Inside or Outside the Pits : Variable Mobility in Conspecific Sea Urchin, Anthocidaris crassispina (A. Agassiz)

The sea urchin Anthocidaris crassispina (A. Agassiz) lives either in small pits or outside the pits on rock surfaces. Difference in movement between individuals living inside and outside the pits was investigated. Sea urchins living inside the pits never moved out of their pits either in summer or winter, or during the day or night, while urchins living outside the pits moved in both seasons, especially during the night. Among urchins living outside the pits, individuals occupying relatively sheltered microhabitats (hollows or edges; see Fig. 1) exhibited less movement than individuals occupying open flats. On transplantation to an open area without pits, some individuals previously occupying the pits showed slight movement. On the other hand, most of the individuals previously outside the pits, when transplanted into an area with many vacant pits, occupied them and showed reduced movement, although neither transplanted groups of individuals adjusted their movement perfectly to their new microhabitats during the observation period. These results suggest that the movement of the sea urchins is, to some extent, flexible and dependent on microhabitats and that they preferred sheltered microhabitats. A morphological and physiological investigation showed that sea urchins inside the pits had similar gut weight (with the content), shorter lateral spines and heavier gonads than sea urchins outside the pits of similar test diameter. The adaptive significance of pit-dwelling in A. crassispina is discussed

Size-related Egg Production in a Simultaneous Hermaphrodite, the Sea Hare Aplysia kurodai Baba (Mollusca : Opisthobranchia)

The relationship between body size and egg production was investigated in a simultaneous hermaphrodite, the sea hare Aplysia kurodai Baba. Individuals of similar size were experimentally paired and the pairs were kept isolated from each other in the laboratory. Under this condition, large individuals produced more eggs per day than small individuals, while the size of eggs did not change with adult body size. It is argued that the positive correlation between body size and egg production rate is likely to occur in field populations of this sea hare

Copulatory Load in a Simultaneous Hermaphrodite Aplysia kurodai Baba, 1937 (Mollusca : Opisthobranchia)

The movement of Apfysia kurodai Baba, 1937 was examined in a field population in relation to copulation. It was confirmed that the mobility of the bottom individual of a copulatory chain is curtailed simply by the total volume of the other individuals it carries (copulatory load)

Nuclear Sex-Determining Genes Cause Large Sex-Ratio Variation in the Apple Snail Pomacea canaliculata

Evolutionary maintenance of genetic sex-ratio variation is enigmatic since genes for biased sex ratios are disadvantageous in finite populations (the “Verner effect”). However, such variation could be maintained if a small number of nuclear sex-determining genes were responsible, although this has not been fully demonstrated experimentally. Brood sex ratios of the freshwater snail Pomacea canaliculata are highly variable among parents, but population sex ratios are near unity. In this study, the effect of each parent on the brood sex ratio was investigated by exchanging partners among mating pairs. There were positive correlations between sex ratios of half-sib broods of the common mother (r = 0.42) or of the common father (r = 0.47). Moreover, the correlation between full-sib broods was very high (r = 0.92). Thus, both parents contributed equally to the sex-ratio variation, which indicates that nuclear genes are involved and their effects are additive. Since the half-sib correlations were much stronger than the parent–offspring regressions previously obtained, the variation was caused by zygotic sex-determining genes rather than by parental sex-ratio genes. The number of relevant genes appears to be small

Distribution of Two Species of Conchoderma (Cirripedia : Thoracica) over the Body of a Sea Snake, Laticauda semifasciata (Reinwardt), from the Kii Peninsula, Southwestern Japan

Two species of Conchoderma were found on a sea snake, Laticauda semifasciata (Reinwardt), collected on the west coast of the Kii Peninsula. A total of 223 individuals of C. virgatum and 6 of C. hunteri in 19 clumps were attached to the snake's body. The barnacles ranged in size from 1.4 mm (cypris larvae) to 18.2 mm in capitulum length in C. virgatum, and from 10.7 to 14.4 mm in C. hunteri. The size of the smallest gravid individuals in both species was between 10 and 11 mm. The distribution of C. virgatum on the snake was non-random both longitudinally and dorso-ventrally, with more barnacles in the posterior region and on the ventral side of the snake, respectively. The proportion of gravid individuals increased towards the tail

Cropping of Sea Anemone Tentacles by a Symbiotic Barnacle

Volume: 197Start Page: 315End Page: 31

Predation on the apple snail, Pomacea canaliculata (Ampullariidae), by the Norway rat, Rattus norvegicus, in the field

Volume: 43Start Page: 349End Page: 35