Scallasis Bate, 1888

Komai, Tomoyuki, Poore, Gary C. B. & Fujita, Yoshihisa, 2020, Redescription of the poorly known ghost shrimp species, Scallasis amboinae Bate, 1888, review of the genus, and description of a new species from the Ryukyu Islands, Japan (Decapoda: Axiidea: Callianassidae), Zootaxa 4766 (3), pp. 401-420 : 402

Genus Scallasis Bate, 1888

Scallasis Bate, 1888: 34 .— Manning & Felder 1991: 780.— Poore et al. 2019: 99.

Callianassa (Scallasis) .— Borradaile 1903: 547–548 (partim).

Cheramus . — Sakai 2011a: 363–365 (partim).

Remarks. The genus was rediagnosed by Poore et al. (2019: 99) after thorough molecular and morphological analysis ( Robles et al. 2020). The genus is recognised by the combination of a narrow rostrum, eyestalks with acute apices, a narrow maxilliped 3 ischium-merus, a simple or bifid spine on the merus of the major cheliped, and strong spiniform setae on the face of the uropodal endopod and on the face of the telson.

As noted above, Poore et al. (2019: tab. 1, 2) included seven species, all from the Indo-West Pacific: S. amboinae , S. andamaniensis , S. caledonica , S. contipes , S. pygmaea , S. spinophthalma and S. tonkinae . Some, however, have been synonymised with others and later removed from synonymy (see S. amboinae , S. caledonica and S. tonkinae below). In his major axiidean review, Sakai (2011a) regarded the four specific taxa, Scallasis amboinae , Callianassa caledonica , Callianassa (Cheramus) pygmaea and Callianassa (Scallasis) tonkinae as valid species of Trypaea Dana, 1852 .

Characterisation of these species is limited without reference to type material – we have relied for most on published works but believe many descriptions and figures omit critical characters not usually noted in callianassid descriptions in the past and which have been often overlooked. For example, the facial spiniform setae on the uropodal rami were unexpected or considered unimportant by earlier authors and are difficult to observe without appropriate lighting or staining. The scaphocerite is acute and short in species we have observed but not mentioned in most descriptions. There appear few reliable differences between species in the rostrum (triangular, sharp, reaching the cornea) and eyestalks (tapering to acute anterior lobes) but we see species differences in maxilliped 3 and in the chelipeds. Adding to the difficulty differentiating species is the paucity of individuals, and differences between sexes, growth stages and individuals. Nevertheless, we have prepared short diagnoses of the seven species of Scallasis recognised here.

Scallasis caledonica , S. inermis n. sp. and S. spinophthalma are known from both sexes. Pleopod 1 is present in males of all three but variously developed, possibly due to the maturity of the specimens examined. Pleopod 2 is present in two species, not in S. caledonica . Chelipeds are apparently sexually dimorphic only in S. inermis n. sp. but this species may be the only one where a fully adult male is known.