A field study was undertaken on the cross-shore distribution and population structure of the sea cucumber Patinapta ooplax (Synaptidae) inhabiting an intertidal sand flat in Awaji, Hyogo Prefecture, western Japan. Two transects of six or seven sampling points each were sampled at two- to three-month intervals from May, 2008 to February, 2009. The sea cucumber inhabited a zone between mean tidal level and mean low water of spring tide, irrespective of season. The gonad index showed a higher value in summer than in other seasons. Autotomized individuals occurred throughout the year, being most abundant in late autumn. Mean body size was smaller in autumn than in other seasons.

抄録全体を表示

The distributions of two closely related camptandriid crabs, Deiratonotus kaoriae and D. cristatus, were investigated in the Kushida River Estuary (Mie Prefecture, Japan) in October, 2008. The two species were distributed differently, with D. kaoriae in the lower reaches and D. cristatus in the upper reaches of the estuary. The physico-chemical environmental conditions of these areas differed in that the areas inhabited by D. kaoriae had a higher water temperature, higher salinity, and higher elevation than those inhabited by D. cristatus. The substrata of the two crabs’ habitats were also different, being dominated by sand in the case of D. kaoriae, but by mud in the case of D. cristatus. The presence of percolating ground water in the substratum was common to the habitat of both species.

抄録全体を表示

The population structure and life history of the goneplacid crab Carcinoplax vestita were investigated in Tokyo Bay, Japan, from November, 2002, to October, 2003. Catch per unit effort (CPUE) was generally constant (<100) with two exceptional peaks in June (171) and August (390), and it was always highest in the central part of the inner bay than elsewhere. The overall sex ratio was significantly biased towards males, but the proportion of males gradually decreased from November and the sex ratio was significantly biased towards females in June, August, and September. The overall size-frequency distributions for males and females were not significantly different, and there is no sexual dimorphism in body size in this species. The size-frequency distributions were bi- and/or trimodal from November to April or May, but unimodal in later months. The small- and large-sized modes consisted of newly recruited individuals and senile, post-reproductive individuals, respectively. Life-span was estimated to be about one year with exceptional individuals surviving longer. The occurrence of soft-shelled small males peaked in December, April, and August, suggesting a four-month molt cycle; however, large males and both small and large females showed no clear peaks or cycles of molting. Larger females were inseminated first, starting in April. In contrast, females concurrently began to spawn and carry embryos regardless of their body size in August. The time lag between summer hypoxia and the ovigerous period has probably contributed to the establishment of a large population in Tokyo Bay.

抄録全体を表示

It has been widely accepted that articulate brachiopod adaptation to and radiation within various benthic environments is reflected in the variability of their attachment organ, the so-called “pedicle”. To evaluate how the pedicle functions, the behavior of the terebratulide brachiopod Laqueus rubellus, harvested from Osezaki, Suruga Bay, Japan, was observed for one month in an aquarium. The behavior analysis focused particularly on pedicle movement and its associated muscular system. Anatomical observations of the muscular system demonstrated that the paired dorsal and ventral adjustor muscles are connected to the root of the pedicle via the mantle membrane, and the two muscles of each side cross in an x-shaped configuration. As a result, the pedicle should rotate when either set of adjustor muscles contracts. A succession of such movements would allow the animal to shift its position on the sea bottom while remaining attached to small pieces of debris, such as pebbles or bioclasts. In addition, the twisting and shaking of this brachiopod as a result of multiple successive pedicle rotations has a lifting effect due to the low density of its body compared to the surrounding sediment. Thus, this movement provides a means of avoiding burial. Previous studies of the relationships between benthic fauna and marine environments have indicated that articulate brachiopods can disperse into any benthic environment and attach to whatever debris exists. These studies have also shown that the abundance of articulate brachiopods depends on competition, nutrient supply, and predation pressure. Given the stable nutrient conditions on the sea bottom off Osezaki, competition and/or predation pressure appear to determine whether or not Laqueus rubellus can flourish in this benthic community.

抄録全体を表示

Carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) are widely used to trace material flow in aquatic ecosystems. In this review, I first summarize the isotope values of potential food sources for benthic invertebrates reported from the Japanese coast. Then I give an example of food source analysis of benthic invertebrates in estuarine lagoons. The food sources exhibit distinctive δ¹³C values [riverine particulate organic matter (POM)/C₃ plants, <-25‰; macroalgae, -25‰ to -13‰; marine POM, -20‰; microphytobenthos/seagrass, -20‰ to -10‰), although spatial and/or temporal variation are observed. In eutrophic estuaries, autochthonous phytoplankton comprise an important component of the estuarine POM pool. Most invertebrate species in the two studied lagoons utilize microalgae as their major diet item: suspension feeders depend mainly on microalgae in the water column (e.g., phytoplankton and re-suspended microphytobenthos), and deposit feeders and grazers depend mainly on benthic and epiphytic diatoms, respectively. Few species utilize river-derived detritus as their major food source. In the lagoons, the diet of the macrozoobenthos varies along two environmental gradients related to the biomass of autochthonous phytoplankton and distance from the river input (i.e., supply of river-derived materials). Stable isotope analyses are useful tools with which to trace material flow among terrestrial, estuarine, and marine ecosystems. In particular, they can help us to assess the food sources and feeding habits of benthic invertebrates in estuarine soft-bottoms．

抄録全体を表示

We studied spatiotemporal variation in the spawning patterns of a local population of the brackish water clam Corbicula japonica in Lake Abashiri and the Abashiri River, over the entire range of this species in this basin. Reproductive patterns were approximately synchronous throughout the local range of this species, with clams maturing in July and spawning in August. However, spatiotemporal variation in reproductive patterns was detected along the brackish water environmental gradient, especially at the edge of the distribution range where a delay in reproduction was observed. Such spatiotemporal variation in reproductive patterns is likely to have arisen due to factors such as water temperature, salinity, turbidity and environmental stress, which are typical of brackish water environments and which are affected by freshwater inputs upstream and tidal inflows downstream. The reproduction and viability of this population are strongly affected by local variations in temperature, salinity and anoxic conditions below the halocline of Lake Abashiri, which is located along the northern edge of the distribution range of C. japonica.

抄録全体を表示