Macandrevia cranium (Müller, 1776)

Figures 1I, 29–31

Terebratula cranium Müller, 1776, p. 249 .

Anomia cranium – Gmelin (1791), p. 3347.

Terebratula euthyra Philippi, 1844, p. 68, pl. 18, figs. 8a–d.

Terebratula glabra Leach, 1852, p. 359, pl. 14, figs. 3–5.

Waldheimia cranium – Gray (1853), p. 58.

Macandrevia cranium – King (1859), p. 261.

Terebratula (Waldheimia) cranium – Lovell (1861), p. 175.

Waldheimia (Waldheimia) cranium – Dall (1870), p. 110.

Waldheimia (Macandrevia) cranium – Davidson (1886), p. 61 –66, pl. 12, figs. 11–23, pl. 13, figs. 1–2.

Magellania (Macandrewia) cranium – Fischer & OEhlert (1891), p. 72 –79, pl. 5, figs. 10a–s.

Macandrevia cranium, new var. novangliae Dall, 1920, p. 355–356.

Waldheimiathyris cranium – Helmcke (1940), p. 275.

Macandrevia novangliae – Cooper (1977), p. 127, pl. 26, figs. 1–11.

Description: Highly variable. Shell equi-biconvex to ventribiconvex. Outline egg-shaped oval to pentagonal with truncated anterior valve margin in large specimens, and typically with dorsal valve about as wide as long. Ventral umbo blunt and short. Hinge line narrow and curved. Anterior commissure generally rectimarginate but can be slightly unisulcate. Small specimens with numerous long and very thin setae along anterior margin, easily falling off during handling. Rudimentary rectangular deltidial plates not joining. Deltidial plates separated from rest of valve by distinct ridges, especially in juveniles. Pedicle short and with a simple attachment base or very rudimentary rootlets. Ornamentation lacking in grown specimens except for very weak and sporadic growth lines. Small specimens often show very fine radiating ridges from which the setae protrude. Shell matrix endopunctate. Colour usually pale brownish, but especially old or empty shells can become grey or whitish. Dental plates supporting hinge teeth in ventral valve. Hinge teeth usually rather small and approximately twice as long as high. Cardinal process in the middle of dorsal hinge area often not developed, but otherwise typically rather broad, triangular and widest posteriorly. Socket ridges (ridges defining sockets for ventral teeth) well developed. Long brachial loop reaching to 2/3 to 4/5 valve length, with triangular crural processes situated close to base, and with small and few anteriorly directed spines in front. Specimens up to 2 mm long lack median ridge or septum. Specimens between 2 and 5–6 mm long develop a high, pillar-like septum highly elevating lophophores. Septum visible on the shell surface as a lighter spot or node around mid-valve length, sometimes extending partway back to umbo. Dorsal septum lost in specimens exceeding 6 mm in length. A study of septal development in this species was conducted by Friele (1877b). Maximum shell length 31 mm.

Depth range: 2–4700 m depth (Dall 1920; Anadón et al. 2022), but with the overwhelming majority occurring at 40–400 m and only one observation from deeper than 3000 m.

Temperature range: -1.1–12.0˚C (Jeffreys 1878; this study).

Salinity range: 31.9–36.0 (this study).

Oxygen range: 55–113% saturation (this study).

Current velocity: Mean current velocity was 1.1–12.0 cm/s, with daily maxima of approximately 0.2–31 cm /s and a maximum velocity of 16.2–42.7 cm /s measured over 1 month (this study).

Substrate: Attached to sand grains, gravel, stones, exposed bedrock, calcareous algae, corals, bryozoans, cirripeds, serpulid tubes, shells, etc. (Thomsen 1990, 2001; this study) (Figure 30). Mostly on sea floor dominated by sand, shell sand, gravel, cobbles or stones (Thomsen 2001), but can occur on silt-/clay-dominated bottoms.

Geography: Greenland, NE America to Rhode Island in the USA, Svalbard, Barents Sea, Novaya Zemlya, Norway, NW Russia, SW Sweden, Iceland, the Faroe Islands, N and W portions of the British Isles, France, Spain, Portugal, W Sahara and the Canary Islands (Jeffreys 1878; Posselt 1898; Derjugin 1915; Grieg 1933; Brunton & Curry 1979; Thomsen 1990, 2001; Gulliksen et al. 1999; Zezina 2014).

Remarks: See under Dallina septigera for distinguishing characters from that species.

This is undoubtedly by far the most common species in Norwegian waters. Despite a wide depth range, this species prefers coastal and inner shelf environments.

Relatively few specimens are found as far north as Svalbard. However, there is no strong evidence of a continuation of the northern expansion of its geographical range that Thomsen (1990) found in association with the strengthening of the warm Atlantic current up along the Norwegian coast during the Holocene warming after the last ice age.

Macandrevia cranium has previously been recorded along the mid and northeastern coast of Greenland (Posselt 1898; Arndt & Grieg 1933; Wesenberg-Lund 1940b; Thomsen 2012). However, four of the seven samples with specimens were located and examined during the present study and solely contained Arctosia arctica . Therefore, it appears likely that the remaining specimens from the area, influenced by the polar water masses of the East Greenlandic Current, also are Arctosia arctica . The presence of Macandrevia cranium along W Greenland is related to the slightly warmed West Greenlandic Current flowing north along the coast.