Pinnaxodes gigas Green 1992
Creators: name: Campos, Ernesto
Publisher: name: Zenodo
Year: 2016
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Pinnotheridae
Pinnaxodes
Pinnaxodes gigas

description: Pinnaxodes gigas Green, 1992
(Figs. 3A–E)
Material examined. 1 male, Bajo Macho, northeast of Consag Rock, upper Gulf of California, Mexico, 31°7'26.38"N, 114°31'50.68"W, May 1995, shrimp trawl; 1 female, Kino Bay, Sonora Mexico, 28°48'33.66"N, 111°54'59.26" W in Pinna rugosa; 10 females, west coast of Baja California, near San Quintín, 30°28'31.58"N, 116°4'30.27"W, 19 Dec. 2011, in Panopea sp. (presumably P. generosa, see below).
Distribution. Gulf of California: Bajo Macho, northeast of Consag Rock, upper Gulf of California; Tastiota estuary, Sonora, Mexico. West coast of Baja California, commercial catches near San Quintín, Baja California, Mexico (Green 1992; Campos et al. 1998; Emparanza et al. 2011; this work).
Hosts. Gulf of California: the bivalves Panopea globosa (Dall, 1898), Pinna rugosa (Sowerby, 1835) (new host) and west coast of Baja California Panopea sp., presumably P. ge n e ros a (new host), (Emparanza et al. 2011; this work).
Remarks. The geoduck crab Pinnaxodes gigas is the largest pinnotherid crab ever recorded for the Americas, with fully developed female having a carapace width of up to 36 mm. Examination of recently dead specimens permitted corroborating the color pattern reported by Green (1992) and Campos et al. (1998): red orange spots (hexadecimal color # d13c00) on the dorsal surface of carapace and ambulatory legs, whereas the ventral surface has shades of dark grayish orange (hexadecimal color #a89d94) (Fig. 3A). Other species including P. floridensis, from the Northwestern Atlantic, and Opisthopus transversus, from the Eastern Pacific, feature such red-spots as well, which Hopkins & Scanland (1964) explained as crabs eating carotenoid-food rich from their host. These species share a suborbicular to subpentagonal carapace, a third maxilliped with a spoon-shaped dactylus proximally inserted on the spatulate propodus, and males with a narrow and triangular abdomen (Figs. 3B–D, F– H). There are nevertheless morphological differences between these species of Pinnaxodes and O. transversus, including the shape of the front, and the meri of ambulatory legs and telson (Campos et al. 1998). Pinnaxodes gigas and P. floridensis have entire the front, meri of ambulatory legs distally swollen, and telson of the males basally expanded (Fig. 3B–E). Opisthopus transversus, in contrast, has an emarginate front, meri of ambulatory legs uniformly wide, and telson of the males not basally expanded (Fig. 3 F–I). Despite females of P. gigas being undescribed, the male diagnostic features of the front and ambulatory legs can be used to separate females of P. gigas from females of O. transversus (pers. obs.).
The present record of P. gigas in Pinna rugosa is unusual. Several decapod symbionts are known from P. rugosa, including the pontoniine shrimps Pontonia pinnae Lockington, 1878, and P. simplex Holthuis, 1951 (Wicksten 1983, 1989; Campos et al. 1995; Paredes-Rios & Balart 1999). The published host data suggests that the preferred host for P. gigas in the Gulf of California is Panopea globosa, (Emparanza et al. 2011; pers. obs.). The strong and firm carapace, chelipeds, and ambulatory legs of this crab are shared with pinnotherids that evolved to live in symbiosis with sea cucumbers, e.g Pinnixa banharti Rathbun, 1918 (host Molpadia arenicola) or Holothuriophilus trapeziformis (Burger, 1895) (host Holothuria inornata Semper, 1868) (Hopkins & Scanland 1964; Campos 2007; Campos et al. 2012). Campos et al. (1998) suggested a sea cucumber as a potential host for P. gigas. The robust morphology of this crab allows to hypothesize that a host change probably occurred in P. gigas, first evolving as a symbiont in the cloaca of a sea cucumber, switching later to a geoduck clam, a burying bivalve that is characterized by large and fused siphons that resemble the body-shape of a sea-cucumber. The strong and firm body of P. gigas does not match the soft carapace and weak pereiopods of pinnotherids that evolved as symbionts of bivalves, e.g. species in Pinnotherinae sensu stricto (Campos 2009). It therefore possible that geoducks clams, Panopea spp., have been more recently exploited as hosts of P. g i ga s.
According to Emparanza et al. (2011) P. globosa from Playa del Sol (open coast), Empalme, Sonora, Mexico were infested with at least one crab each, infestation rate being 100%, but some clams were infested by heterosexual pairs or even three individuals per host recording an average of 1.4 crabs by host. Conversely, a population of P. globosa from Altata lagoon system, Sinaloa, Mexico was uninfested by P. gi gas (Góngora-Gómez et al. 2016; pers. comm). Cáceres-Martínez & Vásquez-Yeomans (2008) recorded an unidentified pinnotherid crab in P. ge n e ros a, presumably P. gigas. Because the known distribution of geoducks in Baja California is putatively disjunt, specimens of P. gi g as from San Quintín herein recorded were probably from P. generosa (see Calderon- Aguilera et al. 2010). The shells of the clam hosts were unfortunately destroyed and their identities were not confirmed. Confirmation of this host is desirable, since several species of subtropical and tropical species of crabs have been recently discovered in San Quintín Bay (Campos & Campos 2012). It is possible that the sub-tropical Panopea globosa may occur within the southern limit of P. ge n e ro s a, between San Quintín and Canoas Point (29°25’N, 115°06’W; González-Peláez et al. 2013) on the west coast of Baja California. Most of the publications dealing with the distribution of Panopea along the west coast of Baja California cut the northern distribution of P. globosa at Magdalena Bay, with only one extralimital record by González-Peláez et al. (2013), and the southern distribution of P. g e ne ro s a in San Quintín-El Tomatal (28°29'9.75"N, 114°4'1.60"W) (Aragón-Noriega et al. 2012; González-Peláez et al. 2013; pers. obser.). Both species may nevertheless overlap their distribution between Magdalena Bay and El Tomatal on the west coast of the Baja California, a transitional biogeographic region for marine biota of the Californian, Mexican, and Cortez provinces (Brusca & Wallerstein 1979) that is periodically affected by El Niño and La Niña oscillations (Wang & Fielder 2006). This may influence the distribution of species of the region including species of Panopea and their symbionts (see Campos & Campos 2012).
description: Published as part of Campos, Ernesto, 2016, The Pinnotheridae of the northeastern Pacific (Alaska to Mexico): zoogeographical remarks and new bivalve hosts (Crustacea, Brachyura, Pinnotheridae), pp. 311-329 in Zootaxa 4170 (2) on pages 320-323, DOI: 10.11646/zootaxa.4170.2.5, http://zenodo.org/record/265679