日本ベントス学会誌 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.

1991年8月から1992年9月にかけて,天草上島,倉江川河口干潟に棲息するチゴガニの個体群構造を調べた.3.95mmより大きい成ガニの密度は,年間を通して約200/m²と安定していた.稚ガニは,7月から10月にかけて定着が見られ,8月にピークがあった.抱卵雌は4月から9月までみられ,7月の抱卵率が最も高かった.さらに,TSUTSUMI ＆ TANAKA(1987,1994)より個体群構造のコホート解析を試みた.その結果,雌は成熟するまで雄よりも早く成長するが,その後は,雄の方が雌よりも成長が早くなることが明らかになった.また,雄ガニの寿命は約3年で生涯に3回の繁殖期を迎えるが,雌ガニの寿命は2年と数ヵ月で2回の繁殖期を迎えることがわかった.これらの個体群特性について,チゴガニの他個体群に関するこれまでの研究結果との比較を行った.

安濃川と志登茂川の河口干潟に隣接した潮下帯において,3種類の二枚貝類(アサリ,イソシジミ,ホトトギスガイ)の浮遊幼生,着底稚貝及び稚・成貝の分布を調査した.3種類とも,浮遊幼生は多く出現するのに反し,底生期の稚・成貝はごく稀であった.着底稚貝と稚・成貝には,干潮の水際に近い定点に分布する傾向が見られた.このことは,浮遊幼生が潮下帯に着底するのを避け,河口干潟の水際近辺に輸送されることを示唆している.

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.