The induction of larval metamorphosis in the sea urchins Anthocidaris crassispina and Pseudocentrotus depressus was investigated in the laboratory, using waters conditioned by 15 different macroalgae combined with the periphytic diatom Navicula ramosissima. Larvae of P. depressus did not metamorphose, but larvae of A. crassispina showed a high incidence of metamorphosis, especially in waters conditioned by coralline red algae or brown algae. High inductive activity for larval metamorphosis was detected in Corallina pilulifera-conditioned water during a 2.5-year investigation, but the activity was relatively low in February or March and in September, the off-growth seasons of the alga. By contrast, Ulva pertusa-conditioned water did not show metamorphosis-inducing activity except in spring or early summer. These results indicate that during their growth phase, red and brown algae release into their environment active substances that are involved in the larval metamorphosis of A. crassispina.

抄録全体を表示

Diatom-based film grown on plastic plates, consisting of several species of periphytic diatom and multispecies of bacteria, is used to induce larval metamorphosis during mass production of sea urchin juveniles. This study attempted to elucidate the roles of diatoms and bacteria in larval metamorphosis of the sea urchins Pseudocentrotus depressus and Anthocidaris crassispina, and to clarify the characteristics of the metamorphosis-inducing cue(s) in the film. Laboratory-grown diatom-based films induced larval metamorphosis in both species of sea urchin, but the response rate was higher in P. depressus. Diatoms collected on glass-fiber filters induced metamorphosis in larvae of both species, although the inducing activity was significantly less than that of the diatom-based films. By contrast, no larva metamorphosed on bacterial films that formed from the filtrates. These results suggest that bacteria alone cannot induce metamorphosis and that diatoms play a major role in the induction. Antibiotic treatment reduced the inducing activity in diatom-based films, even though the treatment did not control bacterial growth. None of five isolated species of periphytic diatom induced larval metamorphosis by itself. Glutaraldehyde (5%), HCl (1N), and heat treatment (45°C) destroyed the inducing activity of the films, but lectin treatments (LCA, SBA, and WGA) had no effect. This suggests that the metamorphosis-inducing cue(s) in diatom-based films is (are) unstable and that carbohydrates do not play a main role in inducing metamorphosis.

抄録全体を表示

Larval culture of the bivalve Barnea dilatata was attempted with artificially fertilized eggs obtained from adults collected on October 3, 1998, from Hayatsue Estuary off Kawasoe-machi, Saga Prefecture. After fertilization, eggs developed in one day to D-shaped larvae with an average shell length (SL) of 60μm. Larvae fed Chaetoceros gracilis grew to 417±66μm, and thereafter no further growth was observed. The cultures provided larvae with a maximum shell length of 490μm, which suggests that the pediveliger shell length was within the range of 400 to 490μm. Experiments revealed that during the process of metamorphosis to plantigrade, planktonic larvae may choose a substratum from the viewpoint of physical factors, e.g, grain size, although biological factors such as bacterial population in the mud flat may also be important. When pediveligers were exposed to mud collected from the flat, they initially swam actively but then started crawling on the mud. The transition from swimming to crawling behavior was reversible and an alternation of crawling and swimming back into the water column was repeatedly observed. Finally, larvae were observed to burrow into themud using their foot. The whole process from initial burrowing in the mud until completion of metamorphosis to plantigrade of the shell and foot took one day.

抄録全体を表示

The population of the green mussel Perna viridis (L.) at Enoshima Island (Sagami Bay, Kanagawa, Japan: lat. 35°30′N) was sampled monthly from April, 1999, to September, 2001, and the annual reproductive cycle was determined by histological examination. In this species, genital tubules containing germ cells occur in the mantle tissue as well as in the body tissue. Already specified above testicular and ovarian activity peaked during August and September in 1999, but from mid-June to August in 2000. We suggest that warmer water in early summer in 2000 compared to that in 1999 caused this shift in the reproductive season. After the end of the spawning season, some gonadal recrudescence occurred in late autumn in both sexes; however, the gonads did not mature and regressed during the winter months, possibly due to low water temperature (10-13°C). In spring, male and female gonads developed once again, and spawning occurred from summer to early autumn. These results indicate that although P. viridis was originally a subtropical species, the Enoshima population of P. viridis is established there throughout the year and is able to reproduce.

抄録全体を表示