local communities to reconnect to nature and promote stewardship over the natural environment.

The second project recognised is the Respectful Returns Initiative in Canada, a partnership between Parks Canada and local and Indigenous communities, which aims to restore damaged rivers and streams in seven national parks along Canada’s Pacific and Atlantic coasts. 

Salmon being released into the Bay of Fundy as part of the Respectful Returns Initiative. Parks Canada.

Since its initiation in 2010, the Respectful Returns Initiative has restored over 650 km² of land and 228 km of waterways, created over 100 jobs, supported research projects by three universities, and formed partnerships with 32 local organisations and community groups. The Salmon population has increased at six of the seven locations where the Initiative works. The Initiative aims to both protect Salmon and to strengthen the connection between the population and their environment. 

The third project recognised is the Thicket Restoration Movement in South Africa, a collaboration uniting over 60 initiatives in Eastern and Western Cape provinces, which aims to restore over 8000 km² of indigenous subtropical thicket by 2030.

Replanting a Kuzuko Thicket in South Africa. AfriCarbon. 

These thickets serve as a grazing resource for both wild Mammals and livestock, particularly under drought conditions, which is an important consideration following the drought of 2023/4, which is the worst the region has suffered in over 100 years. It is a particularly important resource for threatened species such as Black Rhinoceros and African Bush Elephant, and is also a significant carbon reserve, with soils covered by thicket retaining notably more carbon than exposed soils. It is estimated that restoring these thickets sequesters around eight million tonnes of carbon dioxide per year. The project is also predicted to directly create over 1000 jobs in rural communities, as well as improving the lives of around two million people.

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European Flat Oysters return to Belfast Lough.

The demise of the European Native Oyster, Ostrea edulis, from that of a keystone species to an obscure Bivalve, throughout the majority of its natural range has been well documented. A number of factors have been associated with the demise of global estuarine Oyster populations between the mid-1700s and late-1800s; industrial pollutants, coastal development, increases in sewage outflow and growing urbanisation. However, the most prominent drivers related to historical losses in Native Oyster populations have been identified as overfishing and disease. The combination of these stressors devastated stocks to such an extent that the Oyster still remains extinct from many of its historically prolific sites more then 100 years after its disappearance. Instances whereby the Native Oyster has returned unaided are rare and those which have been documented were often the result of an aquaculture spawning event. Pro-active interventions using stock augmentation are therefore considered vital if Ostrea edulis is to make a return to its historic locations. As a result, numerous Ostrea edulis restoration programmes are currently underway throughout the UK and Europe in an attempt to address the Oysters decline. 

Belfast Lough in Northern Ireland once accommodated a substantial population of Native Oysters. The first report of a recognised commercial oyster fishery in the Lough was in 1780 when it was stated that 'the Oyster is dredged from September to May by 27 boats and 123 fishers all of whom can read with the exception of two'. The report also implied that Oyster stocks were in a state of decline as a boat could dredge 800 to 1200 large Oysters a day at a price of 4–7 shillings per 100 in the year of 1800. However, by 1819, a boat was dredging between 100 and 300 Oysters a day at a price of 8–18 shillings per 100. Since the official closure of the Belfast Lough fishery in 1903 there have been no reports of wild Ostrea edulis, no aquaculture stocks and no restoration attempts.

An investigation into the 1897, 1898 and 1901 enteric fever epidemics of Belfast City by the physician Dara Mair stated that, ‘the working class of Belfast were heavy consumers of shellfish Including Periwinkles, Cockles, and Mussels but not Oysters as these have been practically extinct for many years’’. The Irish Fisheries Commission considered the Belfast Lough Oyster fishery officially closed in their report of 1903. The renowned malacologist Nora Fisher McMillan did not record any living Ostrea edulis specimens during meticulous surveys of the Lough between 1928 and 1929. John Gee and Kieth Wilson some fifty years later still failed to document the Oyster on a comprehensive Molluscan species list and more recently, in 1999, malacologists from the Ulster Museum Belfast could not detect any living native Oysters during an extensive subtidal and intertidal investigation. The 2002 Joint Nature Conservation Committee survey of the Lough was also unsuccessful in locating any live specimens and in 2017 the Sanitary Review of Belfast Lough only documented the presence of sub-fossil shell.

