Gelasimus tetragonon (Herbst, 1790)

Michie, Laura A., Barnes, R. S. K., Clark, Paul F., Bennett, Wayne A. & Cragg, Simon M., 2021, Distinguishing ten sympatric species of fiddler crab (Decapoda: Ocypodidae) using a suite of phenotypic characteristics, Zootaxa 5026 (4), pp. 480-506 : 493

Gelasimus tetragonon (Herbst, 1790)

( Figure 8a–j )

Material examined. 10 ♀, 10 ♂, Kaledupa , Indonesia, 5º 29’ 49’’S, 123º 45’ 21’’E, mud flat southwest of boat jetty, coll. L. Michie, 27.08.2014 . 10 ♀, 10 ♂, Kaledupa , Indonesia, 5º 29’ 50’’S, 123º 45’ 21’’E, mud flat between houses and boat jetty, coll. L. Michie, 28.08.2014 .

Diagnosis. Carapace: broader than long; smooth; front narrow; anterolateral angles acute, slightly produced; anterolateral margins short, converging from angles; dorsolateral margins short, sometimes absent; carapace lateral margins slightly converging, practically straight posteriorly. Major cheliped: long, slender dactyl and pollex; pollex almost straight; dactyl moderately arched; dactyl and pollex with one or more sub-distal teeth.

Colouration ( Figure 8a–d ). Adult male. Carapace: blue with black mottled pattern varying in size and shape, anterior ventral surface of carapace generally black, dark brown or navy blue and ventral surface of third maxillipeds normally same blue as carapace, anterolateral angles can be pale straw coloured, occasionally pale anterolateral areas can be extensive. Major cheliped: orange, with dactyl often possessing a lighter tip, proximally pollex with darker hue of orange which extends onto part of manus, proximally manus with dark brown mottled pattern extending to carpus and merus. Minor cheliped & ambulatory legs: black or dark brown, 4 th pair of legs often with black and blue mottled pattern.

Adult female. Carapace: similar to male, often with a darker hue of blue. Chelae & ambulatory legs: orange, often have blue dactyl and pollex.

Juvenile male. Carapace: mottled black and white pattern with colours developing with maturity. Major cheliped: similar to adults but with white distal end to dactyl and pollex. Minor cheliped: blue dactyl, pollex and manus. Ambulatory legs: black.

Juvenile female: same colour pattern as juvenile males.

Gonopod morphology ( Figure 8e–h ). G1 strongly curved in dorso-lateral direction; distal end slightly curved laterally; anterior flange slightly narrower than posterior; pore of sperm channel large; distal protuberance moderately large, not reaching base of flanges.

Gastric mill ( Figure 8i, j ). Urocardiac ossicles moderately complex, with 4 transverse ridges of median teeth, decreasing in size distally, different in shape, first two arched, 3 and 4 straighter, fourth large and attached to plate dorsally; stem with setae present on lateral margins of posterior stem region. Lateral tooth plate with 19 combshaped teeth.

Biology. Gelasimus tetragonon is a medium-size species (largest male CL = 16.8 mm, CB = 23.1 mm, largest female CL = 9.4 mm, CB = 14.3 mm); present in mid and low intertidal zones, often associated with presence of coral rock where crabs were seen feeding on small patches of algae on rock surfaces.

Distribution. South Africa to Iran, Madagascar, Thailand, Malaysia, Australia, Indonesia, Philippines, Papua New Guinea, Taiwan, Micronesia, Melanesia.

Remarks. Gelasimus tetragonon is distinguished by the strong curve of the G1, the long, slender dactyl and pollex and the distinct blue carapace and orange major cheliped. The specimens examined fit the original description and the moderately complex gastric mill conforms to the description by Shih (2015) and is similar to that of G. jocelynae .

Figures

FIGURE 8. Gelasimus tetragonon (Herbst, 1790): a–d Sulawesi Tenggara, Indonesia. Adult male colouration; a, large chela; b, carapace dorsal view. Adult female colouration; c, frontal view; d, carapace dorsal view. Photos credit L. Michie. Line drawing of left G1; e, lateral view; f, mesial view; CLSM images of apical part of G1; g, lateral view; h, mesial view; SEM images of male gastric mill; i, median tooth plate, ventral view; j, left lateral tooth plate, mesial view.