Pachygrapsus Randall, 1840

Poupin, Joseph, Davie, Peter J. F. & Cexus, Jean-Christophe, 2005, A revision of the genus Pachygrapsus Randall, 1840 (Crustacea: Decapoda: Brachyura, Grapsidae), with special reference to the Southwest Pacific species, Zootaxa 1015 (1), pp. 1-66 : 6-7

Pachygrapsus Randall, 1840

Pachygrapsus Randall, 1840 (type species Pachygrapsus crassipes Randall, 1840 , subsequent designation by Kingsley, 1880; gender masculine; type locality was originally indicated as Hawaiian Is. but this is probably an error as it has not been subsequently reported from this locality; the Pacific coast of the United States is more likely).

Goniograpsus Dana, 1851 (type species Goniograpsus innotatus Dana, 1851 , a subjective junior synonym of Pachygrapsus transversus ( Gibbes, 1850) , subsequent designation by Manning & Holthuis, 1981; gender masculine).

Diagnosis

Carapace subquadrate or with lateral margins convergent backwards; dorsal surface slightly convex, feebly or strongly striated. Anterolateral margins entire or with one or two teeth behind exorbital angles. Length of front more than half carapace width. Third maxilliped widely gaping; merus as broad as or broader than long with no oblique setal crest. Inner sub­orbital lobes small, allowing antennae to enter orbits.

Remarks

Pachygrapsus Randall is morphologically close to Planes Bowdich, 1825 , and Metopograpsus H. Milne Edwards, 1853 . The latter two genera include species that have been sometimes placed in Pachygrapsus . Planes differs from Pachygrapsus by having a more convex carapace that is subcircular, smooth or only feebly striated; a natatory fringe of setae on the propodi of the ambulatory legs; and an open­water habitat (among pelagic seaweeds or crawling on floating objects). It includes three species: P. major ( MacLeay, 1838) , P. minutus ( Linnaeus, 1758) , and P. marinus Ratbhun, 1914 . Planes marinus Ratbhun, 1914 is somewhat intermediate in morphology between Planes and Pachygrapsus . Its carapace is more subquadrate and striated, the chelae resembling those of Pachygrapsus , and the natatory fringe of setae on the ambulatory legs can be reduced. This led Chace (1951: 72) to transfer the species to Pachygrapsus , mentioning that, except for its much smaller size, it is very similar to Pachygrapsus crassipes Randall, 1840 . Chace (1966: 647) after observing the well­developed natatory fringes on specimens of Pachygrapsus marinus from St. Helena I., decided that the similarities in shape of the carapace and chelae were probably not very important, and that the presence of natatory fringes and the shape of the G1 were the most important characters for defining Planes . He thus reinstated Planes marinus , and this generic placement is now widely accepted (see Prado & Melo, 2002). These generic changes, however, serve to highlight the close morphological relationship between Planes and Pachygrapsus species.

Pachygrapsus is also morphologically close to Metopograpsus H. Milne Edwards, 1853 , the two being separated only on the basis of whether the antennae are able to enter the orbital hiatus ( Pachygrapsus ) or are excluded ( Metopograpsus ). This character is nevertheless not always reliable. We agree with Tesch (1918: 78), Tweedie (1949: 466, footnote), and Crosnier (1965: 25) that M. thukuhar ( Owen, 1839) may have a distinct gap between the inner sub­orbital lobes and the outer angles of the front, and that it is thus possible for the antenna to enter the orbit. This has also been reported for M. quadridentatus Stimpson, 1858 ( Tesch, 1918; Tweedie, 1949). This minor difference between the genera led to confusion in the past. Tweedie (1936) described P. quadratus from specimens collected from Singapore, but later realised ( Tweedie, 1949) that he had confused it with M. quadridentatus Stimpson, 1858 . Further work is needed to decide if such a variable orbital character is enough to separate Pachygrapsus Randall and Metopograpsus H. Milne Edwards.