We studied the growth, reproduction and body color pattern of the marine isopod Cleantiella isopus in Hakodate Bay, Japan from May 2009 to July 2010. Individuals were collected every month and the sex, body length, color pattern, number of eggs per clutch and developmental stage of embryos for ovigerous females were recorded. Five body color patterns were identified in C. isopus at Hakodate Bay, and their composition was maintained throughout the year. Breeding females (guarded by a male or carrying eggs) were observed in the field from February to August. Newly recruited individuals were first observed in July and grew to mature size by the next breeding season, indicating a lifespan of 13-15 months. Precopulatory mate guarding in which a male holds a female using his pereopods was observed. There was no correlation between body lengths of guarding males and guarded females. The female reproductive cycle was synchronous, and two defined reproductive events were detected during the breeding periods, indicating that females bred twice or more during a lifetime.

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Heteromysis proxima W. M. Tattersall, 1922 (Mysida) was first reported from the Gulf of Manaar, off the south-eastern coast of India. A recent sledge net survey conducted on the north-western coast of Malaysia yielded several specimens of this little-known mysid from a sandy beach located in the mouth area of Merbok River, facing the Malacca Strait. Although the Malaysian specimens have a comparatively smaller body size than those recorded from India, the morphological features of these specimens essentially agree with the typical ones. This paper provides further information on the morphological characteristics of H. proxima on the basis of newly obtained specimens and extends its geographical range to the Malacca Strait.

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Eurydice nunomurai sp. nov. is described from Izu-Ohshima Island, Sagami Sea, Pacific coast of central Japan from a single specimen collected from the body surface of the sea whip coral on subtidal sand bottoms. Eurydice nunomurai is most similar to E. orientalis Hansen, 1890 but distinguished by the longer antennule, large number of antennal flagellar articles, complicated details of the serration and a large number of setae of the pleotelson posterior margin, and having a large body size.

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Distribution of freshwater shrimps and prawns and its relationship with environmental factors were studied in the Hiwasa River, Tokushima Prefecture, southern central Japan, to provide information needed for conserving or propagating them more effectively. Eleven species of decapod crustaceans consisting of three palaemonids, six atyids, and two crabs were collected, of which eight species were diadromous. Results of canonical correspondence analysis showed that substrate coarseness in addition to conventional longitudinal variables largely affects overall decapod distribution. Differences in distribution patterns among amphidromous species were mainly explained by riverbank vegetation coverage and the two aforementioned variables. In contrast, distribution of a non-diadromous atyid, Neocaridina denticulata, was suggested to be determined by relative ease for them to resist flood in that habitat, rather than by longitudinal factors. These results insist on importance of coarse boulder bed and riverbank vegetation as shrimp and prawn habitats at reach scale. The range of Caridina typus within a river differed between the Japanese Archipelago and the Ryukyu Archipelago, and may be explained by limited low-temperature tolerance of this species. Incorporating these results and life history traits of the collected species, we propose a new classification system of distribution patterns for freshwater decapods on the Japanese Archipelago.

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Relative growth of chelae and the area of setal patches was investigated for Hemigrapsus takanoi, in which only males have setal patches on chelae, and for H. sinensis, in which both sexes have setal patches. Chelae and setal patches increased with body size in both species. In H. takanoi, chela size and size increment relative to body size was larger in males than in females. In H. sinensis, chela size relative to body size was larger in males than in females, but size increment was not different between the sexes. Sexual dimorphism of chelae is more prominent in H. takanoi than in H. sinensis. Laboratory observations of intrasexual agonistic interactions revealed that the interaction occurred more frequently in H. takanoi than in H. sinensis without any sexual difference in either species. In agonistic interactions by H. takanoi, males mostly used chelae, while females employed their walking legs. By contrast, both sexes of H. sinensis used chelae more frequently than walking legs in agonistic interactions. In mating behavior, both species made a copulative posture without courtship behavior. Males of H. takanoi handled females with chelae during mating behavior, whereas in H. sinensis, chela handling was infrequent. All the findings suggest the extent of sexual dimorphism of chela is correlated with sexual difference of chela use in social behavior.

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