Austruca triangularis (A. Milne-Edwards, 1873)

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 : 488-491

Austruca triangularis (A. Milne-Edwards, 1873)

( Figure 6a–j )

Material examined. 10 ♀, 10 ♂, Kaledupa , Indonesia, 5º 29’ 51’’S, 123º 45’ 21’’E, mud flat in front of mangroves, under houses to the east, coll. L. Michie, 26.08.2014 . 10 ♀, 10 ♂, Kaledupa , Indonesia, 5º 29’ 49’’S, 123º 45’ 20’’E, mud flat in front of mangroves, under houses to the west, coll. L. Michie, 28.08.2014 .

Diagnosis. Carapace: broader than long; smooth; front broad; anterolateral angles strongly acute and produced; anterolateral margins absent; dorsolateral margins proceeding directly from anterolateral angle strongly converging. Major cheliped: dactyl and pollex relatively wide; pollex arched, dactyl also arched, but less so than pollex; pollex and manus have teeth in proximal and distal portion.

Colouration ( Figure 6a–b ). Adult male. Carapace: mainly black, with white transverse markings varying in size and number, anterior ventral surface of carapace and merus of third maxilliped, black. Major cheliped: dactyl and pollex white/off-white with darker hue extending to manus and carpus, manus, carpus, merus and ischium with dark brown speckled pattern. Minor cheliped and ambulatory legs: black or grey.

Adult female. Carapace: similar to males, black with white transverse markings, with greater degree of variability than males, sometimes white and occasionally posteriorly black, frequently with turquoise transverse markings across posterior region. Ambulatory legs: black or grey.

Juvenile male and female. Carapace: usually white, sometimes light brown/russet with pattern and darker colours developing with maturity. Major cheliped: normally light brown at juvenile stage but becomes darker and develops patterning with maturation. Minor cheliped & ambulatory legs: black or grey.

Gonopod morphology ( Figure 6e–h ). G1 curved in dorso-lateral direction; distal end long and narrow with flanges extend throughout length of distal part of shaft; distal edge of flanges rounded; pore of sperm channel large, extending beyond overlapping flanges; protuberance well developed, elongated, ending above base of flanges; distal end curved in lateral direction.

Gastric mill ( Figure 6i, j ). Urocardiac ossicles complex, with 5 transverse ridges of median teeth, different in shape, 1–4 decreasing in size distally, 5 much larger being twice as broad as other ridges; stem region has 5 pairs of cusps, decreasing in size distally, fifth is much shorter and attached to the plate dorsally; lateral tooth plate with 20 comb-shaped teeth.

Biology. Austruca triangularis is a small-size species (largest male CL = 8.2 mm, CB = 12.1 mm, largest female CL = 8.2 mm, CB = 12.8 mm); one of least abundant species on shore, present in high intertidal zone under stilted houses.

Distribution. Australia, Indonesia, Malaysia, Philippines, Taiwan, China, Papua New Guinea.

Remarks. The specimens examined comply with the description of Shih et al. (2019) specifically with the elongated protuberance of the G1 and the complex morphology of the gastric mill, with the highest number of transverse ridges and cusps of all Austruca species at Ambeua. Austruca triangularis can be distinguished from the other Austruca species by the strongly acute anterolateral angles, the elongated protuberance of the G1, the distinct speckled pattern of the major cheliped and the small size.

Figures

FIGURE 6. Austruca triangularis (A. Milne-Edwards, 1873): 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.