Research Article |
Corresponding author: Paul Valentich-Scott ( pvscott@sbnature2.org ) Academic editor: Graham Oliver
© 2022 Paul Valentich-Scott, Jeffrey H. R. Goddard.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Valentich-Scott P, Goddard JHR (2022) A fossil species found living off southern California, with notes on the genus Cymatioa (Mollusca, Bivalvia, Galeommatoidea). ZooKeys 1128: 53-62. https://doi.org/10.3897/zookeys.1128.95139
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A small bivalve mollusk previously only known from the Pleistocene of Los Angeles County has recently been found living intertidally near Santa Barbara, California. The bivalve has been determined to be Cymatioa cooki (Willett, 1937), a member of the Galeommatoidea J.E. Gray, 1840. We document the habitat for the newly discovered C. cooki, and compare it to C. electilis (Berry, 1963), the other extant member of this genus recorded from the region. Cymatioa cooki is rare, and while many galeommatoid species have been shown to be commensal with other invertebrates, we have been unable to determine any specific commensal relationships for it.
Commensal, intertidal zone, Pleistocene, taxonomy
The invertebrates inhabiting the rocky intertidal zone of southern and central California are among the most studied and documented in the world (
Our recently collected specimens belong to the poorly understood genus Cymatioa Berry, 1964. The only other living representative of this genus in southern California is C. electilis (Berry, 1964). We examined the type specimens of C. electilis and concluded they were not the same as our Naples Point species. With subsequent research, we determined our species matched the holotype of C. cooki (Willett, 1937) from the Baldwin Hills Pleistocene of Los Angeles County.
Galeommatoidean bivalve mollusks have been extensively documented for nearly 200 years (
The environment at Naples Point was described in detail by
The galeommatid bivalve we describe here was collected by hand by the second author at Naples Point, located on the south coast of Santa Barbara County, 24 km west of Santa Barbara (approximately 34.43, -119.95). This area is within the Naples State Marine Conservation Area. The second author also found two living specimens, shell length about 10 mm, on 23 November 2018, under a low intertidal boulder and photographed but did not collect them (Fig.
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.
Abbreviations:
Superfamily Galeommatoidea J.E. Gray, 1840
Family Galeommatidae J.E. Gray, 1840
Crenimargo Berry, 1963, not Cossmann, 1902. Type species (monotypy): Crenimargo electilis Berry, 1963. Recent, eastern Pacific.
Cymatioa Berry, 1964, new name for Crenimargo Berry, not Cossmann.
Shell ovate; subequilateral; exterior surface finely punctate; sculpture of sparse, broad, low, radial ribs; ventral margin undulate; right valve with one anterior cardinal tooth; left valve with two anterior cardinal teeth.
Bornia cooki Willett, 1937: 389, pl. 5, figs 3–6.
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 (
Mantle
: large, reflected, covering most of outer shell surface when fully extended, including umbones (Fig.
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.
Baldwin Hills Pleistocene deposit, Los Angeles County, California; 33.9658, -118.4264;
USA, California, Santa Barbara County, off Naples Point; 34.4339, -119.9500; intertidal zone, in boulders and cobbles.
A–C Cymatioa cooki, shell of left valve collected at Naples Point,
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.
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.
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.
Previously only known from the Pleistocene of Los Angeles, Cymatioa cooki is herein recorded living for the first time. Only three living specimens have been discovered to date. Despite C. cooki’s potential commensal relationship with burrowing invertebrates, we have not sampled the intertidal infauna deeply enough to discover the potential true habitat for this species.
Depending on the lifespan of C. cooki, the adults we observed at Naples Point may have been transported as larvae from much farther south during the marine heatwaves of 2014–2016, which drove northward numerous marine species distributions in the northeastern Pacific (
Other Baldwin Hills Pleistocene bivalves reported by
Given the small size, translucent shell, and cryptic habits of C. cooki, it is not surprising that living instances of the species have been overlooked for over 80 years. We are confident that its description here will lead to discovery of further examples in southern California and likely even further south into Mexico.
Austin Hendy and Lindsay Walker from
Publication costs have been graciously provided by the Southern California Association of Marine Invertebrate Taxonomists Publication Grant (www.scamit.org). We appreciate the assistance of Megan Lilly and Erin Oderlin in facilitating our grant application.