These historical accounts and surveys confirm the European Native Oyster as being absent from Belfast Lough for over 100 years. However, there have recently beeb unconfirmed reports of solitary Ostrea edulis along the intertidal zone of the Lough.

In a paper published in the journal Regional Studies in Marine Science in December 2020, David Smyth, Maria Hayden-Hughes, Jenna Alexander, and Philippa Bayford of the School of Ocean Science at Bangor University Wales, and Louise Kregting of the School of Natural and Built Environment at Queen’s University Belfast, present the results of a study which investigated these unsubstantiated sightings of individual Ostrea edulis to ascertain if an unassisted recovery had indeed occurred after more than a century.

Belfast Lough is a fully marine inshore body of water located on the east coast of Northern Ireland. The Lough is a relatively shallow marine bay roughly 21 km long and 11 km wide, with a max depth of 23 m covering an area of 130 km² with annual seawater temperatures of between 2 to 21°C. The intertidal zones of both shores are characterised by a sandy mud substrate with high volumes of overlaying shell material. The three most abundant contributing species to substrate mixes are; Mytilus edulis, Cerastoderma edule and Artica islandica. The main freshwater input into Belfast Lough is via the River Lagan, at a mean flow of 8.521 cubic metres per second. 

 

Intertidal survey sites, Belfast Lough 2020. Map generated using Arc Map 10.7.1 spatial analysis WGS84 coordinate geometry used throughout. Sites: (1) Shellbank, (2) Interval, (3) Whitehouse, (4) Gideon’s Green, (5) Hazelbank, (6) Whiteabbey, (7) Jordanstown, (8) Greenisland, (9) Carrickfergus, (10) Kilroot, (11) Fujitsu, (12) Kinnegar Treatment, (13) Kinnegar Barracks, (14) Holywood Yacht Club, (15) SeaPark, (16) RoyalNorth, (17) Rock Beach. Smyth et al. (2020).

The River Lagan is impounded by a floodgate, the Lagan Weir, which typically only allows water exchange for 2 hours either side of high tide. According to the water balance index there is a clear dominance of the tidal dynamics against river dynamics in the Lough. Belfast Lough is also subject to intensive anthropogenic stressors. The major industrial shipping port at the head of the Lough manages over 80% of Northern Ireland’s petroleum and oil imports. Furthermore, Belfast Port handles more than 7000 vessels per year with an average freight through flow of 24.6 million tonnes. The inner lough accommodates 21 licenced Blue Mussel, Mytilus edulis, mariculture beds which are fished by dredge. Fishing activity in the outer Lough focuses on pot fishing, Scallop dredging and bottom-trawling for Dublin Bay Prawns (Langoustine), Nephrops norvegicus.

In order to ascertain if Ostrea edulis had settled along the intertidal zone of the Lough, both east and west shores were surveyed between May and June 2020 using belt transects of 5 m carried out 1 m from the low water mark on tides lower than 0.8 m below datum chart. Ten × 1 km transects were completed along the west shore starting at Shellbank and finishing at Kilroot. Dangerous sheer rock terrain and fine sand substrates limited the east shore survey to six × 1 km transects starting at, Fujitsu Shore and finishing at Rock Beach.

If an Oyster was recorded along a transect, a 20-minute timed search was carried out at the site of settlement within a 5 × 20 m survey plot. Substrate type was recorded at all sites and all oyster height measurements recorded in-situ using Vernier callipers. Specimens were photographed using a Canon Powershot G16 on auto setting:19 mm focal length, ISO 100, 1/50 s at f/5.6. 16:9 aspect ratio with a reference in frame for scale. Images were calibrated using Coral Point Count to the suggested known overhead distance of (1 m). The image measuring application in Coral Point Count was applied to each image to obtain morphometric data which could be applied to the shell age associations for Ostrea edulis.

As this was a baseline investigation, statistical analysis was limited. However, a t-test was carried out to examine the total number of Oysters recorded on the east and west shores to determine if settlement was governed by location. In order to ascertain if a difference existed in the size of Oysters recorded at each site a one-way permutational multivariate analysis of variance was carried out between site and size of individual oysters using PAST vr3.4.

