Amakusaxius Sakai, 2011

Komai, Tomoyuki, Chan, Tin-Yam & Shy, Jhy-Yun, 2013, A redescription of a rare axiid shrimp, Amakusaxius amakusanus (Miyake & Sakai, 1967) (Crustacea: Decapoda: Axiidea) from new material collected off Taiwan, Zootaxa 3669 (4), pp. 490-502 : 491-492

Genus Amakusaxius Sakai, 2011

Acanthaxius Sakai & de Saint Laurent, 1989: 66 (part). Amakusaxius Sakai, 2011: 44 .

Type species. Calocaris (Calastacus) amakusana Miyake & Sakai, 1967 . Original designation.

Diagnosis. Rostrum somewhat flattened dorsoventrally, triangular, laterally denticulate, shorter than eyestalks, not depressed below level of carapace, not continuous with lateral carinae. Carapace dorsally granulate; cervical groove deep, visible laterally over half distance to anterolateral margin; supraocular spines prominent; lateral carina divided in two parts, each terminating anteriorly in spine; submedian carina present, spinose; median carina tuberculate or denticulate; low, blunt postcervical carina present. First abdominal pleuron blunt or subacute; second pleuron broad, rounded; third to fifth pleura posteriorly rounded. Telson with lateral fixed spines and posterolateral movable spinules; posterior margin slightly convex, with small median spine. Eyestalk cylindrical, articulating; cornea darkly pigmented. Antennal scaphocerite short, curved; dorsolateral distal spine on second segment of antennal peduncle curved inward. Third maxilliped exopod not clearly bent at base of flagellum. Pleurobranchs absent; podobranchs and arthrobranchs well developed, but former without lamella; epipods present on second maxilliped to fourth pereiopod. Chelipeds asymmetrical, with palm somewhat compressed laterally; merus unarmed on ventral margin in major cheliped, armed with row of spines in minor cheliped; dorsal margin of carpus to dactylus of major cheliped only with small spines, that of minor cheliped with prominent spines. Propodi of third and fourth pereiopods with transverse rows of corneous spinules; dactyli tapering, with longitudinal row of corneous spinules. First pleopod of male absent. Male second pleopod with appendix masculina; second to fifth pleopods each with appendix interna. Uropodal exopod and endopod narrow, with transverse suture on exopod; exopod with middorsal carina terminating posteriorly in strong marginal spine, terminal flap located lateral to this spine.

Composition. Monotypic.

Remarks. Sakai & de Saint Laurent (1989) assigned the following eight species to their new genus Acanthaxius : Axius spinulicaudus Rathbun, 1902 ; Axius spinosissimus Rathbun, 1906 ; Axius miyazakiensis ; Calocaris (Calastacus) amakusana ; Calocaris (Calastacus) hirsutimana Boesch & Smalley, 1972 ; Axiopsis (Axiopsis) polyacantha ; Axiopsis (Axiopsis) caespitosa Squires, 1979 ; and Axiopsis (Axiopsis) pilocheira (type species). Of the eight species, A. spinulicaudus was reassigned to Calocarides by Kensley (1996a). Since the original description, the following nine species have been added to Acanthaxius : A. polychaetes Sakai, 1994 ; A. kirkmilleri Kensley, 1996 b; A. formosa ; A. grandis ; A. clevai Ngoc-Ho, 2006; A. gadaletae Ngoc-Ho, 2006; A. garawa Poore & Collins, 2009 ; A. gathaagudu ; A. ningaloo Poore & Collins, 2009 (Sakai 1994; Kensley 1996b; Kensley & Chan 1998; Ngoc-Ho 2006; Poore & Collins 2009).

Recently, Sakai (2011) reviewed Acanthaxius , and restricted the following seven species to Acanthaxius : A. clevai , A. gadaletae , A. garawa ; A. miyazakiensis ( A. formosa and A. polyacanthus were synonymized with A. miyazakiensis ), A. ningaloo , A. pilocheirus ( A. grandis and A. gathaagudu were synonymized with A. pilocheirus ), and A. spinosissimus . Acanthaxius amakusana was reassigned to the new genus Amakusaxius ; A. kirkmilleri was transferred to the new genus Pillsburyaxius Sakai, 2011 ; and A. polychaetes was transferred to the new genus Bruceaxius Sakai, 2011 .

Sakai (2011) distinguished Amakusaxius from Acanthaxius only by the presence of a postcervical median carina on the carapace in the former. The significance of the presence or absence of the postcervical carina alone is questionable, because interspecific variation is known in Ambiaxius (see Sakai 1995, Sakai & Ohta 2005, Komai et al. 2010). The present newly collected specimens enable the examination of more characters of possible diagnostic significance. Indeed, Calocaris (Calastacus) amakusana is different from other species currently assigned to Acanthaxius in the following characters. Ventral spines of the merus and dorsal spines of the palm of the major cheliped are fairly reduced in the size in C. (C.) amakusana , whereas they are quite prominent in species of Acanthaxius . The uropodal rami are relatively narrower in C. (C.) amakusana compared to species of Acanthaxius , because of the weakly developed mesial convexity in the former species. Furthermore, the mid-dorsal carina on the uropodal exopod is produced into a strong spine posteriorly in C. (C.) amakusana , whereas such a spine is not seen in species of Acanthaxius . Moreover, similarities to certain species of Acanthaxius are also seen. The triangular, somewhat flattened rostrum links C. (C.) amakusana with A. garawa (cf. Poore & Collins 2009) and A. spinosissimus (cf. Ngoc-Ho 2005). The lateral gastric carina on the carapace is interrupted from the rostral lateral margin in C. (C.) amakusana , and similar condition is seen in A. grandis , A. gathaagudu and A. pilocheirus (cf. Sakai 1987; Kensley & Chan 1998; Poore & Collins 2009). In this present study, the validity of Amakusaxius is tentatively accepted because of the existence of some unique features that differentiate the type species from Acanthaxius , but future phylogenetic studies may eventually reveal that this genus could be a synonym of Acanthaxius .