A comparative study of the development ofPhascolosoma agassizii,Golfingia pugettensis, andThemiste pyroideswith a discussion of developmental patterns in the Sipuncula

Abstract

Breeding seasons, eggs, cleavage and development are described for three species of sipunculids, Phascolosoma agassizii, Golfingia pugettensis, and Themiste pyroides, from the San Juan Archipelago on the Northwestern Pacific Coast of the United States. Phascolosoma agassizii breeds from the middle of June to early September, Golfingia pugettensis from late October through January, and Themiste pyroides from March to August. The egg of P. agassizii is the smallest (140 × 110 microns) with a low yolk content, whereas that of G. pugettensis is intermediate in size (160 microns in diameter) and yolk content, and the egg of T. pyroides is the largest of the three (190 microns in diameter) with a dense concentration of yolk. Cleavage in all three species is of the typical spiral pattern except that in the 8-cell stage of T. pyroides the micromeres of the A, B, and C quadrants arc larger than the macromeres. Both P. agassizii and G. pugettensis undergo an indirect development with a pelagic trochophore stage which metamorphoses into a benthi-pelagic larval stage. The benthi-pelagic larva of G. pugettensis is lecithotrophic, losing its cilia at 20 days of age when it begins a gradual transformation into the form of the adult. The benthi-pelagic larva of P. agassizii is planktotrophic and its metamorphosis into a juvenile form has not been observed. Themiste pyroides develops directly within the thick jelly coat of the egg, hatching out as a small, crawling worm. It has no pelagic form, nor do its prototroch cells at any time develop cilia. The developmental patterns of Phascolosoma agassizii, Golfingia pugettensis, and Themiste pyroides are compared with each other as well as with previously studied species. In view of the findings contributed by this study, developmental patterns within the Sipuncula are discussed, special problems in sipunculid development are reviewed, and the terminology regarding metamorphosis and larval types of sipunculids is clarified. Developmental patterns are interpreted in terms of their possible evolutionary significance within the phylum.