Terebratulina septentrionalis ( Couthouy, 1838 )

Hansen, Jesper, 2024, Brachiopods of the Norwegian fauna northern North Atlantic and Arctic, with a focus on, Fauna norvegica 43, pp. 12-68 : 34-36

Terebratulina septentrionalis ( Couthouy, 1838)

Figures 21 , 22

Terebratula septentrionalis Couthouy, 1838, p. 65 –66.

Terebratulina septentrionalis – ( Davidson 1852b), p. 366.

Terebratulina caput-serpensis var. septentrionalis – Davidson (1880), p. 33 –36, pl. 1, figs. 3–9.

Description: This species is very similar to Terebratulina retusa and has overlapping ranges of almost all diagnostic characters. As noted by Sars (1878) and Wesenberg-Lund (1941), T. septentrionalis tends to have the following characters: more egg-shaped outline; rectimarginate or weakly uniplicate anterior commissure; thinner shell-wall; weaker growth lines; denser ribs with 14–22 usually low subtriangular or rounded radiating ribs per 5 mm counted 10 mm anterior of ventral umbo; tubercles on umbonal crest missing or less strongly developed; stronger tubercles usually scattered laterally on umbone; ribs lower and tend to be nearly absent distally on larger specimens; often slightly wider pedicle opening (11–15% as wide as valve). In the present study, the most stable difference from T. retusa was found to be the absence of or denser, lower and more transverse tubercles never overhanging deep interspaces between the ribs on the ventral umbonal crest. The largest measured specimen was 27.5 mm long.

Depth range: 0–3614 m depth ( Dall 1920; Brunton & Curry 1979), but mostly occurring at 10–550 m depth, with the seconddeepest recorded occurrence at approximately 1325 m ( Thomsen 2012).

Temperature range: -1.7–10.6˚C ( Allmen et al. 2010; this study).

Salinity range: 28–35.4 ( Brand et al. 2003; this study).

Oxygen range: 55–104% saturation (this study).

Current velocity: Mean current velocity at four locations measured at 2.7–6.9 cm /s, with a normal daily maximum of 2–18 cm /s and 10.8–24.8 cm /s as the absolute maximum over 1 month of measurement (this study).

Substrate: Commonly attached to serpulid tubes, but also to shells, shell sand, branched bryozoans, the coral Desmophyllum , Porifera, other polychaete tubes, or gravel (this study).

Geography: S, W and E Greenland, Svalbard (Bjørnøya, Spitsbergen, and verified from off northern Nordaustlandet in 2009, and from the Hinlopen Strait), Murmansk, the White Sea, Norway (Hordaland and North), Baffin Bay, East Canada, Jan Mayen, Iceland, Scotland, Ireland and E USA (Grieg 1933; Wesenberg-Lund 1941; Brunton & Curry 1979; Curry & Endo 1991; Thomsen 2001; Allmen et al. 2010; Thomsen 2012; this study).

Remarks: Terebratulina septentrionalis has been synonymised with the species Terebratulina retusa several times (e.g., Jeffreys 1878; Davidson 1880; Posselt 1898; Knipowitsch 1901; Hägg 1905; Zezina 1997a,c) since the species are very similar and intermediate morphotypes are found to occur in places where both species are present (e.g., Wesenberg-Lund 1941; Lüter et al. 2017). Cohen et al. (1991) addressed this question through a molecular study on specimens from British and Icelandic waters. They concluded that there were two distinct species and found no evidence of hybridisation. Recently, in their follow-up study, Lüter et al. (2017) found hybrids in places where both species co-occur in Iceland. Hybridisation could explain some observed morphological trends along Norway, where T. retusa tends to show finer costellae, thinner shells and weaker tubercles in northern regions dominated by T. septentrionalis .

Sars (1878) found that the crural processes in T. septentrionalis were not connected, in contrast to those of T. retusa . This difference was not supported by the examined material in the present study, which included both specimens with disconnected and connected processes for both species. Logan (1979) found there to be an ontogenetic trend with the connection generally developing latter in T. septentrionalis than in T. retusa . Blochmann (1908) used the spicules in the tissue to distinguish the two species. However, Wesenberg-Lund (1941) questioned the usefulness of this method. In the present study, I also failed to separate the two species based on spicules.

The variability and usefulness of all the expected diagnostic features of the two species were rigorously examined based on a large conclusions can be drawn from the absence of data from this area.

Like the previous species, this species is often overgrown by sponges, most likely of the genus Hymedesmia (belonging to Demospongia) ( Tendal & Thomsen 2005; Thomsen 2012).

Figures

Figure 21.Terebratulina septentrionalis. a–d: dorsal, ventral, anterior and lateral views of specimen, Ullsfjord in Troms, the Arctic University Museum of Norway; e–g: dorsal interior and exterior and ventral interior views of shell NHMO H2899, Øksfjord in Finnmark, Norway; h: ventral exterior view of specimen from batch TSZBr-2095, Vardø in Finnmark, Norway, 188 m; i–j: dorsal and ventral views of juvenile specimen, N of Rossøya, Spitsbergen (JH). Scalebar 1mm. Photos: Jesper Hansen.

Figure 22. Distribution of Terebratulina septentrionalis. Colours mark which century the specimens were collected in.