DEVELOPMENT, SUBSTRATUM SELECTION, DELAY OF METAMORPHOSIS AND GROWTH IN THE SEASTAR,MEDIASTER AEQUALISSTIMPSON

Abstract

1. The eggs of Mediaster aequalis (Asteroidea, family Goniasteridae) are 1.0 to 1.2 mm in diameter. Development is lecithotrophic with a wrinkled blastula and modified brachiolaria larva. The metamorphosed juveniles are about 1.3 mm diameter. Estimated growth rates are less than 2 mm per month.2. In the laboratory, M. aequalis larvae settled on tubes of the polychaete Phyllochaetopterus prolifica, but not on other substrata tested, 38 days after fertilization.3. If Phyllochaetopterus tubes were not present, metamorphosis was postponed. Some larvae survived 14 months and still completed metamorphosis when offered tubes of Phyllochaetopterus.4. In the laboratory, juvenile M. aequalis ate settling M. aequalis brachiolaria larvae.5. Juvenile M. aequalis and juveniles of several other asteroid species were found commonly on tubes of Phyllochaetopterus. Their feeding was observed in the field. Tubes of Phyllochaetopterus may play a significant role in the life of asteroids in Puget Sound.6. In asteroids, lecithotrophic development is generally associated with farmation of larger juveniles at metamorphosis (Hyman, 1955). Possible implications of this phenomenon are discussed.