Pinna bicolor is a Pinnidae shellfish, inhabiting shallow waters from temperate to tropical zone. The bivalve possesses large kidneys filled with metal-rich granules. The concentration of manganese in dried kidney tissues of pen shells sampled from Ariake Sea, Hagi, and Kamigoto were 58,500 μg・g^-1, 34,300 μg・g^-1, and 5,700 μg・g^-1, respectively. At the same time, manganese concentrations of 600 to 1,400 μg・g^-1, 260 μg・g^-1, and 130 μg・g^-1 were found for sediment samples from Ariake Sea, Hagi, and Kamigoto, respectively. There were significant correlations between manganese concentration in the kidney and in the mud (p<0.01). These results suggest that manganese concentration in Pinna bicolor kidney can be an indicator of ionized manganese elution due to hypoxia or ooze deposition in the hydrosphere. Because the manganese concentration of samples at Ariake Sea was higher than ever reported. I assume that high elution of ionized manganese is occurring widely in the habitat of Pinna bicolor in Ariake Sea.

The present study is to describe the effect of water temperature on the swimming behavior of Todarodes pacificus paralarvae between 16 and 22.5°C, the range known to be suitable for normal embryonic development, and at 23.5°C, known to be warmer than the optimal range. Paralarvae obtained through artificial fertilization were reared at different temperatures, and those capable of swimming were observed in a 29 cm-deep tank. The swimming rate of paralarvae reared at temperatures between 19.5 and 23.5°C increased at Stage 31, suggesting that this temperature range was optimal for normal swimming, and that swimming ability was developed at this stage. The swimming rates of paralarvae reared at below 18.5°C were low at all temperatures after hatching, implying that egg masses must be maintained above this temperature for successful swimming. However, the swimming rate of paralarvae at Stage 32 dropped at 23.5°C. Paralarvae placed in a 150 cm-deep cylindrical aquarium always swam upward and remained near the surface. This behavior supports previous results of field studies showing hatchlings of T. pacificus were active near the surface. Mantle length of paralarvae at developmental stages 28 and 31 was significantly different from those reared in water of 16–22°C, suggesting that low temperature negatively affects the growth of the paralarvae in regards to swimming ability. These findings may help us improve the predictability of stock fluctuation of T. pacificus affected by environmental changes such as global warming.

To test the hypothesis that vulnerability to predation of small-sized fishes increases during nighttime, the fish species composition, abundance, biomass and stomach contents of potential predators, and the mortality rate during tethering experiments were compared between daytime and nighttime at a seagrass bed in the central Seto Inland Sea, Japan. A total of 494 fishes belonging to 30 taxa were collected using a round seine on 28 and 30 August 2007. Abundance and biomass of potential fish predators (piscivorous fish >82.5 mm in total length) were higher in nighttime than in daytime. Sebastes cheni juveniles and Rudaris ercodes were predated on by Sebastes inermis and Lateolabrax japonicus. Predation rate of juvenile S. cheni, a dominant species in the fish community, estimated from the ratio of the number of juveniles predated to that of population (sum of surviving and predated juveniles) during nighttime (1930-2100 h) was 5.1%. Tethering experiments for 6 h at the seagrass bed indicated that the mortality rate of juvenile S. cheni in nighttime was significantly higher than that in daytime. It is plausible that seagrass beds contribute as foraging habitats for piscivorous fish during nighttime although they are important nurseries for a variety of fish species.