HATCHING RATE AND FACTORS OF INFLUENCE ON EMBRYO MORTALITY OF NEPTUNEA-ARTHRITICA BERNARDI

The hatching rate of the sea snail Neptunea arthritica in Usu Bay, Hok kaido was investigated on the basis of embryonic developmental stages and the hatching process. Factors influencing embryo mortality were al so examined. The development of embryos was classified into four stage s: egg-swallowing, protonch-forming, shell-developing and juvenile. Th e hatching process was investigated by examining seasonal changes in t he proportions of seven egg capsule types, including egg capsules with an embryo in the four developmental stages mentioned above, and three additional egg capsules, one with a hatched juvenile, one with an emb ryo that suffered from predation, and one with an embryo that had ceas ed developing. The hatching rate of embryos was calculated from the ex pectant number of juveniles in September and the number of egg capsule s in July, and estimated to be 18.6%. Of the factors that influence th e hatching rate, such as predation, exfoliation of egg masses, cessati on of development and coating of sponges, predation and exfoliation we re the most common causes for decreased hatching rates, followed by de velopmental cessation. Egg masses deposited on soft substrata, such as ropes and holdfasts of Laminaria sp., were susceptible to detachment more than those on solid substrata, such as rocks and concrete blocks. Cessation of embryonic development decreased with an increae in the n umber of egg capsules per egg mass.