Pelagodiscus atlanticus (King, 1868)

Figures 2–3

Discina atlantica King, 1868, p. 171 –173, figs. 1–2.

Discina (Discinisca) (?) atlantica, Jeffreys – Dall (1873), p. 201.

Discinisca atlantica – Davidson (1888), p. 200 –202, pl. 26, figs. 18–22.

Pelagodiscus atlanticus – Dall (1908), p. 440.

Description: Shell subcircular with semitransparent conical dorsal valve and flat ventral valve. Ventral valve very thin, attached to a hard substrate and markedly smaller than dorsal valve. Large subperipheral pedicle opening in ventral valve. Holoperipheral growth with apex slightly behind valve centre. Surface marked by dense concentric growth lines. Shell chitinophosphatic, impunctate and in most cases partly transparent. Juvenile shells nearly white or colourless, becoming yellow, reddish brown or even dark brown with increasing size. Dorsal valve margin with numerous radiating mantle setae, very long along anterior margin, shorter posteriorly. Ventral mantle margin lined with dense, short setae. No hinge. Maximum length 8 mm.

Depth range: Minimum range is 658–5530 m depth for live specimens (Davidson 1880; Zezina 2014). Although Dall (1920) reported two specimens from 294 fathoms (or possibly metres) off the coast of Florida, these should preferably be verified. Zezina (2010) also reported specimens from as shallow as 366–525 m depth near the Scandinavian Peninsula, in the South Ocean and the Malay Archipelago; however, more detailed information is missing and verification would thus be preferable.

Temperature range: - 1.2–3.5˚C (Jackson 1912; this study).

Salinity range: 34.9–35.1 (4 measurements) (this study).

Substrate: Attached to rock, bones or shells, but typically on bottoms dominated by ooze (Davidson 1880; Wesenberg-Lund 1941; Brunton & Curry 1979; this study).

Geography: Panoceanic deep-water species mainly found along the mid-ocean ridges as well as along the bases of continental slopes. Presently not recorded from the Greenland Sea and the Norwegian Sea (Figure 3). Since the species is reported from both the North Atlantic and an empty shell from the Canadian Basin in the Arctic Ocean (e.g., Wesenberg-Lund 1941; Brunton & Curry 1979; Zezina 1997a, 2010), it is likely also living in the deep parts of the Norwegian and the Greenland Sea.