Cyclothyca pacei Petuch, 1987

Cyclothyca pacei Petuch, 1987

Cyclothyca pacei Petuch, 1987

Beach drift, Boca Raton, Palm Beach County, Florida (4 mm.). Digital images by David Kirsh.

    The photographer, David Kirsh, reported that he recently collected from drift in Boca Raton, Florida and found a 4 mm. cap-like shell that he thought he knew. He recalled that the species name had "tri-" in it, and possibly "Ama-" something but that he couldn't find it in Abbott's American Seashells (1974), Colin Redfern's Bahamian Seashells, or Harry Lee's Marine Shells of Northeast Florida. He subsequently checked Okutani's Marine Mollusks in Japan and there it was: Amathina tricarinata (Linnaeus, 1767). He wondered whether others have found this species in the western Atlantic.

Response by Scientific Advisor Harry G. Lee 

    David's specimen is referable to Cyclothyca pacei Petuch, 1987 (22; pl. 4, figures 6,7). It seems to prosper on the shells of living pectinids and Spondylus americanus. According to the original description, in which the genus name is consistently misspelled "Cyclotheca," the favored host is Chlamys imbricata. [= Caribachlamys pellucens (Linnaeus, 1758); see below]

    Cyclothyca was placed in the Capulidae by Stearns (1891: 212) and Petuch (loc. cit.) whereas the affinities of the usually much larger Amathina tricarinata (Linné, 1767: 1259: sp. 762; see below for URL), the monotype of its genus, Amathina Gray (1842: 63; see Gray, 1847: 157; genus 267 <>), are with the only remotely-related Heterobranchia.

    But wait! I have just examined some juvenile C. pacei shells and discovered that, like the Heterobranchia, they are heterostrophic. That is, the embryonic shell is sinistral, and the axis of coil rotates through about 270 degrees at the metamorphosis to the teleoconch. The shells of the two nominal taxa are in fact very close! So close that, allowing for differences in size, and accounting for the natural variability of shell form induced by irregularities of substrate topology, they could be the same species! In any event the resemblance between "Cyclothyca" pacei Petuch and Cyclothyca Stearns 1891 [Type species Capulus corrugatus Stearns, 1891* <>; <>] is infinitesimal compared to it and A. tricarinata <>.

    In its specialized niche, "Cyclothyca" pacei is a common species. In the original description it was noted that Bob Pace found up to five C. pacei on a single living Caribachlamys pellucens (Linnaeus, 1758) <> (Petuch, loc. cit.). Wayne Harland (pers. comm., 21 June, 2011) recalls finding it commonly on Spondylus spp. and certain Caribachlamys off Broward Co., and I have found several dozen shells of this ectosymbiont in Spondylus americanus <> grunge from off SE FL provided me by Tom Honker and by Jeff Whyman. Given that Caribachlamys spp. and S. americanus have been ultra-favorites with collectors for a long time, it is astounding that C. pacei could have escaped detection from the dawn of snorkel and SCUBA collecting in that region as long as it did. Could it be that this snail is simply an ecophenotypic descendant of an immigrant A. tricarinata? The latter is known to be associated with oysters and pen shells in Japan. Maybe, when introduced to, say, Biscayne Bay in bilge water pumped out from one of the Japanese merchant fleet sometime in the middle third of 20th Century, it took up with the most available hosts in the vicinity and rapidly adapted to those substrates?

* Although Petuch (loc. cit.) reported Cyclotheca corrugata Stearns [sic (orthography; neither parentheses nor date)] as being based solely on a 19th Century lot from Nicaragua and its Panamic provenance had been questioned earlier (Keen, 1971: 467), any doubt had been removed with James McLean's 1974 collection of the species in Costa Rica (Keen and Coan, 1975: 22) and its subsequent rediscovery in Nicaragua (Garcia, 1996). More recently it has been found in Ecuador (Kate Clark, pers. comm. Feb. 26, 2000.

Garcia, E.F., 1996. The rediscovery of Cyclothyca corrugata Stearns, 1890. American Conchologist 24(4): 19. December.

Gray, J.E., 1842. [Mollusca] pp. 48-92 in: Synopsis of the Contents of the British Museum 44th edition. G. Woodfall and Son, London. [not seen; see Gray (1847: 157; genus 267: <>)].

Gray, J.E., 1847. A list of the genera of Recent Mollusca, their synonyma and types. Zoological Society of London, Proceedings for 1847 [15](178): 129-219. Nov. 30. <>.

Keen, A.M., 1971. Sea shells of tropical west America. Stanford Univ. Press, CA, pp. 1-1064 incl. numerous figs. + 22 pls. Sept. 1.

Keen, A.M. and E. Coan, 1975. Sea shells of tropical west America: additions and corrections to 1975. Western Society of Malacologists Occasional Paper 1. 66 pp. June 22 [p. 22; vidi 1988 reprint]

Linné, C. von, 1767.
Systema Naturae, seu per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus II Editio duodecima, reformata. Laurentius Salvius, Holmia (Stockholm). Pp. 533-1327. <>

h, E.J., 1987. New Caribbean molluscan faunas. Coastal Education Research Foundation, Charlottesville, VA, pp. 1-154 incl. 29 pls. + A1-A4.

Stearns, R.E.C., 1891. Scientific results of expeditions of the U.S. Fish Commissions Steamer Albatross. Proceedings of the United States National Museum 13: 205-225, pls. 15-17. [These are the proceedings for the year 1890, but, based on Library of Congress imprint, not published until 1891
; see URL's in text above.]