On sandy beaches of southern Shikoku, one temperate-zone crab, Ocypode stimpsoni, and two tropical and subtropical crabs, O. sinensis and O. ceratophthalma, co-occur, dwelling in individual burrows. Their distribution patterns, food habits, and physiological condition were investigated seasonally. Among the three species, O. sinensis was distributed in the highest zone vegetated with Vitex rotundifolia and Calystegia soldanella and was well segregated from the others except during its juvenile stage. It fed on terrestrial invertebrates and terrestrial plants. The distribution of O. ceratophthalma overlapped with that of O. stimpsoni; O. ceratophthalma mainly fed on terrestrial and aquatic invertebrates whereas O. stimpsoni fed on detritus and diatoms together with terrestrial and aquatic invertebrates. Ocypode ceratophthalma was poor in hepatopancreas index late in autumn and rarely overwintered, with the result that almost the entire population consisted of immature individuals. It is likely that the population of O. ceratophthalma in southern Shikoku is maintained by a supply of larvae from more southern regions. It is possible that O. stimpsoni, which is competitively inferior to O. ceratophthalma due to its smaller adult size, may become extinct in southern Shikoku if the recent rise in air temperature during winter enables more individuals of O. ceratophthalma to thrive there.

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To better understand the relationship between human-induced disturbance and the meiobenthic community in a harbor environment, sediment samples were collected at four stations (Sts. MP1 to 4) in Manazuru Port in Sagami Bay on eight occasions, approximately monthly between July, 2003, and March, 2004. Qualitative meiofaunal samples were collected by a simple cylindrical dredge. In addition, chloroplastic pigment equivalents (from October, 2003, to March, 2004), organic carbon and nitrogen content (in April and June, 2004), and grain size composition (in April, 2004) of the sediment were analyzed. Organic carbon and nitrogen were much higher at St. MP2 than at the other three stations. The sediment samples at St. MP2 frequently included dead fish and shellfish, evidently dumped from a nearby fish market. Station MP2 thus was subject to heavy organic loading. Multivariate community structure analyses using only higher taxonomic levels (phylum, order and/or class) of the meiobenthos did not show significant differences among the stations and sampling months. In contrast, based on species and feeding-types of nematodes, a distinct sample group form St. MP2 was revealed by multidimensional scaling (MDS) and cluster analyses. R-type analysis of the species association of dominant nematodes and correspondence analysis of dominant species and feeding-types of nematodes both also clearly showed the distinctness of the sample group from this station, which mainly consisted of facultative predators collected exclusively there. These results suggest that two species of facultative predators, Meyersia sp. and Metaparoncholaimus sp. belonging to the family Oncholaimidae, and four other nematode species are good indicators of a harbor environment affected by heavy organic loading.

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In Japan, the hermit crabs Pagurus minutus, Diogenes nitidimanus and Clibanarius infraspinatus are common in inner bays, sandy tidal flats and shallow subtidal sediment, respectively. We investigated the distribution of these three species in summer at six sampling sites in the Waka-River Estuary, Wakayama Prefecture, Japan. The sampling sites were placed along the length of the estuary, with Site 1 situated high up in the brackish water of the estuary, Site 6 at the river mouth, and the other four sites at intermediate locations along the estuary gradient with the number of each site. Pagurus minutus was the dominant species at five out of six sites. Diogenes nitidimanus was mainly found at intermediate St. 3 and 5, relatively close to the river mouth, and had sandy substrates. Clibanarius infraspinatus was mostly found at St. 6. The sex ratio of D. nitidimanus was highly biased to males at St. 3 and to females at St. 5, while the sex ratio of the other two species did not vary across sites except for P. minutus at St. 4. Carapace and chelae length of males were larger than those of females in P. minutus for their right hands and in D. nitidimanus for their left hands, but not in C. infraspinatus for their right hands. In D. nitidimanus, small males were especially abundant at St. 3, while males of various size including large ones, were collected at St. 5. Females of various sizes were found at both sites. However, there were very few females at St. 3, while many females, including ovigerous ones, inhabited St. 5. The existence of the small male population distant from the reproductive population in D. nitidimanus may be an interesting aspect with respect to the life history strategy of the males and deserves further investigation.

