Amamiku, new genus

Candidiopotamon— Sakai 1976: 562 (English text), 347 (Japanese text) (part); Yoshigou 1999: 23 (part).

Type species. Candidiopotamon amamense Minei, 1973, by present designation.

Etymology. The generic name is derived from the name of the goddess “ Amamiku ”, who is credited with creating the Okinawa and Amami groups in Ryukyuan myth. Gender feminine.

Diagnosis. Carapace quadrate, dorsal surface flat, rough, postorbital crista interrupted to epibranchial tooth. Antenna reaching about proximal half of antennal basis when folded backward. Chelae almost symmetrical both males, females. G1 slender, simple, distal end beyond sternal knob in length at natural position; inner margin of terminal segment relatively long, outer margin simply curved outward, distal end with no accessory structure; outer margin of subterminal segment slightly concaved, distal inner margin angulated; synovial membrane elongate, lying distal part of dorsal side of subterminal segment. G2 slender, simple, flagellum short, distal end of flagellum reaching proximal inner margin of terminal segment of G1 at natural position, total length of G2 subequal to total length of G1.

Remarks. Amamiku, new genus, is closest to Candidiopotamon Bott, 1967 by the appearance of the carapace (Fig. 1), but Amamiku can be distinguished from Candidiopotamon by the shapes of their G1 (Fig. 2 A–C). The distinguishing characters of the G1 are the shape of the distal end of the terminal segment (simple, with no accessory structure in Amamiku in contrast to bottle­necked in Candidiopotamon), the shape of the outer margin of the terminal segment (simply curved outward in Amamiku in contrast to inroading into the ventral surface in Candidiopotamon), the length of the inner margin of the terminal segment (long vs. short), and the shape of the outer side of the distal end of the subterminal segment (slightly rounded inward vs. distinctly rounded inward) (this study; Bott 1967; 1970; Dai 1999). Indeed, Bott (1967, 1970) designated the shape of the distal end of the G1 as one of the diagnostic characters of Candidiopotamon, and the difference from Amamiku in this character is distinct.

G1 of Amamiku is similar to that of Ryukyum Ng and Shokita, 1995, but the G1 of Ryukyum differs in having a bulb­like structure on the ventral side of the terminal segment, shorter inner margin of the terminal segment, and the smooth distal inner portion of the subterminal segment (this study; Ng and Shokita 1995).

Geothelphusa Stimpson, 1858 is also close to Amamiku by the simple terminal segment of the G1 and the elongated synovial membrane (Fig. 2 E). Amamiku, however, is clearly differentiated from Geothelphusa by the characters of the G1, viz. relatively long terminal segment with oblique proximal margin (relatively short terminal segment with transverse proximal margin in Geothelphusa), the distinct groove lying from the proximal inner to distal outer margin of the ventral side of the subterminal segment (indisitinct or shallow in Geothelphusa), and strongly tapered in general shape (gently tapered in Geothelphusa). Geothelphusa can be also differentiated from Amamiku by the generally indistinct epibranchial tooth of the carapace and indistinct postorbital crista (Shy, Ng &Yu 1994: 783).