Montacuta cf. substriata (Montagu, 1808)

Fig. 3 A – F

Ligula substriata Montagu, 1808: 25.

Material examined.

Four specimens from 122 m off Agulhas Bank, ~ 110 km south off Mossel Bay (35.196 ° S, 22.056 ° E).

Description.

Shell thin, fragile, moderately inflated, opaque; inequilateral, much longer anteriorly; anterior and posterior ends broadly rounded (Fig. 3 A, B); shell margins only weakly gaping, if at all; prodissoconch length ~ 300 μm; dissoconch sculpture of commarginal striae, irregular widely-spaced radial striae, plus 1–3 low, broad, irregular radial undulations in some; umbos narrow, pointed, slightly projecting; hinge plate narrow; both valves with short, stout anterior cardinal tooth, and long, thin posterior cardinal tooth (Fig. 3 C, D); ligament in oblique resilifer between cardinal teeth. Length up to 3 mm.

Mantle not reflected.

Foot. Large, equal to or slightly longer than the length of the shell when fully extended, trigonal, without heel; long ventral byssal groove extending to end of smooth foot tip. Can attach to the host by byssal threads.

Ctenidia. With one demibranch on each side, comprised of ~ 20 widely-spaced filaments in larger specimens.

Type.

Lost; Devon coast, United Kingdom.

Commensal relationship and habitat.

Found crawling on the oral surface of the heart urchin Spatangus capensis Döderlein, 1905. Up to 20 specimens have been observed byssally attached to the host.

Locality information.

Collected in 122 m off Agulhas Bank, ~ 110 km south off Mossel Bay (35.196 ° S, 22.056 ° E). Voucher specimens deposited as SBMNH 467288, SAMC-A 096818, and UCM 60478.

Discussion.

Montacuta substriata is a well-documented species in the North Atlantic (Oliver et al. 2016) and the Mediterranean (Gofas et al. 2011). Barnard (1964 a) reported a single valve of this species in 100 fathoms (182 m). Cosel and Gofas (2019) did not report the species from tropical West Africa, nor have there been any other records from the African Atlantic or Indian Oceans. We acknowledge that there is likely limited gene flow between the populations of Montacuta substriata in the North Atlantic and Cape Town, especially in light of the apparent absence of the species in tropical West Africa. However, the two populations match conchologically in all details although their hosts are different; Spatangus capensis in South Africa and Spatangus purpureus Müller, 1776 and Enchinocardium flavescens Müller, 1776, in the northeast Atlantic. Unfortunately, we have been unable to extract DNA from our specimens from the Agulhas Bank, so we are unable to completely confirm this identification. It is possible that the South African specimens represent a new species.