Melliteryx mactroides ( Hanley, 1857 )

Valentich-Scott, Paul, Griffiths, Charles, Landschoff, Jannes, Li, Ruiqi & Li, Jingchun, 2024, Bivalves of superfamily Galeommatoidea (Mollusca, Bivalvia) from western South Africa, with observations on commensal relationships and habitats, ZooKeys 1207, pp. 301-323 : 301-323

Melliteryx mactroides ( Hanley, 1857)

Fig. 5 A – H , Suppl. materials 3, 4

Pythina mactroides Hanley, 1857: 340.

Material examined.

Three specimens from Miller's Point , False Bay, South Africa (34.231 ° S, 18.476 ° E) .

Description.

Shell trigonal, thick for size, moderately inflated, cream colored; subequilateral; umbos narrow, pointed; anterior and posterior ends broadly rounded; shell margins not gaping; periostracum thick, adherent, yellow to dark brown; exterior sculpture of fine commarginal striae, some with micro-pits; hinge plate broad; both valves with an anterior and posterior lateral tooth with a wide gap between them; left valve small central pseudocardinal tooth; anterior lateral tooth in right valve with small thickening near umbos; ligament oblique, narrow, in shallow resilifer. Length up to 6 mm.

Mantle. Translucent, only slightly reflected, forming a facultative siphon posteriorly (see Suppl. material 3).

Foot. Long, broad, translucent, with distinct heel. This species is an active crawler (see Suppl. material 3).

Ctenidia. With one demibranch on each side, comprised of ~ 75 narrowly spaced filaments in larger specimens.

Type.

NHMUK 1967994, Cape of Good Hope, South Africa.

Commensal relationship and habitat.

Found in small groups of 10–20 animals, clinging to the underside of rocks in the lower intertidal. We have found no directly associated hosts.

Comparisons.

Huber (2015) documented two additional species of Melliteryx in South Africa, Melliteryx jaeckeli Huber, 2015 , and Melliteryx fortidentata ( Smith, 1904) . Melliteryx jaeckeli has a much weaker hinge plate than Melliteryx mactroides , and we question whether this is the correct genus for the Smith species. Melliteryx fortidentata is inequilateral, with the umbos placed well off the center, compared to the subequilateral Melliteryx mactroides .

Locality information.

Specimens were collected in the intertidal at Miller's Point (34.231 ° S, 18.476 ° E). Voucher specimens were deposited as SBMNH 665156, SAMC-A 096820, and UCM 60477.

Discussion.

We are following Huber (2015) and Cosel and Gofas (2019) with the placement of this species into the genus Melliteryx . As our molecular results indicate below there is little resolution of the genera due to lack of taxon sampling globally. With further genetic data it is possible that our South African species might fall into an undescribed genus.

Cosel and Gofas (2019) reported Melliteryx mactroides from tropical West Africa. The specimens they illustrate are more elongate than our specimens and have a weaker hinge plate. Additional study must be completed to determine if these are indeed the same species, or if the tropical West Africa specimens represent a new species.

On the shell exterior in living specimens of Melliteryx mactroides we observed a dense layer of filamentous biofilm (see Suppl. material 4). A similar biofilm was reported by ( Gillan and De Ridder 1997) and ( Gillan et al. 2000) in the North Atlantic galeommatid Tellimya ferruginosa ( Montagu, 1808) .

Figures

Figure 5. Melliteryx mactroides A – F SBMNH 665156, length = 6.0 mm, height = 5.0 mm A exterior of right valve B exterior of left valve C interior of left valve D interior of right valve E hinge of left valve F hinge of right valve G living animals crawling on a rock H living animal with foot and mantle extended (note biofilm present in G and H).