Bathymetric distribution data were compiled on benthic Medusozoa (Cnidaria) of the Bay of Biscay and nearby seas. The area of study extended from 42°N to 48°30′N, and westwards to 10°W. The depth range of each species was traced in a review of the literature. The number of species in each of several depth zones is summarized here: intertidal, intertidal–30 m, 30–100 m, and thereafter at 100 m depth intervals throughout the entire column (0–5000 m). Some 200 species were included (six Scyphozoa, four Staurozoa, 190 Hydrozoa). Of these, 196 could be ascribed to the Coastal Realm (0–200 m) (118 exclusives; 60% of the fauna) and Deep Benthic Realm (200+m) (31 exclusives;16%), with 47 species inhabiting both (24%). Eighty-eight species (45%) were present intertidally. Biodiversity was highest above the summer thermocline (30 m) (133 species, 67%), and no species have been recorded from depths greater than 4706 m. Beyond the thermocline, biodiversity dropped with increasing depth. A significant change occurred in the 30–100 m interval (123 to 78 species; Distinctiveness=51%), although all intervals down to the 300 m isobath exhibited substantial changes. Most species in the Deep Benthic Realm thrive on the upper part of the slope (200–800 m; 50 species). Biodiversity appeared homogeneous across wide bathymetric ranges in deep bathyal and abyssal regions, perhaps due to unique vertical biocenological units. Thus, 15 species are known between 1400 and 2000 m, and four between 3100 and 4300 m. Fifty-four species were considered eurybathic (34%), with some showing impressive bathymetric ranges of over 4000 m, but most species were stenobathic (106, 66%).

As with data on benthic medusozoans from South Africa, the north-western Atlantic, the Arctic, and the warm western Atlantic (Bermuda), bathymetric biodiversity was highest in the first 100 m, with a substantial drop below that in species numbers on the shelf and at the beginning of the bathyal. While numbers of species may vary widely from one geographical region to another, such variations are due to differences in biodiversity in the upper 100 m. Hydroid species richness in the deep bathyal and abyssal is similar and always low. Worldwide, benthic medusozoan biodiversity is highest at shallow depths, and these organisms normally seem to be minor components of deep benthos. Very likely, the graph of biodiversity at increasing depth is similar worldwide for the Hydrozoa.