The invertebrates inhabiting the rocky intertidal zone of southern and central California are among the most studied and documented in the world (Morris et al. 1980; Ricketts et al. 1985; Carlton 2007). The bivalve mollusks of this region and in this habitat have also been extensively researched (Coan et al. 2000; Coan and Valentich-Scott 2007). It is thus a surprise that a bivalve previously known only from the Pleistocene has been recently discovered living on the underside of intertidal rocks at Naples Point in Santa Barbara County, California. This small, translucent bivalve is clearly identifiable as a member of the frequently cryptic yet exceedingly diverse superfamily Galeommatoidea J.E. Gray, 1840.

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 (Turton 1825; Deshayes 1856; Morton and Scott 1989; Goto et al. 2012; Li et al. 2012). Members of the superfamily are exceptionally diverse, with both free-living and commensal species (Li et al. 2016). Those with commensal relationships have been documented living in association with many different invertebrate hosts, including echinoderms, crustaceans, and annelids (Morton and Scott 1989; Goto et al. 2012).

Willett (1937) documented the molluscan fauna at Baldwin Hills, central Los Angeles, during the time a sewer line was being installed. The sewer trench uncovered a 20–30 cm thick Pleistocene deposit of invertebrate and vertebrate fossils, approximately four feet below ground level. In his publication, Willett recognized 296 species of mollusks and described two new species of galeommatoidean bivalves, Rochefortia reyana and Bornia cooki [now Cymatioa cooki].

Bandy and Marincovich (1973) estimated the Baldwin Hills deposits to be between 36,000 and 28,000 years before the present. The deposits range from 78 to 146 m above current sea level and are approximately 10 km from the modern coastline.

The environment at Naples Point was described in detail by Sousa (1979), who conducted ecological research there (termed the Ellwood Boulder Field) and by Goddard et al. (2020), who conducted a long-term study of heterobranch sea slugs at the point. Common macro-invertebrates observed under boulders and cobbles by the latter included Striped Shore Crabs Pachygrapsus crassipes Randall, 1840, juvenile Bat Stars Patiria miniata (Brandt, 1835), juvenile Purple Sea Urchins Strongylocentrotus purpuratus (Stimpson, 1857), the Banded Turban Snail Tegula eiseni Jordan, 1936, the chitons Stenoplax conspicua (Dall, 1879), Lepidozona pectinulata (Carpenter in Pilsbry 1893), and Leptochiton rugatus (Carpenter in Pilsbry 1892), the Tidepool Ghost Shrimp Neotrypaea biffari (Holthuis, 1991) and its commensal goby Typhlogobius californiensis (Steindachner, 1879), the Peanut Worm Phascolosoma agassizii Keferstein, 1866, the brittle star Ophioplocus esmarki Lyman, 1874, and juvenile two-spot octopuses (Octopus sp.). It was near the end of the study by Goddard et al. (2020) that he found the living galeommatid bivalve described herein.

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. 1C, D). On 4 March 2019, the second author found a third specimen, shell length 7.4 mm, on the underside of a low intertidal boulder, about 10 m east from where the first two specimens were found. After the third specimen was photographed in situ and collected, additional images were taken following relaxation in MgCl₂ (Fig. 1A, B). On 10 December 2019, a fourth specimen, a left shell valve 8.8 mm long, was found underneath a low intertidal boulder (Fig. 2A–C). Subsequent visits to the same locality did not yield any additional shells or living animals.

Abbreviations: CASIZG, Invertebrate Zoology and Geology, California Academy of Sciences, San Francisco, California, USA; SBMNH, Invertebrate Zoology, Santa Barbara Museum of Natural History, Santa Barbara, California, USA; LACMIP, Invertebrate Paleontology, Natural History Museum of Los Angeles County, Los Angeles, California, USA.

Superfamily Galeommatoidea J.E. Gray, 1840

Family Galeommatidae J.E. Gray, 1840

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 (Cavole et al. 2016; Sanford et al. 2019), including populations documented specifically at Naples Point (Goddard et al. 2016, 2018). This might explain why the second author did not find C. cooki at this site prior to 2018, despite intensively searching the same under-rock habitat for heterobranch sea slugs at Naples Point since 2002 (Goddard et al. 2020).

Other Baldwin Hills Pleistocene bivalves reported by Willett (1937) have been documented as living in southern California. Mytilus adamsianus [= Brachidonties adamsianus (Dunker, 1857)] is a common modern rocky intertidal species from Santa Cruz Island, California, to northern Peru (Coan and Valentich-Scott 2012). Ensis californicus [= Ensis myrae (Berry, 1953)] and Petricola “tellimyalis” [= Petricola hertzana (Coan, 1997)] are also found intertidally in southern California with the former in sandy protected environments and the latter associated with giant kelp holdfasts (Coan and Valentich-Scott 2012). The Cymatioa specimen described by Willett (1937) was named for a Miss Edna T. Cook, who collected the specimens.

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.