LEYS, S.P.*; EERKES-MEDRANO, D.I.; The University of Alberta, AB; University of Victoria, BC: Gastrulation in the calcareous sponge Sycon

Haeckel�s studies of development in the calcareous sponge Sycon (1872) led him to develop the �Gastraea Theory�, which proposes that during the development of all metazoans, including sponges, gastrulation occurs by invagination to produce a functional gut. The corollary is that the primary germ layers are homologous among all metazoans. His observations, that gastrulation in the Calcarea occurs by invagination of a ciliated larva upon settlement and metamorphosis, are repeated in texts today. We have re-examined embryogenesis, larval development and metamorphosis in Sycon using scanning and transmission electron microscopy in order to understand how and when gastrulation occurs. Oogenesis and fertilization occurs in the mesohyl. Cleavage gives rise to a flattened, hollow, blastula-like embryo, which already has at least two differentiated cell types and cilia that face into the �blastocoel�. The embryo is pulled through the choanoderm, everting to turn the ciliated cells outward, and cells immigrate into the new blastocoel. The free-swimming larva has columnar ciliated cells on the anterior hemisphere, and globular non-ciliated cells at the posterior hemisphere. Several cell types are embedded in a dense extracellular matrix in the larval interior. Although the settled juvenile initially has an unusual indentation on its anterior end that is rapidly lost as the outer pinacoderm forms, no stage has been found that represents the Gastraea described by Haeckel. Considering the pivotal position occupied by the Calcarea as the possible sister-group to all other Metazoa (Borchiellini et al., 2001), these results call for a reinterpretation of Haeckel�s observations and of the relationships of the primary germ layers in basal metazoan phyla. Haeckel E. 1872. Bd. 1-3. Verlag von Georg Reimer, Berlin. Borchiellini C. et al., 2001. Evol. Biol. 14: 171-179.