Thyasira “ frigida, Oliver, 2025

Thyasira “ frigida ” Ockelmann MS

Figs 2–4 View Fig View Fig View Fig

In Ockelmann’s monograph (1958) on the bivalves of East Greenland, he illustrated a shell from Jan Mayen as Thyasira equalis ( Verrill & Bush, 1898) ( Ockelmann 1958: fig. 7). He noted that this species was widely distributed in East Greenland and was in general a little larger than T. equalis from NE America. In McIntyre (1961), it became evident that Ockelmann changed his mind and that the T. equalis illustrated by him was probably undescribed. In his collections, Ockelmann labelled this form as “ Thyasira “ frigida ” n. sp. ” and selected a prospective holotype (NHMD-1175760) from “Ingolf” stn 126; 67°19′ N, 15°52′ W, at a depth of 535 m, N of Iceland. The Danish Ingolf Expedition of 1895/96 was reviewed by Wolff (2008). Ockelmann’s map of distribution for T. “ frigida ” gives a widespread range across the cold regions of the North Atlantic into the Arctic Ocean at 80° N. Then, around 1975/76 he corresponded with Irene Lubinsky who subsequently described T. dunbari Lubinsky, 1976 from Arctic Canada. Ockelmann was gifted a specimen of T. dunbari (NHMD-1175909) and from then on dropped his name T. “ frigida ” in favour of T. dunbari .

I find this surprising as the shapes of the shell are quite different, T. “ frigida ” being polygonal rather than the pyriform shell of T. dunbari . Furthermore, the prodissoconch sizes, which Ockelmann gave emphasis to throughout his studies, are quite different with a mean of 165 µm in T. “ frigida ”, but 193 µm in T. dunbari . Perhaps he considered T. dunbari to represent a larger, gerontic form of his T. “ frigida ”, but there are no shells resembling T. dunbari in any of the collections in the Natural History Museum of Denmark from East Greenland, northern Norway or Svalbard. Here, I present his illustrations ( Fig. 2 View Fig ) and map ( Fig. 4 View Fig ) from the archive along with photographs of the type specimen of T. “ frigida ” from Ockelmann’s collection ( Fig. 3B View Fig ) and a paratype of T. dunbari ( Fig. 3A View Fig ). From this simple comparison, I would not consider T. dunbari and T. “ frigida ” to be conspecific. However, samples of T. “ frigida ” do vary and are close to T. rotunda ( Jeffreys, 1881) , a taxon not considered by Ockelmann in his monograph. A full consideration of T. dunbari , T. “ frigida ” and T. rotunda will be presented in a forthcoming paper on the Thyasiridae of the UK Atlantic margin (Oliver in prep).