A new species of the genus Chiromantes Gistel, 1848 s. str. is described from the Ryukyu Islands. The new species is distinguished from another species of Chiromantes s. str., C. haematocheir (De Haan, 1833), by having proportionately longer and less setose ambulatory legs, a relatively flatter carapace, larger eyes, proportionally narrower front, relatively broader male sixth abdominal segment, less inflated male chela, and distinct colouration. The present study also regards Holometopus serenei as a junior subjective synonym of C. haematocheir.

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While records of seasonal abundance patterns are available for gnathiids (Crustacea, Isopoda, Gnathiidae) in temperate zones and cold water populations, none exist for tropical and subtropical populations. We found the gnathiid isopod Elaphognathia aff. cornigera inhabiting sponges on a subtropical tidal flat in the Haneji Island Sea, Ryukyu Archipelago, southwestern Japan. To examine seasonal abundance patterns of this species, we conducted monthly sampling from December 2004 to November 2006 of the demosponge Haliclona sp. in which the gnathiids dwelled. All of the adult males, adult females in various reproductive phases, and larvae of Elaphognathia aff. cornigera were found throughout most of the year, suggesting that the species reproduces year-round. However, gnathiid density showed a remarkable seasonal pattern, with the maximum abundance occurring in spring and the minimum abundance in summer. These results were quite different from temperate species, in which female development and reproduction are restricted to warmer months and the highest densities are often recorded in the summer. Warm subtropical winters would likely not disturb gnathiid growth and reproduction, but extreme environmental conditions, including summer high water temperatures and typhoons, may affect gnathiid density.

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Vertical distribution and reproductive biology of the hermit crab Pagurus nigrofascia were researched from March 2003 to February 2005 along an intertidal boulder shore in Fukuoka, a warm temperate region of Japan. In the spring, P. nigrofascia was widely distributed throughout the intertidal area but moved to the upper intertidal area in the summer and to the middle intertidal area in the autumn. In the winter, most crabs were found in the lower intertidal area, as many had moved into the subtidal area. Most females produced a brood in March and released larvae in December after an extremely long incubation period of 9 months. From March to October, the embryos were in diapause, and the eggs were completely filled with yolk. In November, all embryos began to develop, and most broods hatched in December. New recruits appeared in March and April and grew slowly. These reproductive characteristics of P. nigrofascia were similar to those observed in a previous study in Hakodate, a cold temperate region of Japan. However, in Fukuoka, brood production and larval release occurred approximately 2 months earlier and females exhibited higher fecundity than in Hakodate.

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A re-description of a little-known Asian mysid Gangemysis assimilis (Tattersall, 1908), which has previously been recorded from estuarine and fresh waters of eastern India and Bangladesh, was made, and, subsequently, the monotypic genus Gangemysis Derzhavin, 1924 was redefined. This study also provided a new locality record extending its geographical range south to Peninsular Malaysia.

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Four species of squat lobsters (Galathea amboinensis de Man, 1888, G. continua new species, G. inflata Potts, 1915, and G. leptocheir new species), all associates of comatulid crinoids, are described from material collected on coral reefs of the Ryukyu Islands. Galathea continua is found in association with Capillaster multiradiatus, Clarkcomanthus littoralis, Comatella maculata, C. nigra, C. stelligera, and Comanthus gisleni. It is distinguished from the other three species by the absence of a spine on the hepatic region lateral to the first transverse ridge of the carapace, the anterior margin of sternite 3 submedially produced into two spines instead of roughly bilobed, and epipods present on pereopod 1 only instead of pereopods 1-3. Galathea leptocheir associated with Phanogenia gracilis is distinguished from G. amboinensis by much more slender pereopod 1, the carapace with four uninterrupted and three interrupted transverse ridges placed alternately instead of one interrupted and six uninterrupted ridges on the posterior half surface, and abdominal somites 2-4 with two uninterrupted instead of three uninterrupted transverse ridges. Coloration is also different in each of these species. A key to the four species is provided.

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Ilyoplax pusilla has been known to exhibit a unique mudusing territorial behavior termed barricade building. Most of the barricade builders are large males that can pair with females, which suggests that the barricade is built by males to aid courtship. We conducted this study to clarify whether the barricade building improves mating success. Barricade abundance and waving frequency was highest in June, corresponding to the peak of male courtship behavior. Males that paired with females built barricades significantly more frequently than did neighboring males that did not pair with females. These results suggest that barricading in I. pusilla enhances in pair formation.

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Allocation of energy between reproduction and growth is a fundamental principle in life history theory. In a seasonal environment, animals with indeterminate growth are predicted to change their allocation between reproduction and growth according to the season. We examined the seasonal variation in the allocation to reproduction and growth within a breeding season in the hermit crab Pagurus nigrivittatus. Females of P. nigrivittatus produced smaller clutches during the early part of the reproductive period than later although the frequency of prenuptial molting was lower early in the period. We suggest that environmental factors, such as temperature, may affect the development rate of embryos, as well as the metabolism and surplus energy in females, which have consequences for seasonal variation in the energy allocation to reproduction and growth.

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We analyzed a total of 3,784 young fiddler crabs of Uca arcuata (De Haan, 1833) with carapace widths (CW) between 1.45 and 9.95mm, collected from a site on the Ohno River in Central Kyushu, Japan. All crabs with CW<3.0mm possessed two small symmetrical chelipeds. Young male crabs with a CW between 3.0 and 6.0mm exhibited one of four stages of cheliped differentiation: 1) two small chelipeds; 2) two large chelipeds; 3) only one cheliped; and 4) asymmetrical chelipeds (one large and one small). We identified 646 male and 632 female crabs with a CW between 6.0 and 9.95mm. No statistical difference from the 1:1 ratio is evident. We conclude that the basic sex ratio for U. arcuata is unity.

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Surface structures of Pugettia quadridens and P. intermedia were examined by scanning electron microscopy. Both crabs attach pieces of sea algae to six hooked setae sites on the carapace. In addition to the hooked setae, the crabs develop plate setae, which cover the entire surface except the tips of the chelae. The plate setae of P. quadridens were flat and played no role in decoration, whereas those of P. intermedia were variously modified in shape. For example, the setae on the abdominal carapace surface were flat, whereas those on the walking legs and the dorsal carapace were conical and in some cases considerably elongated. These modified plate setae served as attachment sites for algae and other small materials on the carapace and walking legs, blurring the outline of P. intermedia.

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