Abstract: Year-to-year changes in the distribution of rocky intertidal molluscs were monitored at Bansho Cape on the southern Pacific coast of mainland Japan from 1985 to 1994. All the molluscan species except opisthobranchs were recorded from each of 69 quadrats(8x8m)in April or May each year. Over the ten years, 183 species of molluscs were recorded, of which the Gastropoda made up 74%, Bivalvia 20%, and Polyplacophora 6%. Species with a southern geographic range (northern limit35°c N) constituted the remainder. Species distribution patterns in relation to height on the shore and wave exposure were consistent throughout the ten years, and the non-southern species always exceeded the southern species in terms of mean number of quadrats in which they occurred. On the other hand, the southern species increased significantly over the ten years in terms of both number of species and cumulative number of quadrats occupied. Air and water temperatures increased significantly in winter near the study site, and the distance from the shore to the warm Kuroshio Current decreased significantly during the period from 1980 to 1994. The increase in near-shore water temperature in winter was 1.7-1.9° from the early 1980s to the early 1990s. The present and previous results suggest that temperature and/or the Kuroshio affected the structure of the molluscan assemblage in the study area by controlling the abundance of southern species. Low mortality in winter due to the temperature rises, and an enhanced supply of larvae from the Kuroshio, are proposed as mechanisms for the recent increase in prevalence of southern species.

View full abstract

Abstract: Many invertebrates inhabit the shallow subtidal zone in subtropical regions, and some of them also emerge at times into the water column. To detect any synchrony of their emergence patterns with the day/night or tidal cycle, and to monitor their abundance in the surface and bottom waters, small invertebrates were collected continuously for one month at Iriomote-jima in the Ryukyu Islands, Japan. Two pumps were set to collect surface and bottom water, respectively. The daily patterns of emergence in 24 major taxa, mostly crustaceans, were analyzed by visual inspection and statistical methods. A notable feature was a clear nocturnal pattern in all the taxa except Corycaeus sp.; however, the nocturnal pattern varied among taxa with regard to its strength and the influence of the tidal cycle upon it. The emergence patterns of each taxon were much the same between the surface and bottom. Some animals (e.g. Labidocera pavo and Lucifer hanseni) were more abundant in the bottom water, but other animals (e.g. crab zoeas comprising several species) in the surface water. Almost all the animals collected by the pumps were also collected from the bottom substrate in the daytime. Therefore, absence of these invertebrates by day from pump samples does not imply their escape from the area where the pumps were installed, but merely that by day they hide within the bottom substrat or just above it, where very few specimens could be collected even by the bottom pump. Dispersing or swimming into the water column at night, they could then be collected by both pumps. ‘Daily emergence/dispersal’ may be a better term than ‘daily vertical migration’ for this behavior pattern in subtropical marine invertebrates. Furthermore, because of similar patterns of periodic activity, the distinction between ‘benthos’ and ‘plankton’ is blurred for small subtropical invertebrates.

View full abstract

Abstract: Laboratory experiments were carried out to observe the day/night activity patterns and habitat preferences of two gastropods, Gibbula umbilicaris and Jujubinus exasperatus, on leaves of the seagrass Posidonia oceanica. Both G. umbilicaris and J. exasperatus preferred old (apical) parts of leaves to young (basal) ones, and vertically oriented leaves rather than horizontal ones. Four types of behavior (rest, shaking epipedalic tentacles without any other motion, directional movements, and shell twisting mainly for foraging) were observed. Also, interspecific differences in the day/night activity pattern were recognized; G. umbilicaris mainly rested during the day and actively moved and foraged at night, whereas J. exasperatus continued to move about and forage during both day and night, resulting in a less marked day/night pattern. These results partly explain the observations of previous studies, in which both species were more abundantly sampled on seagrass leaves at night than by day.

View full abstract

Abstract: Natural, stable, sulfur-isotope tags were used in Tsuyazaki Inlet (Kyushu, Japan) to interpret sulfur uptake by the prevailing Gastropoda and Bivalvia communities. The isotopic analyses showed that biomass with light sulfur (lowδ 34S values) derived from bacterial reduction of seawater sulfate forms the largest part of the biomass on the feeding grounds of benthos that prevail in and/or on the anoxic sediments with free sulfides, in contrast to the uptake of seawater sulfate by common benthos communities close to the open sea.

View full abstract

Abstract: Many fishermen of the common brackish water clam Corbicula japonica report that in the warm season, the clams inhabit shallow depths of the sediment and are easily caught by scraping the sediment with fishing gear, but in the cold season, they burrow more deeply and become harder to catch. We examined, first, this seasonal pattern of burrowing of clams in Lake Ogawara, a mixo-oligohaline lagoon in northern Honshu (Japan), and showed that the clams perform a clear seasonal vertical migration; in cold months, the clams burrowed up to 12cm into the sand sediment, but in warm months, they occurred exclusively within 3cm of the sediment surface. Secondly, an experiment of clam burrowing was conducted at different temperatures in the laboratory. The results were similar to those of field experiment; burrowing depths were deeper at lower temperatures. This suggests that temperatures may be the proximate cause of the seasonal vertical migration. Possible ultimate factors include physical turbulence, such as rough wave action, freezing of the shallow sediment layer, and predation by birds in cold months. In cold months, when the clams may have a low activity level, they would avoid physical turbulence and birds' predation, and increase survival rates by burrowing deeper into the sediment. These results suggest that the lowered fishing efficiency for clams that occurs in winter is closely related to the observed deep burrowing within the sediment, and which becomes more compact and, hence, more difficult to dredge due to the scattered vertical distribution of clams in the sediment.

View full abstract