Brachiomya ducentiunus, Valentich-Scott & Griffiths & Landschoff & Li & Li, 2024

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

Brachiomya ducentiunus sp. nov.

Figs 1 A – H , 2 A – F , Suppl. material 2

Type locality.

Miller’s Point Lagoon, in False Bay, Western Cape Province, South Africa; 34.231 ° S, 18.477 ° E; 3 m; attached to spines, or crawling amongst spines of Spatagobrissus mirabilis ( Clark, 1923) , collected by Charles Griffiths, July 2018.

Type material.

Holotype (Fig. 1 A, B ), SBMNH 713162 , length 2.50 mm, height 1.75 mm, preserved in 70 % EtOH . 13 Paratypes, SAMC-A 096817 , same locality as holotype, largest specimen length 2.5 mm, height 1.7 mm, preserved in 70 % EtOH, collected by Jannes Landschoff and Craig Foster, 9 June 2016 . 7 Paratypes, SBMNH 666970 , same locality and collector as holotype (Fig. 1 C – H ), dried specimens mounted on SEM stub; length 2.17 mm, height 1.38 mm; length 2.40 mm, height 1.56 mm; length 1.87 mm, height 1.19 mm; length 2.42 mm, height 1.67 mm . 1 Paratype, UCM 60476 ; length 1.5 mm, height 1.0 mm .

Description.

Shell extremely thin, fragile, moderately inflated, translucent; inequilateral, slightly longer anteriorly; ovate-elongate; anterior end obliquely truncate in larger specimens; posterior end broadly rounded; ventral margin straight, slightly invaginated in some; dorsal margin gently sloping from umbos; shell margins weakly gaping; prodissoconch well defined, umbonate, smooth, subcircular; prodissoconch length ~ 350 μm; external sculpture of commarginal striae, with few widely spaced radial striae, especially anteriorly; umbos low, wide; hinge plate extremely narrow, with one minute pseudocardinal in each valve; ligament internal, very short. Length up to 2.7 mm.

Mantle. Large, reflected, covering ~ 95 % of outer shell surface when fully extended, but not fully covering umbos; mantle can be almost completely retracted into the shell; reflected portion with low papillae; mantle near shell margin with longer tentacles; anterior end with large cowl, serrate on end; cowl can be greatly extended (Fig. 2 E, F ; Suppl. material 2).

Foot. Of moderate size, ~ the length of the shell when fully extended, vermiform, with slight heel. The species is an active crawler, and can also attach to the host by byssal threads. The foot has been observed to frequently wrap around the urchin spines as the bivalve crawls.

Ctenidia. One demibranch on each side, comprised of ~ 30 narrowly spaced filaments in larger specimens.

Brooding. Up to ten shelled juvenile specimens observed brooding in the dorsal portion of ctenidia in mature specimens.

Distribution.

Only known from the type locality in False Bay, South Africa, and only found attached to the echinoid Spatagobrissus mirabilis ; not observed free-living.

Commensal relationship and habitat.

Found crawling on the oral surface of the heart urchin Spatagobrissus mirabilis . This host species was found to be living in a specialized microhabitat of coarse gravel and half-buried cobbles or boulders (at least at the type locality associated with kelp forests). At the type locality in 2018, of 10 sampled heart urchins, all had associated Brachiomya on their oral surface. Densities of Brachiomya ranged from 38 to 172 specimens on a single host. Two other commensal species were also recorded on these same urchins, a small but very common unidentified amphipod of family Lysianassidae , and a large, scale worm (family Polynoidae ), of which only a few specimens were found. The amphipod and polychaete species also both appear to be new to science.

Discovery.

Initially discovered via free-diving in 2016 at the type locality, collected by Jannes Landschoff and Craig Foster.

Etymology.

The name ducentiunus is from Latin, meaning “ 201. ” The species was discovered while preparing and working on the ‘ 1001 Seaforest Species’ project, a research and storytelling program aimed at increasing awareness of regional kelp bed ecosystems colloquially referred to as ‘ the Great African Seaforest’ (see www. seachangeproject. com). The number 201 was chosen as a unique identifier for the 1001 program, with the goal to link each hundredths species to a species described as new to science.

Comparisons.

The Pacific and Asian Brachiomya stigmatica , which is the only other known species in the genus, is more evenly rounded anteriorly, has a strong rust-colored stripe medially, lacks radial striae, and has more developed teeth.

Figures

Figure 1. Brachiomya ducentiunus sp. nov. A, B holotype, SBMNH 713162, length 2.50 mm, height 1.75 mm A exterior of right valve B exterior of left valve C – H paratypes, SBMNH 666970, crystallization on exterior and interior of shell is from dried bleach, not a sculpture element C exterior of right valve, length 2.17 mm, height 1.38 mm D exterior of left valve, length 2.40 mm, height 1.56 mm E interior of left valve, length 1.87 mm, height 1.19 mm F interior of right valve, length 2.42 mm, height 1.67 mm G hinge of right valve H hinge of left valve.

Figure 2. Brachiomya ducentiunus new species, living animals A, B crawling on hard substrate C overview of specimens crawling on the aboral surface of urchin Spatagobrissus mirabilis D crawling on urchin spine with mantle and foot extended E, F extended mantle between urchin spines.