Uca (Petruca)

Uca (Petruca) View in CoL subgen. nov.

Minuca Bott, 1954: 160 View in CoL (part); Bott 1973: 323 (part); Crane 1975: 154 (part)

Leptuca Bott, 1973: 324 View in CoL (part); Beinlich & von Hagen 2006: 26 (part); Ng et al. 2008: 241 (part). Celuca Crane, 1975: 211 View in CoL (part); Rosenberg 2001: 848, 852 (part).

Type species. Gelasimus panamensis Stimpson, 1859 , by present designation.

Etymology. From the Greek petra for “rock or stone”, for the stony habitat ( Fig. 8 View FIGURE 8 C) of the type species, in arbitrary combination with the genus name Uca . Gender feminine.

Diagnosis. Carapace flat, widest distance of carapace between anterolateral angles, angles acute, produced obliquely outwards, anteriorly; anterolateral margins relatively long ( Fig. 1 View FIGURE 1 A); boss (raised tuberculate ridge) on carapace behind dorsolateral margin ( Fig. 1 View FIGURE 1 B); front wide, about 1/3 carapace width ( Fig. 2 View FIGURE 2 C, D); suborbital crenulations strong, each truncate, separate, increasing in size towards antero-external angle ( Figs. 1 View FIGURE 1 C, 2A–C); orbital floor without mound or pile, but with tubercle at inner corner adjacent to antennule ( Fig. 2 View FIGURE 2 A–C). Male with pleonal clasping apparatus in abdominal cavity. Urocardiac ossicles of gastric mill ( Fig. 5 View FIGURE 5 ) simple, median tooth with 3 pairs of similar transverse ridges, separated by gaps, not reaching central ridge on posterior tooth plate; first pair of ridges strongest, next 2 lower, with 2 or 3 weak pairs of cusps on stem region; stem region with long, wide median part ( Fig. 5 View FIGURE 5 A). Major pollex, dactylus with tips blunt, without or with weak tooth on cutting margins ( Figs. 3 View FIGURE 3 A, B, 8A, B). Adult male major manus with outer surface smooth, proximal outer part disproportionately thick, with posterior extension beyond distal part of carpus ( Figs. 3 View FIGURE 3 A, 8A); inner surface with vestigial oblique ridge, without or with low, rounded tubercles ( Fig. 3 View FIGURE 3 B). Adult minor cheliped with manus rounded, broad almost as long; gape wide; fingers with numerous long stiff setae on tip ( Figs. 3 View FIGURE 3 C, D, 8B). G1 ( Fig. 4 View FIGURE 4 ) with anterior flange (before genital pore) broader than posterior one (after genital pore); inner process large, broad, becoming thicker distally, bent forwards at right angles, extending beyond tip of posterior flange; thumb slender, ending below base of flange.

Remarks. One of the diagnostic characters of Petruca subgen. nov. is that the carapace is widest between the tips of the anterolateral angles ( Fig. 1 View FIGURE 1 A). In this feature, it is similar to, but more pronounced than in any Leptuca species. The maximum carapace width in Minuca species is usually behind the anterolateral angles ( Crane 1975).

Beinlich & von Hagen (2006) suggested that the function of the orbital armature at the inner corner of the orbital floor is to secure the eyestalk in its depressed position. This character of U. panamensis ( Fig. 2 View FIGURE 2 B; Beinlich & von Hagen 2006: fig. 7j) is considered to be ancestral because it is only present in the subgenera Uca ( Crane 1975: figs. 26E, 31D, 32J–K; Beinlich & von Hagen 2006: fig. 7d) and Afruca ( Crane 1975: fig. 27D–F; Beinlich & von Hagen 2006: fig. 7i), and is also seen in Ocypode ( Ocypodidae : Ocypodinae ; Orchard 2012: 232–251; Wong et al. 2012: fig. 6b) and Heloecius ( Heloeciidae ; Beinlich & von Hagen 2006: fig. 7c). Other fiddler crabs do not have this character, or have instead evolved other structures that may secure the eyestalks (such as broadening of the margins of the orbits, and one or more rows of tubercles, partly on raised “mounds” behind the lower orbital border) (see Beinlich & von Hagen 2006). The supposedly more plesiomorphic orbital characters of U. panamensis supports our hypothesis that Petruca subgen. nov. separated earlier from the other two American broad-fronted subgenera, Minuca and Leptuca ( Fig. 9 View FIGURE 9. A , see below).

