In 1993, a small population of Uca arcuata was found in the Sashiki tidal flat of Nanjo city, Okinawajima Island, facing Nakagusuku Bay. This location was far isolated from the main distributional range of the species, and a genetic study indicated that this Okinawajima population had a low genetic diversity and a different genetic structure from other populations. We report here that small populations of this species have become newly established in artificial tidal flats adjoining the recently reclaimed Shinko district of Uruma city about 20 km away from the Sashiki population. The genetic structure of the new populations, as assessed by comparison of 3,700 bp sequences of the mitochondrial DNA using PCR-RFLP, is the same as that of the Sashiki population. This suggests that the new populations are derived from the Sashiki population through gene flow by larval transport within Nakagusuku Bay.

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The macrofauna of three estuarine tidal flat areas at the mouth of the Chikugo River, Rokkaku River, and Shiota River in the inner part of Ariake Bay is described. Sampling was done from a boat during high tide using a Smith-McCintyre grab sampler (22.5×22.5 cm) at 11 stations in each area in December, 2005, and May and August, 2006. Sediment was sieved through a 1-mm mesh screen and the residue was fixed, stained with Rose Bengal, and sorted. Multivariate analyses showed that community structure differed significantly between the tidal flats. This was mainly due to differences in the abundance of the semelid bivalve Theora fragilis, capitellid polychaetes, the arcid bivalve Scapharca kagoshimensis, the venerid bivalve Ruditapes philippinarum, and stenothyrid gastropods Stenothyra spp. The distribution of endangered species and invasive species also differed between tidal flats. The invasive corbulid bivalve Potamocorbula laevis and nassariid gastropod Nassarius sinarus, and the endangered tellinid bivalve Tellina iridescens, arcid bivalve Tegillarca granosa, and camptandriid crab Camptandrium sexdentatum occurred mainly in the mudflats of the Rokakku River and/or Shiota River estuaries, while the brachiopod Lingula anatina and mytilid bivalve Modiolus metcalfei were mainly distributed in the Chikugo River estuary. Differences in sediment grain size were likely the main factor resulting in differences in the macrofauna and the distribution of endangered species among the tidal flats.

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Reproduction carries costs, in terms of growth, survival, and future reproduction, because reproduction entails the expenditure of energy and time that could be used for other functions. Male Pagurus hermit crabs engage in precopulatory mate guarding behavior for a period of up to several days, and this guarding behavior may entail costs for males. We investigated whether precopulatory mate guarding depresses feeding rate and molting frequency as a cost of reproduction for males in the hermit crab Pagurus nigrofascia. Males were discriminated into the following three size classes: small males that were not observed to guard females, middle-sized males, and large males that guarded females. In the field, molting frequency of the large males was significantly lower than that of the other two size classes. The feeding activity of males was greatly depressed while they were engaged in guarding behavior. The duration of guarding was affected by sex ratio in laboratory experiments, the total guarding time increasing along with the number of mates. Molting frequency did not differ significantly among male crabs under the different sex ratios, however. We suggest that precopulatory mate guarding may entail a reproductive cost for males of P. nigrofascia due to depression of feeding activity although this cost may be compensated for by higher feeding activity before and/or after guarding.

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Long-term macrobenthic faunal change was investigated in the Shigetomi-higata tidal flats of northern Kagoshima Bay by means of surveys conducted in spring and autumn of 1994 and 2005. Density and biomass of all species of macrobenthic animals and sediment characteristics were surveyed along four transect lines covering the entire intertidal area of the flats. In total, 37 and 38 species were collected from this area in 1994 and 2005 respectively. Density and biomass of the various benthic animals have changed differently among the transects, but gastropods remarkably increased in both density and biomass during this period along most of the transects. Especially, Batillaria multiformis dominated the flats in 2005 but were not found in 1994. In contrast, the density, biomass, and species number of bivalves decreased, and decrease of Ruditapes philippinarum in both density and size of individuals was remarkable. The dominant polychaete changed from Notomastus sp. with a larger body size to Armandia lanceolata with a smaller body size. In general, epibenthic and smaller species increased whereas endobenthic and larger species and larger individuals decreased during this period. We could not identify the environmental factors responsible for causing these faunal changes, but topographic changes in the tidal flats may have played a role in the biomass change of R. philippinarum.

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