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Patches of Mytilus edulis 1. composed of similar sized mussels were surveyed in order to investigate the effect of patch size on community structure of associated animals. Each patch included common components such as byssus, shell fragments, sediments. algae and associated animals. Volume of each component increased with patch size. With increasing patch size. species richness and number of individuals of associated animals increased, but decreased per unit patch size. Patch area affected population density more than patch volume. Volume of associated animals per unit patch area was relatively constant, but volume per unit patch volume decreased with increasing patch size. Mean body width of the polychaete Typosyllis adamanteus Jrun'lensis Chlebovitsh increased with patch size, Large individuals of the winkle Littorina brevicula Philippi were not found in smaller patches. At high tides, large numbers of the amphipod Hyale grandicomis Kroyer, which live among algae attached to the patches, were seen on rock surfaces covered by the barnacle Chthamalus challengeri Hoek around the patches. These results can be mainly explained in relation to accumulation of sediments and shell fragments of M, edulis in the patches, and the community structure of the Mytilus island is discussed with reference to species-area relations.

Islands of Mytilus edulis as a habitat for small intertidal animals: effect of Mytilus age structure on the species composition of the associated fauna and community organization* Marine Biological Station, Tohoku University, Asamushi, Aomori 039-34, Japan Laboratory of Animal Ecology, Department of Zoology, Kyoto University, Kyoto 606, Japan ABSTRACT: Community structure of the associated animals of low intertidal patches of Mytilus edulis L. with different age structure was investigated at Asamushi, northern Japan. Each patch was composed of similar components i.e. live M. edulis, byssal threads, shell fragments, sediments, algae and associated animals. The amounts of sediments, shell fragments and byssal threads were greater in the periphery (MP) and central part (MC) of adult mussel patches than in patches of young (Y) and old (0) mussels. Total organic matter within sediments increased in the following order: 0, MC, MP and Y. Algal growth was luxuriant in Y and this supported abundant amphipods. Species richness was higher in older patches and species diversity (H') and equitabihty (J') were higher in MP and MC than in Y and 0. Sediments, shell fragments and byssal threads seemed to play an important role in increasing heterogeneity of environments and thus species diversity. Similarities in species compositions among MP, MC and 0 were high, but those between Y and other patches extremely low. This fact is probably caused both by the presence of a large number of epiphytic animals in Y and the difference in the composition of creeping fauna among the patches. Based on these results, the processes controlling community organization of Mytilus islands are discussed.

SynopsisWe determined the species-specific habitat associations of coral reef fishes and environmental characteristics in an Okinawan coral reef in Japan. We focused on three families (Pomacentridae, Gobiidae and Labridae) and attempted to determine differences in habitat utilization. We selected six sites along the coast of Amitori Bay, from the entrance to the innermost part, in order to cover a wide range of habitat characteristics (exposed habitat, semi-exposed habitat and sheltered habitat). The species diversity of coral assemblages was greater at the exposed and semi-exposed habitats, whereas branching coral mostly covered the sheltered habitat. The environmental factors that determine the species-specific spatial association in fishes differed among families. Both biological characteristics (coral morphology and coral species diversity) and physical characteristics (water depth and wave exposure) affected the spatial association of pomacentrids and gobiids. In contrast, physical characteristics such as substrate complexity and water depth affected the species-specific spatial association of labrid species. Further study is needed to determine the ecological factors that regulate the species-specific habitat preference in Okinawan coral reefs.

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