Because the bivalve molluscs of estuaries and lagoons are sedentary yet lack permanent attach-ment to the substratum, they can be manipulated in an even wider diversity of experimental treatments than their invertebrate counterparts on rocky intertidal shores. Manipulations of local density of suspension-feeding bivalves in soft sediments have revealed an almost universal pattern of reduced individual growth with increasing local density. Despite reductions in growth rate, mortality rate is generally invariant with increasing density except where density-dependent predation is operating. Several lines of evidence suggest that food limitation is the cause of the reduction in growth at higher local densities. Consequently, the outcome of competition for food among suspen-sion-feeding bivalves in marine soft sediments is a reduction in growth rates but no competitive exclusion. A series of experiments assessing the role of interspecific competition in contributing to the observed segregation of species along the elevation gradient on an intertidal flat demonstrated that competitive exclusion does not occur and thus does not cause the observed habitat segregation. The absence of competitive exclusion may be general when food rather than space is limiting because food is a renewable resource : even less able competitors will get access to some food, whereas loss of living space is absolute and implies mortality. Current theory on the community-level conse-quences of competition should better reflect this dichotomy between the outcomes of interference and exploitation competition. This analysis is intended to illustrate how a study of soft-sediment ecology can be framed in a broader ecological context so as to contribute directly to construction and testing of general theory in community ecology. Future studies in marine soft sediments would have greater impact on the field of ecology if their motivation were broadened to address ecological issues intrinsic to all communities and to include meaningful attempts to synthesize system-specific results.

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A biological community is organized by interspecific interactions among many species. The interactions form the structure of community through two main organizing systems, i.e., trophic and habitation systems. Habitation system consists of colonizers' classes graded by intensity of influences on environments. The primary inhabitant includes two types having opposit-directing influence to environments. As an example for destabilizer, one of them, thalassinid shrimps have been known. A removal experiment of the shrimps was conducted during over 2 years in Yamada Bay tidal flat, Japan. The results with obscure influences of the shrimps suggested the importance of combined effects with physical stresses. Another type of primary inhabitants was stabilizer like seagrasses. In a pure stand of Syringodium isoetifolium in Fiji, it was indicated that a huge biomass of filter-feeders was supported by the existence of a seagrass canopy. Through these considerations, mechanisms of community organization combined among functional groups, filter-feeders, deposit-feeders, symbionts, etc., were discussed.

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This article reviews recent studies on filtration and ammonium excretion rates of filter-feeding bivalves. Many of the studies emphasized the controlling role of filter-feeding bivalves in material cycling between the water column and the sediments. However, the application of metabolic rates measured in vitro to in situ nitrogen cycling includes some problems. For example, measurements of metabolic rate without sediments cause an artificial stress on infaunal bivalves. Concentration of suspended materials also affects the rates of filtration and ammonium excretion of filter-feeding bivalves. Constructing a desirable apparatus for measuring bivalve metabolism therefore requires considerable effort to simulate in situ environment. From this point of view, batch type experiments are not suitable because they require a measurable change in concentration of the objectives in a closed system, which usually induces artificial stress on bivalves. Continuous incubation technique in an open system is presumed to create less stress, and it also enables the repeated measurement with same animal. Another problem in recent works is that they mostly concentrated on the effect of bivalves on either taking up or supplying nutrients to Phytoplankton. Since both effects take place simultaneously, studies must pay attention the concomitant analysis of both effects in the natural environment.

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Monthly measurement of growth rings along the shell margin of the North Pacific cockle Clinocardium californiense revealed that a ring is formed annually in September (above one year-old) or in October (0 year-old), when the bottom temperature is high. Age was determined by counting the number of growth rings on the shell. The VON BERTALANFFY growth equation was fitted to the age-shell length relation, and the equation, L_T=57.14(1-exp(-0.526(T-0.143))) is obtained, where L_T is the shell length (mm) at age T (year). Seasonal growth pattern was estimated by measuring the monthly shell increment from the last growth ring, and the equation obtained is L_T=57.35[1-exp(-0.522(T-0.241)+0.085sin(2π(T-0.476)))]. These equations show nearly the same growth rates with those previously reported for cardiid cockles, but are higher than those for other bivalves.

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Lithophyllum yessoense, a dominant crustose coralline alga on lsoyake (urchin-dominated barren) ground in southwestern Hokkaido, was experimentally fed to the sea urchin Strongylocentrotus nudus (3cm in test diameter) and the limpet Acmaea pallida (2cm in shell length) in the laboratory. Grazing marks of the urchin were composed of pentagonally-arranged five strips, and scattered on the crust surface. The wound was 0.01mm deep within sacrificial epithallium, and crust surface of 0.20mm² was removed by one biting. The limpet grazed more densely and deeply (0.04mm) enough to remove distal columnar cells as well as epithallial cells. Crust surface of 0.43mm² was removed by one biting. The urchin-grazing marks (shallow wounds) became vague by the usual production of epithallial cells in 5 days. On the contrary, the limpet-grazing marks (deep wounds) recovered by regeneration of the intact columnar cells in 10 days. The production of epithallial cells and the regeneration from columnar cells must be the important nature to elucidate the vigorous coverage of this species under high urchin-grazing pressure in the Isoyake ground.

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In Dokai Bay, effluents from factories which were located along the bay, and the sewage from Kitakyushu City (population ; approximately one million) had resulted in serious pollution of the water and bottom sediment of the bay since 1930. However, in the past two decades, the conditions of the water and bottom sediment of the bay have been much improved, owing to recovery projects of the environmental conditions of the bay. In this study, the physico-chemical conditions of the bottom sediment and the faunal aspects of benthic community in the nearshore area of the bay are described to evaluate the present stage of improvement of the bottom environment in this area. Results obtained in July, 1989 revealed that the nearshore area of the bay is clearly divided into two subareas in terms of the bottom environment in summer. One is mud or muddy sand bottom with the reduced condition due to organic pollution, and the other is sand bottom with the less organically polluted and oxygenated condition. Although a gradient of environmental conditions from the bay mouth to the innermost part of the bay was not obvious in the bottom of the nearshore area, the abundance and the biomass of benthic community markedly decreased at the innermost part of the bay. In the mud or muddy sand bottom, Prionospio pulchra (polychaete), Neanthes succinea (polychaete) and Nebalia bipes (Nebaliacea) predominated. In the sand bottom, both Capitella capitata (polychaete) and Corophium spp. (amphipod) were dominant. However, from autumn (or winter) to spring of the next year, the dominant species of the sand bottom in the last summer occurred with high densities in the mud bottom at this area. Seasonal change of bottom environment with the reduced and/or oxygenated condition in the sediment may influence the distributions of benthic organisms in the nearshore area of the bay.

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