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The spatial distribution of the golden mussel, Limnoperna fortunei, was investigated in the Tone River System from November, 2007, to February, 2008. Investigations were conducted up to 180 km from the mouth of the Tone River and the mussel was found up to 120 km upstream. The highest density sites, where one researcher was able to collect 1820 individuals in 10 minutes, are 26 km from the river mouth. The mussel was also found in some other rivers (the Kokai, Nagato, Kurobe, Hitachitone, and Edo Rivers), ponds and lakes (Lakes Teganuma, Sotonasakaura, Ushikunuma, and Yotaura), and canals. The mussel is thus widely distributed in the Tone River System. The density of the golden mussel in the Tone River rises starting at its confluence point with Ibaraki Prefecture's Kokai River, in which the mussel was found abundantly downstream of the water division facility of the Kasumigaura Canal. This spatial distribution of the mussel suggests that it was introduced from Lake Kasumigaura to the Kokai River through the Kasumigaura Canal.

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The macrobenthos and meiobenthos communities at five stations in Hiroshima Bay were investigated by simultaneous sampling over the couse of a year (1998) to estimate the production of their major animal groups. Polychaeta was the most abundant animal group of the macrobenthos throughout the investigation. Mollusca, consisting mainly of Theora fragilis, was the second most abundant animal group, and their density increased from April to July. Nematoda was the dominant animal group of the meiobenthos, comprising more than 75% of the total abundance of meiobenthos at all stations throughout the investigation except for St. 1 in March. Although the abundances of macrobenthos and meiobenthos varied overtime, the average abundances of macrobenthos as well as meiobenthos were highest at the innermost station in Hiroshima Bay. The mean biomass of macrobenthos in Hiroshima Bay was 3.26 g dry weight/m², and that of meiobenthos was 0.49 g dry weight/m². The production calculated from these values in biomass was 9.79 g dry weight/m²/year for macrobenthos and 4.93 g dry weight/m²/year for meiobenthos. These results indicate that the production of meiobenthos relative to that of macrobenthos is high in this eutrophic area.

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To examine effects of seagrass Zostera japonica on benthic animals, community structures of the benthos was compared among two Z. japonica meadows and a bare sandy area on intertidal flats at Takinai and Uchinoura, Tanabe Bay, Wakayama Prefecture, Japan, from August, 2004, to July, 2005. The mean number of species (N/0.0625 m²) was higher in the Z. japonica meadows (Takinai: 18.5-26.8, Uchinoura: 16.0-28.7) than in the bare sandy area (6.0-13.5). In addition, higher mean densities (N/m²) and mean wet weight (g/m²) of benthic animals were found in the seagrass meadow (Takinai: 6,408-10,763 and 147.0-533.5, respectively; Uchinoura: 1,632-9,061 and 162.0-562.0, respectively) than in the bare sandy area (368-1,424 and 12.1-49.1). Almost all of the species (82.5%) collected from the bare sandy area were also present in the Z. japonica meadows. Appearance rate and abundance of the benthic species were different between the Z. japonica meadows at Takinai and Uchinoura. The number of gastropod species was lower at Uchinoura than at Takinai, but the reverse was true for gastropod abundance. Although the number of species was higher in seagrass meadows than in the bare sandy area, the lower density of benthos at the latter site resulted in a higher evenness value (J') there than in the Z. japonica meadows, and the diversity index (H') at all three sites was, therefore, similar. Lower evenness in the Z. japonica meadows is also related to the notable dominance of certain species, such as Pullucina pisidium and Cirriformia tentaculata. From these results, it is concluded that Z. japonica meadows provide a valuable and hospitable environment for benthic animals, especially infauna, in sandy and muddy bottoms in the intertidal zone.

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Since Clements' ‘phytometer’, bioindicators have been used in various fields of ecological research. Bioindicators include both indicator species and biological indices (e.g., diversity index) that indicate a particular physical environment, community or partial ecosystem. In this paper, I examine some logical problems with the concept of indicator species and evaluate its utility. Supporters of the indicator species concept emphasize that it can detect the average, cumulative or extreme effect of the physical factor in question often by a simple short-term study, and that the results are more suggestive of the impacts on living organisms, including humans. Critics point out that indicator species are less decisive than physicochemical parameters and are sometimes invalid in different geographic sites. The problems of logic associated with the indicator species concept are summarized as follows: Its definition is inductive, its application amounts to affirming the consequent, and the logic is circular. Concerning the criticism of induction, perpetual in-situ tests of the indicator species are needed. The problem of affirming the consequent could be solved by repeated confirmation of the coincidence of the indicator species and the factors concerned in various situations, for which statistical significance tests are necessary. Circularity of logic is avoided by excluding the indicator species itself from its definition and basing the definition only on physical factors. When an indicator species is used for falsification of some previous assertion, or when plentiful background information is already available, such as in the case of temperature indicator species based on geographic distribution (e.g., tropical species), its interim employment is plausible. But in general, the use of indicator species is inappropriate without the process of standardization, because of the latent logical problems of the concept.

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