Arctosia arctica ( Friele, 1877a )

Hansen, Jesper, 2024, Brachiopods of the Norwegian fauna northern North Atlantic and Arctic, with a focus on, Fauna norvegica 43, pp. 12-68 : 29-31

Arctosia arctica ( Friele, 1877a)

Figures 15 , 16

Terebratula arctica n. s. Friele, 1877a, p. 221, fig. 1, 1a–c.

Liothyris arctica – Davidson (1886), p. 10, pl. 1, figs. 17–18.

Liothyris (Terebratula) arctica – Grieg (1907), p. 540.

Gryphus arcticus – Dall (1920), p. 317.

Liothyrella arctica – Thomson (1927), p. 198.

Arctosia arctica – Cooper (1983), p. 250 –251, pl. 58, figs. 1–8, pl. 60, fig. 9, pl. 65, figs. 7–8.

Salinity range: 38.1–38.5 ( Allmen et al. 2010).

Current velocity: 25–100 cm /s ( Emig 1987; Emig & García-Carrascosa 1991).

Substrate: Rock, gravel, artificial substrates and shells ( Fischer & OEhlert 1891; Logan 1979; Allmen et al. 2010; Toma et al. 2022).

Geography: Southern margin of the Celtic Sea, Bay of Biscay, Portugal, the Mediterranean, off North Africa, Cape Verde ( Fischer & OEhlert 1891; Logan 1979; Brunton & Curry 1979; Zezina 2014).

Description: Shell moderately to strongly equi- to ventribiconvex. Outline subtriangular in smaller specimens, changing to broadly subpentagonal and somewhat truncated. Posterior margins form an angle of 75–80°. Dorsal valve without median fold or depression.Valves rarely with shallow sulcus. Anterior commissure rectimarginate. Relatively dull and thin-shelled, semitransparent to opaque, and endopunctate. Ornamentation generally lacking except for faint growth lines, but faint, dense, radiating striation can occur. Colour whitish. Beak short and rounded in lateral profile, slightly incurved, and often with short triangular deltidial plates narrowly conjoining in larger specimens. Foramen moderately large. Deltidial plates are bordered by ridges, especially in smaller specimens. Pedicle collar thick. Pedicle typically with short rootlets distally. Brachial loop very small and short, with a nearly straight, thin transverse band. Crural process just anterior to mid-loop and loop reaches to 1/4 to 1/3 valve length. No median ridge in dorsal valve. Dorsal muscle scars reaching 1/4 to 2/5 (typically 1/3) valve length. Dental plates absent. Shell up to 16.4 mm long, 13.5 mm wide and 9.2 mm deep.

Depth range: 35–481 m depth ( Friele 1877a; this study). Wesenberg-Lund (1941) reported it occurring down to 1500 m depth; however, this was based on misidentified material.

Temperature range: -1.8–1.5˚C (this study).

Salinity range: 33.7–34.9 (this study).

Substrate: Clay or sand with stones, fine mud, or on coarse gravel and small stones ( Friele 1877a; Posselt 1898; Hägg 1905; this study). Specimens were found attached to gravel, bryozoan branches, tubes of serpulid worms and to other specimens of Arctosia (this study).

Geography: Greenland, Jan Mayen and N and NE Iceland ( Friele 1877a; Grieg 1933; Zezina 1997b). Zezina (1997b) reported the species from just north of Franz Josef Land, bordering the Barents Sea to the North. Reports from off West Spitsbergen and NE Barents Sea by Grieg (1924), latter referred to by Gulliksen et al. (1999) were, as stated by Grieg (1933), misidentifications of young specimens of Hemithiris psittacea . The same is true for reports from SW of Iceland, which proved to be a new species of Xenobrochus following re-examination.

Comment: In many new publications, the year of publication is given as 1878 or even 1879. However, the publication year is specified as 1877 in the original preprint, which, according to the International Code of Zoological Nomenclature (Chapter 5, Article 21.8), is the date of publication.

This species is easily confused with Macandrevia cranium but is externally most easily distinguished by the shape of the pedicle opening and thick pedicle collar.

The rather few verified occurrences of this species are from 1973 or earlier. It has thus not been possible to evaluate whether there have been any changes in the abundance and distribution of this species. Since this is a truly Arctic species, its geographical range is likely shrinking due to increasing oceanic temperatures.

Figures

Figure 15. Arctosia arctica. a–d: dorsal, ventral, lateral and anterior views of specimen from batch TSZBr-2480, Kap Petersen, E Greenland, 200 m; e: dorsal interior of shell from batch TSZBr-2480, Kap Petersen, E Greenland, 200 m; f: ventral valve interior from batch TSZBr-2480, Kap Petersen, E Greenland, 200 m; g: lateral view of specimen from batch TSZBr-2480, Kap Petersen, E Greenland, 200 m; h: dorsal view of specimen NHMD- 1201199, Forsblad Fjord, E Greenland, 35 m; i–j: dorsal and ventral interior views of empty shell NHMO H93, type locality SW of Jan Mayen, the Norwegian North Atlantic expedition 1877 st. 237, 481 m; k–l: dorsal and ventral interior views of juvenile specimen, Jan Mayen, the Danish East Greenland Expedition 1891, 301 m, the Zoological Museum of Copenhagen. Scalebar 1mm. Photos: Jesper Hansen.

Figure 16. Distribution of Arctosia arctica.