The posterior extension of the major manus ( Fig. 1 View FIGURE 1 A, C) is a unique character of the new subgenus, as previously noted by Crane (1975). The posteriorly extended rounded protuberance prevents a full lateral extension of the manus and Crane (1975: 158) suggested that males strike the ground with this part of their claw when they court producing a seismic signal. Subsequent observations (J. Christy & U. Schober unpublished), however, revealed that the claw is held well above the substrate during the entire wave. Crane also seldom saw courtship waving. Indeed, courtship waving by this species on a cobble beach near the Pacific entrance of the Panamá Canal occurs during a relatively short interval of approximately an hour beginning late in the tidal cycle well after low tide and after the crabs have fed, and on only a few days each semi-lunar breeding cycle. Males, which typically are very dark in colour at this location, lighten on the carapace to a khaki or cream colour when they court ( Fig. 8 View FIGURE 8 B), typically from the highest point on a stone, one male per perch. Males that court from elevated positions may be seen more easily by prospective mates but also by their predators including great-tailed grackles [ Quiscalus mexicanus (Gmelin, 1788) ] and the furtive but quick grapsid crab Geograpsus lividus (H. Milne Edwards, 1837) . When disturbed, courting males drop from their perch landing at the seam between the stone and the sand or shell hash matrix where the openings to their burrows are located.

The significance of the smooth inner and outer surfaces of the major chela ( Fig. 3 View FIGURE 3 B) is unknown. This feature is also present in species inhabiting sandy or muddy habitats, e.g., U. (Cranuca) inversa (Hoffmann, 1874) , U. (Austruca) sindensis (Alcock, 1900) , U. (Minuca) argillicola Crane, 1941 , U. (M.) zacae Crane, 1941 , U. (Leptuca) latimanus (Rathbun, 1894) , U. (L.) panacea Novak & Salmon, 1974 , U. (L.) pugilator (Bosc, 1802) , U. (L.) subcylindrica ( Stimpson, 1859) , and U. (L.) tenuipedis Crane, 1941 (see also Rosenberg 2001). Christy & Wada (2015) noted that the claws of male U. pugilator often slip when males fight and that the relatively dry sand can be difficult for a challenger to grip. They suggested a smooth claw may allow males in their role as challengers to persist in a fight at the entrance to a burrow without being thrown by the defender, which has the positional advantage and is able to grip the burrow shaft. Similarly, a defending male U. panamensis in a burrow under stones may have a decided positional advantage over challengers attempting to grip sand or shell hash. Too little is known about possible positional asymmetries during combat in the other species to know whether selection for proficiency in combat when males are challengers may explain why they too have smooth claws.

The brush of long stiff setae on the finger’s tips of minor cheliped ( Fig. 3 View FIGURE 3 C, D) is a very unusual character not present in other fiddler crab taxa; and is apparently an adaptation for scraping algae from the hard rock surface ( Crane 1941, 1975; Takeda & Murai 2003).

The urocardiac ossicles of gastric mill in Petruca subgen. nov. are of the simple form, i.e., with fewer transverse ridges of median teeth, especially those on the posterior tooth plate ( Fig. 5 View FIGURE 5 ; Shih 2015). Another character is the swollen median part of the stem region of the gastric mill in this subgenus ( Fig. 5 View FIGURE 5 A), which is present in the subgenera Xeruca, Afruca , and Uca (Shih 2015: fig. 8A, B; Franklin Barnwell, personal communication), but not yet observed in Minuca ( Fig. 6 View FIGURE 6 ) and Leptuca (except in U. panacea , Fig. 7 View FIGURE 7 ). Although the median part of the stem region of U. panacea is swollen, it is not wider than the median teeth in Petruca subgen. nov., and the median teeth on the posterior tooth plate also are more complex in structure (with 5 transverse ridges) ( Fig. 7 View FIGURE 7 E).

The urocardiac ossicles of Minuca show a more consistent pattern than those of Leptuca ( Figs. 6 View FIGURE 6 , 7 View FIGURE 7 ). In U. burgersi , U. minax , U. mordax , U. pugnax , U. rapax , U. victoriana , U. cf. virens , and U. vocator of Minuca ( Fig. 6 View FIGURE 6 ), there are 6–9 transverse ridges on the median teeth of the posterior tooth plate and 2–4 weak pairs of cusps on the stem region; except for U. brevifrons , which has only four ridges on the posterior tooth plate and a pair of cusps on the stem region. Species of Leptuca show a more diverse pattern of the urocardiac ossicles ( Fig. 7 View FIGURE 7 ), with 3–8 transverse ridges on the posterior tooth plate and 1–10 pairs of cusps on the stem region, e.g., 3 and 2 (= three ridges on posterior tooth plate and two pairs of cusps on stem region, hereinafter the same) of U. subcylindrica ; 4 and 1 of U. terpsichores ; 4 and 2 of U. uruguayensis ; 5 and 1 of U. spinicarpa ; 5 and 2 of U. panacea ; 7 and 10 of U. pugilator ; 8 and 3 of U. speciosa ; 8 and 4 of U. umbratila ; and 8 and 6 of U. thayeri .

Crane (1975) treated U. panamensis as a member of Minuca , in part because of the similarity of their G1 structures, especially with U. vocator , U. ecuadoriensis , and U. pygmaea ( Crane 1975: fig. 66). The G1 of U.

panamensis is still unusual, with the large inner process bent at right angles to cover the flanges ( Fig. 4 View FIGURE 4 B), a character not seen in any Minuca or Leptuca species (see Crane 1975: figs. 66–71).