REPRODUCTION AND DEVELOPMENT OF THE HERMAPHRODITIC SEA-STAR,ASTERINA MINORHAYASHI

Abstract

1. The breeding season of Asterina minor is estimated to be during the month of May in Kushimoto, Japan. A. minor shows a characteristic breeding assemblage and the eggs are laid on the substratum in a mass spawning. The eggs are not protected by the adults. 2. A. minor is a spatial hermaphrodite, where ovaries and testes in an individual become mature simultaneously. Isolated individuals are capable of self-fertilizing and the self-fertilized eggs develop normally. 3. The spawned eggs are spherical, yellow, and 437 µm in average diameter. They attach to the substratum with a sticky jelly layer. Cleavage is total and radial. 4. Eggs through the wrinkled blastula stage develop into a pear-shaped brachiolaria bearing three brachiolar arms within the fertilization membrane. 5. About four days after spawning, the brachiolariae hatch from the fertilization membrane and creep on the substratum with well-developed brachiolar arms. There is no evidence of pelagic life in the present species. 6. One day after hatching, brachiolariae attach firmly to the substratum with the brachiolar arm and undergo a rapid transformation of the body (metamorphic climax). Metamorphosis is completed with the opening of the mouth about 10 days after spawning. The newly metamorphosed juvenile is about 700 µm in diameter and each arm bears two pairs of the tube-feet and one red eye-spot at the base of the terminal tentacle. 7. The reproduction and larval development of A. minor are unique, and the study may prove a good guideline for understanding the evolution of reproduction and development in Asteroidea.