Cymatioa cooki (Willett, 1937)

Valentich-Scott, Paul & Goddard, Jeffrey H. R., 2022, A fossil species found living off southern California, with notes on the genus Cymatioa (Mollusca, Bivalvia, Galeommatoidea), ZooKeys 1128, pp. 53-62 : 53

Cymatioa cooki (Willett, 1937)

Figs 1A-H , 2A-C

Bornia cooki Willett, 1937: 389, pl. 5, figs 3-6.

Description.

Shell: thin, fragile, subovate; inequilateral, posterior end much longer; anterior and posterior ends broadly rounded; dorsal margin gently sloping on each side of umbos; ventral margin broadly gaping in living animal; beaks small, sharply pointed; prodissoconch 200 µm in diameter; sculpture of irregular, slightly wavy commarginal striae, and fine, dense punctae; ventral margin with sparse, broad, low radial undulations; periostracum thin, light beige, silky; hinge plate narrow; right valve with one short anterior cardinal tooth, one elongate posterior lateral tooth; left valve with two minute anterior cardinal teeth, one elongate posterior lateral tooth; ligament internal, opisthodetic, elongate; resilifer narrow, elongate; ventral margin slightly wavy internally; adductor muscle scars subovate, subequal; pallial line entire; strong accessory muscle scars dorsal to pallial line. Length to 11.4 mm ( Willett 1937).

Mantle: large, reflected, covering most of outer shell surface when fully extended, including umbones (Fig. 1A ); mantle can be mostly retracted into the shell; reflected portion of mantle sparsely papillate (Fig. 1A ); slightly fused posteroventrally; two anterior and two posterior tentacles, short, slightly extending past shell margins (Fig. 1A, B ).

Foot: large, translucent, exceeding the length of the shell when fully extended, spathate, with distinct pointed heel; bright white stripe extending from the tip of foot to the shell margin, presumably related to byssal formation (Fig. 1A ). This species is an active crawler (Fig. 1C ).

Type locality.

Baldwin Hills Pleistocene deposit, Los Angeles County, California; 33.9658, -118.4264; LACMIP locality 59.

Locality of living specimens.

USA, California, Santa Barbara County, off Naples Point; 34.4339, -119.9500; intertidal zone, in boulders and cobbles. SBMNH 629938, conjoined shell and anatomy, length 7.4 mm, height 4.5 mm (Fig. 1A, B ); SBMNH 641848, (Fig. 2A-C ), one left valve length 8.8 mm, height 5.5 mm.

Habitat and potential commensal relationships.

All three living specimens were found near the seaward edge of a boulder field centered at 34.4339, -119.9500 and located on a broad, gently sloping, wave-cut bench of Monterey Shale. This boulder field extends vertically from a tidal height of approximately +0.3 m above mean lower low water to -0.4 m. The surfgrass Phyllospadix torreyi S. Watson, 1879, dominates much of the surrounding bench. At low tide, a shallow lagoon lies just landward of the boulder field, and behind that are more shale bench, a narrow sand beach, and then cliffs up to 20 m high consisting of Monterey shale overlain by terrestrial deposits. Sand levels on the beach and in the lagoon fluctuate seasonally, with nearly all of the beach scoured away in winter, but the boulder field as a whole is never significantly inundated, especially at its seaward edge where the Cymatioa was found. Vertical relief in the boulder field is fairly low, with most boulders under 0.5 m diameter. A few rock outcrops just to the west are only about 1 m high.

The specimens found on 23 November 2018 were on sand underneath a boulder (Fig. 1C, D ). One of these was found at the entrance to a burrow of unknown origin, with its foot extended and tentaculate inhalant siphon extending into the burrow opening. The burrow may have been constructed by the Tidepool Ghost Shrimp, Neotrypaea biffari (Holthuis, 1991), which occur frequently under boulders at this site, usually with commensal Blind Gobies Typhlogobius californiensis Steindachner, 1879. This sighting is vouchered in eight images at https://www.inaturalist.org/observations/18597683, with the last image showing one of the specimens as first observed, next to the burrow entrance described above.

The specimen found on 4 March 2019 was on the underside of a boulder, among scattered tubes of the annelid Spirorbis sp. and small, scattered patches of an unidentified tan-colored encrusting sponge. Two small dorid nudibranchs, Conualevia alba Collier & Farmer, 1964; a single mussel, Mytilisepta bifurcata (Conrad, 1837); and an adult chiton, Stenoplax conspicua (Dall, 1879), were also present, all within a few centimeters of the C. cooki . Burrow openings of unknown origin and 3-5 mm in diameter were also present on the undersurface of the boulder. This sighting is vouchered in six images at https://www.inaturalist.org/observations/20962245.

Comparisons.

The shell morphology of C. cooki is closest to C. electilis , with both species sharing a commarginal and punctate sculpture and an undulate ventral margin (Fig. 2A-F ). Cymatioa cooki is subquadrate and inequilateral, with a much longer posterior end (Fig. 2A ), whereas C. electilis is subovate with a slightly longer posterior end (Fig. 2D ). The cardinal teeth in both species are quite small and similar; however, the posterior lateral tooth in C. cooki is longer and more robust (Fig. 2C ) than that of C. electilis (Fig. 2F ). Because the living animal is undocumented for C. electilis , we are unable to provide anatomical comparisons. However, based on other galeommatid taxa, many differences in mantle tentacles and papillae are likely.

Figures

Figure 1. Cymatioa cooki. A, B living animal from Naples Point, SBMNH 629938, length = 7.4 mm A lateral view with extended foot, note mantle papillae anteriorly and dorsally B ventral view with wide, long mantle gape C, D animals on native substratum E-H holotype, LACMIP 59.2., length = 9.7 mm E exterior of right valve F exterior of left valve G interior of left valve H interior of right valve.

Figure 2. A-C Cymatioa cooki, shell of left valve collected at Naples Point, SBMNH 641848, length 8.8 mm A exterior of valve B interior of valve C close up of hinge D-F Cymatioa electilis, left valve D, E holotype, CASIZG 043976, length = 16 mm D exterior of valve E interior of valve F paratype, SBMNH 34017, close up of hinge.