Fabia subquadrata Dana 1851

Fabia subquadrata Dana, 1851

(Figs. 1A–C, E, 2B)

Material examined and new hosts. 1 female (hard stage), El Coyote Estuary, Punta Abreojos, Baja California Sur, Mexico, 26°48'43.62"N, 113°27'54.98"W, 16 July 1999, free-living; 1 adult female, intertidal, Los Bungalos, Tortugas Bay, Baja California Sur, Mexico, 27°41'55.52"N, 114°52'45.45"W, 8-9 April 2000, in Modiolus capax.

Revised distribution. Akutan Pass, Alaska, U.S.A. to El Coyote Estuary, Punta Abreojos, Baja California Sur, Mexico (Campos 1996).

Hosts. Common hosts include the following bivalves: Modiolus capax, M. modiolus (Linnaeus, 1758), Mya arenaria Linnaeus, 1758, Mytilus californianus Conrad, 1837, and M. edulis Linnaeus, 1758. Occasional hosts include: Cyclocardia ventricosa (Gould, 1850), Leukoma staminea (Conrad, 1837), Saxidomus gigantea (Deshayes, 1839), Tivela stultorum, Tresus capax (Gould, 1850), and T. nuttallii (Conrad, 1837) (Pearce 1966; Garth & Abbott 1980; Campos 1996).

Other hosts. Garth & Abbott (1980) recorded the Atlantic bivalve Cyclocardia borealis (Conrad, 1832) as an occasional host; however, the presence of this bivalve in the eastern Pacific should be confirmed. The record of Mytilus edulis also needs confirmation because this species is validly reported only from embayments in California U.S.A. Mytilids from open coastal area may belong to M. galloprovincialis Lamarck, 1819, or M. trossulus Gould, 1850 (D. B. Cadien, pers. comm.).

Remarks. Species of Fabia can be separated from other genera of Pinnotheridae by the possession in adult females of two deep sulci on the carapace that extend from the orbit to the gastric region (Fig. 1A) and males having a smooth, shiny, porcelain-like carapace and two or more fused abdominal somites (Campos 1996). Four species have so far been recorded from the eastern Pacific: Fabia carvachoi Campos, 1996, F. concharum (Rathbun, 1893), F. malaguena (Garth, 1948), and F. subquadrata. Both F. subquadrata and F. concharum overlap in their distribution and may inhabit the same host in Southern California, U.S. A and on the west coast of Baja California, Mexico. Females of both species can be separated using the morphology of the chela and third maxilliped. Fabia concharum has one row of setae on the ventral margin of the cheliped propodus (Fig. 2A) and the length of the dactyl of the third maxilliped is less than one-half the length of propodus (Fig. 1 D). Fabia subquadrata, instead, has two rows of setae, one marginal and other submarginal, on the ventral region of the hand of cheliped (Fig. 2 B), and the length of the dactyl of the third maxilliped is more than one-half the length of propodus (Fig. 1B, C). Males can also be separated by differences in the abdomen. Somites 2–4 are fused in F. subquadrata, the lateral margin of somite 5-6 with pubescence, and the telson is subcircular (Fig. 1 F). In contrast, F. concharum has fused abdominal somites 3–5 (Rathbun, 1918; but this needs confirmation), the lateral margin of the sixth somite is hairless, and the telson is subtrapezoidal (Fig. 1 E). Campos (1996) and Campos & Manning (1998) pointed out how these species can be separated from the Pacific Ocean congeners F. carvachoi of the Gulf of California, Mexico and F. malaguena of Malaga Bay, Colombia.

The present record extends the southern distribution limit of F. subquadrata approximately 600 km from Ejido Eréndira, Baja California to El Coyote Estuary, Abreojos Point, Baja California Sur, Mexico and adds Modiolus capax as a new host for the adult female. Other pinnotherids commonly found in the mantle cavity of M. capax include Opisthopus transversus and F. concharum. This last species only has been recorded subtidally in Todos Santos Bay, Baja California and Tortugas Bay, Baja California Sur, both localities located on the west coast of Baja California (Campos et al. 1992; pers. obs.). Fabia subquadrata commonly is found in the mantle cavity of the mussel Mytilus californianus in California, U.S.A. and Baja California; nevertheless, M. modiolus is the common host in Puget Sound, Washington, U.S. A (Pearce 1966).