Kabutos, Komatsu, Hironori & Ng, Peter K. L., 2011

Komatsu, Hironori & Ng, Peter K. L., 2011, Kabutos, a new genus for the western Pacific leucosiid crab, Merocryptus durandi Serène, 1955 (Crustacea: Decapoda: Brachyura), Zootaxa 2881, pp. 31-38 : 32-33

Kabutos n. gen.

Diagnosis. Carapace distinctly broader than long, dorsal surface with distinct median, epibranchial ridges; front produced; mesogastric region longitudinally positioned, broadly convex; gastro-cardiac region raised, with 2 gastric tubercles, single cardiac tubercle; intestinal region produced; branchial region expanded laterally, gently sloping anterolaterally from ridge. Basal segment of antennule not occupying entire fossa when closed. Basal segment of antenna lodged in orbital hiatus. Maxillule with tongue-shaped endopod. First, second maxillipeds with exopodal flagellum. Third maxilliped exopod with triangular tooth at proximal end of lateral margin. Chelipeds relatively slender, not elongate. Ambulatory legs relatively short. Thoracic sternum covered with numerous rounded granules; sutures between s3/4, s4/5, s5/6, s6/7, s7/8 interrupted medially; medial suture absent. Male abdomen with first, second somites free, third to fifth somites completely fused, sixth somite, telson free. Female abdomen with second, third somites free, fourth to sixth somites fused, telson free. G1 relatively slender, straight, compressed. G2 short, about 0.3 times as long as G1.

Type species. Merocryptus durandi Serène, 1955 , by monotypy.

Etymology. The name of the genus is derived from kabuto, the Japanese word for a Samurai helmet, alluding to the form of the carapace. Gender masculine.

Remarks. Serène (1955) discussed the characters and composition of Merocryptus at length. He commented that while there existed substantial variation in the form of the antero- and posterolateral margins and the structure of the carapace regions in the four species (including M. durandi ), they are nevertheless related and the differences are only in degree. The main generic characters according to Serène (1955) are the adjoined base of the orbit to the elongated antennular peduncle; the 5-lobed lateral margins of carapace; the well developed pterygostomial region forming a distinct tooth or lobe; and the swollen or raised branchial regions forming a prominent lateral lobe or projection on the carapace. He noted that the diagnosis for the genus by A. Milne-Edwards (1873), later modified by Miers (1886), was generally effective, but with the description of M. durandi , the posterior carapace margin may be entire (not with two teeth or projections) and the anterior distal margin of the endostome may be emarginated (not entire).

Comparison of the present material with the type species, M. lambriformis , however, revealed major differences between the two species, indicating they are not congeneric. In M. durandi , the basal segment of the antenna fills the orbital hiatus (leaves a gape in the orbital hiatus in M. lambriformis ), the exopod of the third maxilliped has a well developed tooth on the lateral margin (absent in M. lambriformis ), and the arrangement of the fused male and female abdominal somites are quite different (Table 1). These characters, together with the different carapace features, argue for the establishment of a separate genus for M. durandi Serène, 1955 . On the basis of the type descriptions of the Atlantic M. boletifer and M. obsoletus (see also Guinot & Ribeiro 1962; Manning & Holthuis 1981), their key generic diagnostic characters are the same as those of M. lambriformis . Forest & Guinot (1966) had suggested that the Atlantic genus Atlantotlos Doflein, 1904 (type and only species A. rhombifer Doflein, 1904 ) was synonymous with Merocryptus but Manning & Holthuis (1981) argued that there were several notable differences, especially in the form of the G1, that indicated both were distinct genera. These differences are also valid for Kabutos n. gen.

Table 1. Comparison of Kabutos n. gen. with its allied genera. Abbreviations: R = fused somites; T = telson.

Kabutos n. gen. Merocryptus Merocryptoides Oreotlos

Orbital hiatus filled with basal segment of leaving a gape filled with basal filled with basal

antenna segment of antenna segment of antenna Second maxilliped exopod with flagellum with flagellum without flagellum with flagellum Third maxilliped exopod with triangular tooth without tooth with triangular tooth without tooth Chela normal normal normal blade-like Male abdomen 1+2+R+6+T 2+R+T 1+2+R+T 1+2+R+6+T Female abdomen 2+3+R+T 1+2+R+T 2+R+T 2+3+R+T References Serène, 1955; present study Serène, 1955; Komatsu & Takeda, Tan & Ng, 1995

present study 2001

Kabutos n. gen. shows intermediate morphological features between Merocryptoides Sakai, 1963 , and Oreotlos Ihle, 1918 , but can be differentiated from them by the structure of their second and third maxillipeds and chelae as well as the degree of fusion of male and female abdominal somites (see Tan & Ng 1995; Komatsu & Takeda 2001; Ng et al. 2009) (Table 1).

Comparative material. Merocryptus lambriformis A. Milne-Edwards, 1873: 1 male (10.0 × 12.0 mm), 1 ovigerous female (12.3 × 15.0 mm), NSMT-Cr 13196, Tosa Bay, Japan, 153–155 m, coll. RV Kotaka Maru, 21 January 1998. Merocryptoides frontalis Sakai, 1963: 2 males (3.5 × 3.7 mm, 4.8 × 4.4 mm), 1 ovigerous female (5.3 × 6.6 mm), NSMT-Cr 10406, off Kisami, Shimoda, Japan, 18–45 m, coll. K. Nakamura, 23 August 1982. Oreotlos angulatus ( Rathbun, 1906) : 1 ovigerous female (10.7 × 16.5 mm), NSMT-Cr S 928, Chichi-jima I., Ogasawara Is., Japan, 27°04.55ʹN 142°09.16ʹE – 27°04.73ʹN 142°09.31ʹE, 83– 81 m, coll. RV Koyo, 28 October 2008. Oreotlos heuretos Tan & Ng, 1995: 1 male (4.1 × 6.0 mm), 1 ovigerous female (7.5 × 12.3 mm), NSMT-Cr S 931, northwest of Ototo-jima I., Ogasawara Is., Japan, 27°13.09ʹN 142°09.19ʹE – 27°13.19ʹN 142°09.23ʹE, 135.8– 135.5 m, dredge, coll. RV Koyo, 15 July 2009.