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Abstract. A seagrass bed in Otsuchi Bay, northeastern Japan, consists of three seagrass species (Zostera marina, Z. caulescens and Z. caespitosa) that differ in morphology and phenology. We studied the mobile epifaunal community in the seagrass bed to examine (1) whether seasonal and between‐substrate variation in the epifaunal community agrees with variation in seagrass abundance and morphological complexity, and (2) whether patterns of seasonal and between‐substrate variation vary among epifaunal species. We collected mobile epifauna from each of Z. marina and Z. caulescens on 11 occasions between October 1995 and November 1996 (at 1 – 1.5 month intervals) using a hand‐closing net. A total of 9 842 individuals of mobile epiphytic animals were collected and they were classified into 80 taxa. Abundance and species richness of the epifaunal community were high in summer to autumn and low in winter to spring, and diversity index and evenness were higher in Z. marina than Z. caulescens. However, patterns of seasonal and between‐substrate variation in these parameters did not parallel those in seagrass abundance and complexity. Most of the dominant epifaunal species showed significant seasonal and between‐substrate variation in abundance, although their patterns varied greatly among individual species. A cluster analysis recognized several distinct groups of animals showing similar seasonal and between‐substrate variation. Our findings suggest that the observed variation in the epifaunal community is not determined by a single or some strong external factors but by complex interactions of multiple factors operating differently for each component species.

Male fiddler crabs (genus Uca) have an enlarged major claw that is used during fights. In most species, 50% of males have a major claw on the left and 50% on the right. In Uca vocans vomeris, however, less than 1.4% of males are left-clawed. Fights between opponents with claws on the same or opposite side result in different physical alignment of claws, which affects fighting tactics. Left-clawed males mainly fight opposite-clawed opponents, so we predicted that they would be better fighters due to their relatively greater experience in fighting opposite-clawed opponents. We found, however, that (i) a left-clawed male retains a burrow for a significantly shorter period than a size-matched right-clawed male, (ii) when experimentally displaced from their burrow, there is no difference in the tactics used by left- and right-clawed males to obtain a new burrow; however, right-clawed males are significantly more likely to initiate fights with resident males, and (iii) right-clawed residents engage in significantly more fights than left-clawed residents. It appears that left-clawed males are actually less likely to fight, and when they do fight they are less likely to win, than right-clawed males. The low-level persistence of left-clawed males is therefore unlikely to involve a frequency-dependent advantage associated with fighting experience.

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