The Otranto Channel (South Adriatic Sea), a hot-spot area of plankton biodiversity: pelagic polychaetes

Rosanna Guglielmo,Antonia Granata,Vincenzo Bonanzinga,Nunziacarla Spanò,Genuario Belmonte,Roberta Minutoli,Giacomo Zagami,Letterio Guglielmo,Francesco P Patti,Alessandro Bergamasco

doi:10.1038/s41598-019-55946-6

Abstract

Composition, density and specimen sizes of pelagic polychaete assemblages were analyzed in the Southern Adriatic Sea. The study was based on finely stratified vertical (0–1100 m) and spatial sampling (17 stations) representing spring conditions. Holoplanktonic polychaetes were distributed in both neritic and pelagic waters, although the highest densities were observed along the Otranto Channel. Analysis of the size frequency distribution revealed a trend with depth only for some species. Spatial distribution of holoplanktonic polychaete density was not related to bottom depth, being the organisms mainly concentrated in the epipelagic layer (0–100 m). The most abundant species showed maximum values below or within the thermocline and within the Deep Chlorophyll Maximum or just above it. Relations between polychaete presence and the underlying oceanographic mechanisms regulating the circulation in the Otranto Channel were discussed. The presence of several non-determined polychaete larvae (e.g. Syllidae) in the pelagic waters at 800–1100 m depths suggests the importance of the role of Levantine waters as main actual and potential carrier of species in the area, though a relevant contribution comes also from North Adriatic dense waters through deep spilling and cascading in the Southern Adriatic pit. These findings increase the knowledge on holoplanktonic polychaetes ecology within the South Adriatic Sea, and represent significant data in the monitoring of changes in biodiversity.

Highlights

Polychaetes are a large and diversified group, widely distributed in freshwater and marine habitats, from the intertidal zone to the deepest sediments
The intermediate layer hosts the Modified Levantine Intermediate Waters (MLIWs) and the deep layer is occupied by Adriatic Deep Water (ADW) originated in the Southern basin (SAdDW) or formed in the Northern basin (NAdDW)
The 146 BIONESS samples were labeled to form 4 clusters: 5 samples were assigned to Adriatic Surface Water (ASW), 71 to Ionian Surface Water (ISW), 28 to MLIW and 42 to ADW

Summary

Introduction

Polychaetes are a large and diversified group, widely distributed in freshwater and marine habitats, from the intertidal zone to the deepest sediments. Pelagic polychaetes inhabit the entire water column, most species are found in the 100 m thick surface layer[9], even if several forms have bathypelagic habits[10] In spite of their low abundance in the plankton, but in consideration of a wide range of feeding strategies, holoplanktonic polychaetes play an important role in the pelagic food web[11,12] and in organic matter mineralization[13]. Their trophic ecology is complex: (i) most species are active predators and use their quickly eversible proboscis to attack other zooplankters, such as fish larvae, siphonophores, chaetognaths and appendicularians (ii) some others are filter-feeders or phytophagous and (iii) only few species are detritivorous[13]. Two species of both Typhoscolecidae (Sagitella kowalewski Wagner, 1872 and Typhloscolex muelleri Busch, 1851) and Iospilidae (Phalacrophorus pictus Greeff, 1879 and Iospilus phalachroides Viguier, 1886), already included in the checklist of polychaetes of the Italian seas[22], have been reported for the first time in superficial waters (20–50 m depth) along the northwestern coast of Isola d’Elba (Tuscany)[23]

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