Hydroid diversity and species composition along a gradient from shallow waters to deep sea around Bermuda

Hydroids were examined in 88 qualitative samples from a depth range of 0–4550 m in the Bermuda area. Of 89 species represented overall, 54 were present in samples from 0 to 25 m. Thirty-one of these 54 taxa were found only within that bathymetric zone. Hydroid species numbers remained relatively high (36) in collections from shallow neritic bottoms (26–100 m), although the faunal assemblage of the zone was quite distinct from that of overlying waters. Numbers of species dropped to 23 in the 101–200 m range and declined further from 21 to three in samples from the 201–500 and 501–1000 m zones. Only seven species (Tubularia sp., Eudendrium sp. 2, Egmundella superba, Opercularella sp., Acryptolaria longitheca, Filellum serratum, Lafoea coalescens) were collected at depths exceeding 1000 m in the study area. Mean numbers of species per sample likewise indicated that the hydroid fauna was richest in shallow waters (<100 m) of the Bermuda Platform and poorest in the deep sea, and that changes in diversity along the bathymetric gradient were greatest over the upper 500 m. From submersible observations and collections between 730 and 3550 m, hydroids appeared to be scarce at great depths on the slope of the Bermuda Pedestal, even where hard substrates (rocky outcrops, calcareous rubble, megafaunal invertebrates) were present. Thecocodium brieni, an infrequently collected anthoathecate species, is reported from Bermuda and the western Atlantic for the first time.