Within the intertidal zone of Belfast Lough, live Ostrea edulis were recorded at six sites. The west shore had five sites and the greatest number of Oysters with 32 individuals, the smallest being 27 mm in length and the largest 112 mm. The size and age variations between sites indicated that recruitment has been on-going within the Lough over the past 8–10 years. The average age of Oysters on the west shore were between 3–4 years with an average shell height of 64 mm. The east shore had one settlement site with nine individuals, the smallest being 33 mm and the largest 72 mm. The average age within the assemblage was 1–2 years with an average shell height of 50 mm. However, it must be emphasised that there may be variations within these age determinations these estimates were based on Ostrea edulis specimens from Essex which sits three lines of latitude below the sites in Belfast Lough.

  

Intertidal Ostrea edulis assemblages: (4) Gideon’s Green, (5) Hazelbank, (6) Whiteabbey, (7) Jordanstown, (8) Greenisland and (13) Kinnegar Barracks. Map generated using Arc Map 10.7.1 spatial analysis WGS84 coordinate geometry used throughout. Smyth et al. (2020).

A t-test was carried out using PAST vr3.4 between the total Oyster abundance of west and east shore assemblages, no significant difference was detected. Furthermore,  permutational multivariate analysis of variance analysis did not detect any significant differences between Oyster size and site. A basic substrate description was assigned to each survey transects and live Oyster sites. 

 

Substrate composition at survey sites. Oyster assemblages were recorded at sites (4)–(8) on the west and site (13) on the east. Map generated using Arc Map 10.7.1 spatial analysis WGS84 coordinate geometry used throughout. Smyth et al. (2020).

Unsubstantiated reports of solitary European Flat Oysters on the shores of Belfast Lough suggested that Ostrea edulis had returned to the waterway after more than a century of absence and the findings presented by Smyth et al. have categorically shown that this is indeed the case. A total of 42 Oysters were recorded with sizes ranging from 28–112 mm indicating a population consisting of both 0–1-year juveniles and adults of over 8 years. These variations in age are significant and indicate that recruitment had been taking place unnoticed within the Lough for at least 8-10 years.

 

Examples of settled Ostrea edulis at sites; (A) Gideon’s Green, (B) Hazelbank, (C) Whiteabbey, (D) Jordanstown, (E) Greenisland and (F) Kinnegar Barracks. Smyth et al. (2020).

While the number of Oysters does not signify a sustainable population, it does raise questions as to; where did they come from and what allowed the settlement of larvae to occur? There have been no attempts at Ostrea edulis aquaculture within the Lough’s catchment and therefore the recent settlements must have been incidental. An understanding of recent abiotic and biotic changes within the Lough may offer some explanations as to how Ostrea edulis has managed to make a return. An, important recent development within the Lough to be noted before assumptions are made occurred in 2016 with the dredging of the central channel. This was carried out to accommodate an increase in shipping traffic which had been steadily growing since 2000 when 5336 vessels visited Belfast Port to more than 6800 in 2018. As a result, over 400,000 m³ of dredge spoil were removed to widen the main shipping routes and accommodate deeper drafts.

The hypothesis that shipping ballast water spreads Molluscan species has been postulated since the late 1800s and is now a well-documented route for invasive and native species into new areas. The transportation of the Pacific Oyster,  Crassostrea (or Magallana) gigas, larvae has been periodically detected in ballast water sources. It has been hypothesised that ballast water transfer in the 1990s was the cause of Crassostrea gigas introductions into the east coast of Scotland as there had been no aquaculture ventures for the species in the region.

The increases in shipping to Belfast Port over the last decade offer the possibility for a ballast water induced spread via the transfer of pelagic larvae through ballast deposition. However, the reality of this is doubtful as the global sources of planktonic Ostrea edulis larvae are extremely rare and mortalities within the pelagic stage of the life cycle are considerable. 

