A Mediterranean mesophotic coral reef built by non-symbiotic scleractinians

Abstract

This is the first description of a Mediterranean mesophotic coral reef. The bioconstruction extended for 2.5 km along the Italian Adriatic coast in the bathymetric range −30/−55 m. It appeared as a framework of coral blocks mostly built by two scleractinians, Phyllangia americana mouchezii (Lacaze-Duthiers, 1897) and Polycyathus muellerae (Abel, 1959), which were able to edify a secondary substrate with high structural complexity. Scleractinian corallites were cemented by calcified polychaete tubes and organized into an interlocking meshwork that provided the reef stiffness. Aggregates of several individuals of the bivalve Neopycnodonte cochlear (Poli, 1795) contributed to the compactness of the structure. The species composition of the benthic community showed a marked similarity with those described for Mediterranean coralligenous communities and it appeared to be dominated by invertebrates, while calcareous algae, which are usually considered the main coralligenous reef-builders, were poorly represented. Overall, the studied reef can be considered a unique environment, to be included in the wide and diversified category of Mediterranean bioconstructions. The main reef-building scleractinians lacked algal symbionts, suggesting that heterotrophy had a major role in the metabolic processes that supported the production of calcium carbonate. The large amount of available suspended organic matter in the area could be the main nutritional source for these species, as already suggested in the literature referred to Mediterranean cold-water corals.

Massive bioconstructions built by Neopycnodonte cochlear (Mollusca, Bivalvia) in a mesophotic environment in the central Mediterranean Sea

“…In mesophotic environments, the congeneric N. cochlear was reported to be able to make biogenic formations scattered over both soft and hard substrates 24 or build thick bioconstructions on the walls of submerged karst dolines along the northern Apulian coast 16 . In addition, N. cochlear was one of the secondary bioconstructors in the coral reef recently described on the northern Apulian coast 5 .…”

“…These explorations have emphasized the high species richness and diversity of the benthic assemblages that thrive in such ecosystems 33,34 and the notable role of engineer animal species in building 3D animal forests 35 . Most such studies have stressed the role of scleractinians as main reef-building organisms [5][6][7] , highlighting the paramount ecological role of such calcifying bioconstructors 36 . 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 [37][38][39] .…”

“…Cladopsammia rolandi Lacaze-Duthiers, 1897 and Leptopsammia pruvoti Lacaze-Duthiers, 1897 formed large facies mainly in shaded Mollusca. The living mollusc fauna sampled at OT and SML was represented by 9 species belonging to the classes Gastropoda (5) and Bivalvia (4) ( Supplementary Table S1). Among the Gastropoda, 4 species belonged to the order Littorinimorpha and 1 to the order Lepetellida.…”

“…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 [16][17][18] . In addition, the role of zoobenthic taxa as builders in Mediterranean mesophotic environments has been emphasized in a recent paper 5 describing an outstanding carbonate bioconstruction built mainly by scleractinians along the southern Italian Adriatic coast.…”

“…Available studies reveal that Mediterranean mesophotic bioconstructions can represent notable biodiversity hotspots 5,[19][20][21] and include species of considerable economic and ecological importance 20,22 . Moreover, as a result of their vulnerability, habitats associated with mesophotic bioconstructions are protected by international agreements 23 .…”