Embryonic and larval development of the peanut worm Themiste pyroides (Sipuncula: Sipunculoidea) from the Sea of Japan

Abstract

Summary Embryonic and larval development are described for the sipunculan Themiste pyroides from the Peter the Great Bay, the Sea of Japan. The eggs are 120 μm in diameter and coated by a 2 μm thick envelope, with a dense concentration of yolk. Egg cleavage is complete, spiral, and unequal. T. pyroides from the Sea of Japan is characterized by an indirect development with two pelagic larval stages—a lecithotrophic trochophore and a lecithotrophic pelagosphera that is short lived. At the blastula stage, the embryos begin to move in the near-bottom layer of water. At this stage, the embryo resembles a dense sphere with a wide ciliary band extending along its equator. The actively swimming trochophore after 3 days of development metamorphoses to a lecithotrophic pelagic larva, the pelagosphera. The pelagosphera of T. pyroides does not feed but relies on stored yolk, whose large granules are visible in the gut. At day 14, larvae finally settle on the bottom and creep peristaltically. Results of our study of the development of T. pyroides from the Sea of Japan substantially differ from the earlier published data (Rice, 1967) on development of the same species in British Columbia. The species from the East Pacific develops directly within the jelly coat, the embryo hatching from the jelly as a small, crawling worm (see Rice, 1967). These differences correlate with the differences in size of the eggs, time of breeding season and water temperature in the localities of this amphi-pacific species in the East and West Pacific.