Meeting Abstract
Gastrulation, the formation of multiple germ layers resulting in an epidermis, a middle layer, and a feeding gastrodermis, is disputed to take place in sponges, the earliest diverging metazoans. Glass sponges (Class Hexactinellida) are one of only two poriferan groups that form a feeding epithelium – flagellated chambers – during embryogenesis, as larvae. Can this be equated to gastrulation? The fate of the larval chambers and of other tissues during metamorphosis is unknown due to the difficulty of obtaining larvae from this deep-sea group. Also, understanding metamorphosis in glass sponges is challenging as very little tissue is present in late stages of metamorphosis and the syncytial nature of the tissue makes it difficult to trace cell fates. We studied metamorphosis in Oopsacas minuta a small hexactinellid that inhabits caves in the Mediterranean. We used three-dimensional models to map the fate of larval tissues through metamorphosis. Intriguingly, multiciliated cells that form a belt around the larva are discarded in the first stages of metamorphosis and do not appear to be involved in forming the flagellated chambers of the juvenile sponge. We found that larval flagellated chambers are retained throughout metamorphosis and become the first pumping and feeding chambers of the juvenile sponge. As O. minuta settles, larval flagellated chambers are enlarged by using the larval yolk supply. These observations suggest that in O. minuta the feeding epithelium – the flagellated chambers – are internalized, and the outer epidermis – the reticular syncytium – is externalized during embryogenesis and this relationship is retained through metamorphosis into the adult. In this way development of the germ layers in this hexactinellid sponge strongly resembles gastrulation processes in other animals.