A more probable explanation is that adult Oysters were introduced through the commercial Blue Mussel fishery. The Lough has 21 licenced subtidal Mytilus edulis mariculture sites spread over a number of plots located either side of the shipping channel. The fishery plots are seeded with juvenile Mussels dredged from various locations in the Irish Sea. It may be that within some of the dredged seed that a number of fecund Ostrea edulis were collected and relayed. However, the translocation of seed Mussel has been carried out for over 30 years with no Ostrea edulis settlements recorded. Therefore, if the mariculture plots have been the source of Oyster larvae, it must have been from seed deployed within the last ten years. This is significant as records of the seed Mussel site locations for the 2008, 2009 and 2010 relays may reveal a possible undiscovered Irish Sea population of Native Oysters.

In addition, abiotic or biotic changes within Lough over the last 10 years could have created conditions conducive to promoting the pooling and settlement of Oyster larvae. Recent bathometric changes through the deepening of the central shipping channel and the resulting subsequent alterations to fine-scale hydrodynamics, may offer an explanation. However, fine-scale particle tracking and hydrodynamic modelling of the watercourse would be required to confirm this theory and unfortunately this was economically beyond the resources of Smyth et al.'s short survey.

Belfast Port sits at the head of the Lough and is the largest commercial harbour in Northern Ireland. Anthropogenically introduced disturbances such as dredging are often associated with detrimental changes in marine habitats. However, in the case of Belfast Lough the increases in shipping and the subsequent dredging induced bathymetrical changes may have created a situation whereby the hydrodynamic regimen is now in a position to once again permit Ostrea edulis larval settlements.

Increases in vessel activity and their subsequent wakes in particular have been shown to be instigators of environmental change within soft sediment benthic communities. Wake generated water velocity has led to instances of increased siltation in shallow estuarine systems and the subsequent smothering of bivalve beds and the burying of hardshell substrates. However, this is not always the situation and a number of factors need to be considered when assessing the effects of vessel wakes. Sediment composition, channel geometry, distance to shore, the number of successive vessels and their speed of passage can all influence wake effects.

In 2012 Kyle Demes, Rebecca Kordas, and Jennifer Jorve, showed in British Columbia that primary production actually increased on rocky shore sites which lay favourable distances from intermittent ferry wake pulses. The distance from the wake’s origin can have a significant influence on the degree of environmental stress. In some instances, Crassostrea virginica Oyster reef systems in shallower estuarine regions were displaced by wakes with heights as shallow as 2 cm. However, wakes were shown to have had no impact on recruitment. Indeed, the dislodgement of Oyster assemblages may have actually aided the geographical expansion of Crassostrea virginica within estuaries.

The combined effect of regular wake travel to and from the shore can result in cleaning substrate surfaces of sediment through a washing effect. This process can lead to increases in species abundance and richness along intertidal zones which are situated sufficiently far enough from the initial high velocity water pulse. 

Smyth et al.'s studty has revealed no Oyster settlement close to the Port entrance even though shell substrate was substantial. Therefore, the speculation that the mariculture plots were a source of larvae becomes ever more likely as settlements were localised to sites within the boundaries of the Mussel fishery lays and between 3–7 km from the head of the Lough. A distance which appears to have been suitably far enough from initial wake amplitude to allow shell substrates to remain clear of siltation, while tidal water retention remained sufficient to enable Ostrea edulis pediveligers the opportunity to undergo cementation. The provision of clean settlement material and sufficient water retention have been recognised as optimal for larvae to settle and metamorphose.

The substrate mixes along Lough’s intertidal zones provide some excellent settlement areas, however the findings of the survey identified assemblages in close proximities to each other. Suggesting, that the assumptions as to how the Native Oysters returned is not confined to one factor but to a combination of the proposed hypothesises. The unassisted reoccurrence of Native Oysters in Belfast Lough is undoubtably a unique event with many questions needing addressed before a pathway of re-establishment can be confirmed.

The 2020 documented settlements of Ostrea eduli  in Belfast Lough have raised a number of interesting hypotheses. Further research as to how the return of the Oyster was induced is required. In depth investigations into the effects of pollution and dredging impacts were practically and financially beyond the scope of Smyth et al.'s study. The main aim of this manuscript was to document that Oysters are once again present in Belfast Lough after a century of absence. The initiation of future studies should be considered a matter of urgency, as a better understanding of abiotic and biotic parameters within the Lough could greatly benefit numerous European Native Oyster restoration projects which are currently underway.

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Mesophotic Oyster Reefs in the Mediterranean Sea.

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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