A Mediterranean mesophotic coral reef built by non-symbiotic scleractinians

Frine Cardone,Senem Onen Tarantini,Eliana Valenzano,Stefania Lisco,Carlotta Nonnis Marzano,Maria Flavia Gravina,Massimo Moretti,Cataldo Pierri,Caterina Longo,Maria Mercurio,Francesco De Giosa,Giuseppe Corriero,Maria Mastrodonato,Adriana Giangrande

doi:10.1038/s41598-019-40284-4

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.

Highlights

This is the first description of a Mediterranean mesophotic coral reef
A high acoustic impedance bedrock bottom occurs in the upper flat sector, while poorly consolidated sedimentary deposits overlay the bedrock in the deeper part of the system
Mesophotic coral ecosystems are widespread and diverse and their investigation is quickly increasing, they are still little explored in most part of the world[38]

Summary

Introduction

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. Coral reefs mainly occur in the oligotrophic waters of the western Atlantic and Indo-Pacific regions, within the latitude of 30°N and 30°S3,4 They are mainly composed of stony corals, helped in the bioconstruction by several species of invertebrates with carbonate skeletons and coralline algae. It is known that mesophotic coral reefs (MCRs) are widespread and diversified worldwide They are found at depths ranging from 30–40 m and extending to the limit of the photic zone, which may be up to 200 m in adequate environmental conditions[9,10]. Associated sessile invertebrates with calcified skeletons (i.e., mainly cnidarians, annelids and bryozoans) contribute to the bioconstruction, as they increase and consolidate the carbonate structure, sometimes becoming more abundant than the encrusting algae; coralligenous can be considered a mosaic of different communities[19]. Along the Apulian continental shelf (e.g. South Adriatic and northern Ionian Sea), coralligenous extends over approximately 450 km[2], likely representing the most relevant habitat in this area[31]

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