BENTHOS RESEARCH Vol. 1996(1996) No. 50

Sixteen species of starfish were collected at depths between 0 m and 220 m in Wakasa Bay, and the effects of sediment and depth upon the species composition were examined. The sandy area was populated by fewer Luidia quinaria and more Astropecten scoparius than the muddy area. One reason for this may be that L.quinaria preys upon sea urchins, which are abundant in the muddy area, while A.scoparius eats small bivalves, which are abundant in the sandy area. The species composition shifted abruptly in the 100-160m depth zone, while the water temperature decreased more rapidly in the 90-200m zone than in the other zones. The species sampled from the colder depth range in Wakasa Bay are geo-graphically distributed in colder regions. These results show that water temperature restricts their distribution.

A unique foraging behavior of the carnivorous polychaete Halla okudai (Lysaretidae) is described in detail. Living bivalves, Ruditapes philippinarum were supplied to adult H. okudai, which were burrowing in sandy sediment in aquariums. The foraging behavior was divided into 4 stages ; responding, searching, handling, and feeding. Responding and searching were considered to be dependent on the olfactory sense, and searching was significantly responsible for the variation in total foraging time. Handling was performed by inhibiting the escape of the bivalve and then opening its shell while secreting a jelly-like material. Bivalve size (23.8-26.3mm shell length) showed a significant correlation to handling time, which ranged from 0.87 to 6.67hr. During feeding, the ploychaete continued to secrete the jelly-like material, which was different in hue from that secreted during handling.

The population structure and reproduction of Ilyoplax pusilla (DE HAAN) (Brachyura:Ocypodidae) were studied from August, 1991, to September, 1992, in the Kurae River Estuary (32°30' N, 130°24' E), Amakusa Kamishima Island. The density of mature crabs (adults) was ca. 200/m² throughout the study period, with no strong seasonal change. Juveniles occurred from July to October, with maximum numbers in August. The ovigerous females occurred from April to September, with a peak in July. Cohort analysis revealed the following: maturity was attained at an age of 1 year; females grew faster than males before maturity, whereas males grew faster after attaining maturity; males were estimated to survive for at most 3 years and to experience 3 reproductive seasons during their lives, whereas females were estimated to survive for 2 years plus a few months and to experience 2 reproductive seasons. Compared with other local populations in temperate and subtropical Japan, the population of the Kurae River Estuary was characterized by smaller maximum sizes of both sexes, smaller minimum sizes of mature females, and the earlier appearance of ovigerous females. Settlement of juveniles occurred over a longer period than in the other populations.

Spatial distributions of planktonic larvae and newly-settled and benthic stages of three species of bivalves (Ruditapes philippinarum, Nuttallia oliuacea, Musculista senhousia) were investigated in the subtidal area adjacent to the tidal flats of the Ano and Shitomo Rivers. In all three species the benthic juveniles and adults were very scarce in the subtidal area, while the larvae occurred abundantly there. The newly-settled and benthic bivalves tended to occur at sites near the low water of tidal level water. This suggests that larvae avoid settling on the bottom in the subtidal area but are transported to the lower margin of the tidal flats.

Fish farming using net pens in Japanese coastal waters has become increasingly common over the past two decades. In this new style of fishery, however, large amounts of material are discharged from the net pens. The increased organic input to the bottom immediately below the net pens tends to result in organic enrichment of the sediment. This enrichment causes a catastrophic environmental disturbance on the bottom during summer, resulting from hypoxic bottom water conditions and the development of reduced conditions in the sediment. In March, 1993, we assessed the chemical conditions of the bottom sediment and the composition of the Benthic community in a cove where fish farming has been taking place since 1973. In this cove, assessments of the benthic conditions have been conducted since 1968, prior to the start of fish farming. We compared the results of the study in 1993 with the earlier data to describe the f aunal changes in the benthic community, caused by the organic enrichment of the sediment. The most serious organic enrichment of the sediment occurred only at the areas adjacent to the fish farms. The benthic f aunal community, however, changed dramatically in the whole area of the cove after the onset of fish farming. Various molluscs predominated prior to the start of fish farming. As the sediment became organically enriched, the abundance of molluscs markedly declined, their biomass decreased, and the molluscan community species composition was extremely simplified. In 1993 only three species of small polychaetes (Capitella sp. 1, Pseudopolydora paucibranchiata, Euchone sp.) and one species of amphipod, Aoroides columbiae, predominated in the Benthic community in the most organically enriched areas. These species were very rare or not found at all in the cove prior to the start of fish farming. Thus, the organic enrichment of the sediment caused by fish farming for two decades has resulted in drastic changes in the benthic faunal community of the cove, to the extent of the replacement of molluscs by previously rare species.