Typhlocarcinops arcuatus (Miers, 1884)

Ng, Peter K. L. & Rahayu, Dwi Listyo, 2020, A synopsis of Typhlocarcinops Rathbun, 1909 (Crustacea: Decapoda: Brachyura: Pilumnidae), with descriptions of nine new species from the Indo-West Pacific, Zootaxa 4788 (1), pp. 1-100 : 92-94

Typhlocarcinops arcuatus (Miers, 1884)

( Figs. 78–80)

Ceratoplax arcuata Miers, 1884: 243 , pl. 25B.

Typhlocarcinops decrescens — Tesch 1918: 213, pl. 13 fig. 4 (not Typhlocarcinops decrescens Rathbun, 1914 ). Typhlocarcinops arcuata — Ng 1987: 78, 88.

Typhlocarcinops arcuatus — Ng et al. 2008: 144.

Material examined. Holotype: male (6.5 × 6.0 mm) ( NNM 1882.7 ), Port Darwin , Australia, coll. H.M.S. Alert, 1882.

Diagnosis. Carapace ( Figs. 78A, B, 80A) slightly broader than long, width to length ratio 1.1, surface covered with short pubescence; anterolateral margin gently arcuate, with 3 low notches forming 4 broad lobes. Front ( Fig. 78A, B) slightly deflexed with shallow but distinct median cleft; margins distinctly convex. Orbit ( Figs. 78B, C, 80A) shallow, ocular peduncles filling orbit, immovable, cornea small. Third maxilliped ( Fig. 80B) broad, merus transverse, anteroexternal angle prominent, protruding, auriculiform. Cheliped ( Fig. 79 A–D) moderately large, sur- face covered with small granules to smooth, palm slightly longer than broad, outer surface of major chela mostly smooth, that of minor chela covered with small granules; fingers gaping when closed; cutting edges of fingers lined with sharp teeth and denticles; inner angle of carpus with distinct tooth. P2‒P5 ( Figs. 78A, 79 E–G) slender; P2–P4 dactylus slender, gently curved; P5 dactylus straight. Male pleon ( Fig. 80D) narrow, telson short, 1.4 times as long as somite 6, triangular with rounded distal margin. G1 ( Fig. 80E, F) slender, sinuous, upper and lower halves subequal, distal part slender, curved, tapering to pointed tip.

Remarks. Miers (1884) described Ceratoplax arcuata from one small 6.6 × 6.0 mm male collected from northern Australia and Miers (1886: 235) subsequently recorded it from Papua New Guinea. Ng (1987) noted that Miers described the anteroexternal angle of the merus of the third maxilliped as “prominent” (Miers 1884: 244) and the figure of the male pleon ( Fig. 78D) shows a narrow and extremely wide somite 1; and as such, commented that the species should be transferred to Typhlocarcinops instead. The examination of the type specimen confirms this generic assignment.

With regards to the more subovate carapace and strongly auriculiform anteroexternal angle of the third maxilliped, T. arcuatus is probably most closely related to T. angustipes . The two species, however, differ markedly in other characters. In T. arcuatus , the anterolateral margin has four low lobes ( Fig. 80A) (versus only one low lobe in T. angustipes ; Fig. 77A); the ambulatory legs are relatively longer and more slender ( Figs. 78A, 79 E–G) (versus shorter and stouter in T. angustipes ; Fig. 76A, B); the male pleon is proportionately transversely more narrow ( Fig. 80D) (versus proportionately broader in T. angustipes ; Fig. 77C); and the G1 is distinctly more sinuous ( Fig. 80E, F) (versus relatively stouter in T. angustipes ; Fig. 77D, E).

As discussed above, Tesch’s (1918: 213, pl. 13 fig. 4) records of “ T. decrescens ” from Talaut Island in the Moluccas and Roma Island north of Timor should probably be referred to T. arcuata instead.

With regards to the female specimen of Miers (1886) (1 female (11.5 × 9.3 mm) (NHM), station 188, south of New Guinea, 9º59’0”S 139°42’0”E, 28 fathoms, coll. H.M.S. Challenger) ( Fig. 81), he noted that “This example is of much larger size than the small male described in the Report cited; the whole animal is more pubescent and the carapace proportionately broader; the subdistal tooth or prominence on the upper margin of the merus of the chelipedes, which is obscurely indicated in the original type, is more distinctly developed.” (Miers 1886: 235). Although a female, examination of this specimen shows that it is not a species of Typhlocarcinops . The female thoracic sternum is relatively narrow transversely and pleonal somite 1, while almost reaching the coxae of the last ambulatory legs, is not extremely wide ( Fig. 81 B–D). This is more akin to the condition observed in Heteropilumnus species (cf. Fig. 4C, D). The general appearance of the carapace, third maxillipeds and chelipeds ( Figs. 81, 82A, E–H) are also more similar to species of Heteropilumnus (cf. Ng et al. 2018) ( Fig. 82).

Type locality. Port Darwin , Australia, 12 fathoms .

Distribution. Known only from type locality.