Post‐Embryonic Development and Formation of the Heterocoelic Aquiferous System in Two Species of Calcareous Sponges (Calcarea, Porifera)

We characterized the morphogenetic processes of larval metamorphosis and the development of the olynthus and heterocoelic aquiferous system (AS) in Sycettusa hastifera (syconoid) and Paraleucilla magna (leuconoid) (Porifera, Calcarea). Metamorphosis and development up to the olynthus were similar in both species. During metamorphosis, apparently, the juvenile's cell lineages were established by invagination of the ciliated pole into the larval cavity, while posterior pole cells covered the juvenile. Ciliated cells seemed to differentiate into an inner cell mass (ICM) (forming choanoblasts and scleroblasts), while granular cells seemed to form mainly the pinacoderm. Then, cavitation of the ICM formed the asconoid AS of the olynthus. Choanocyte chamber morphogenesis for the syconoid and leuconoid AS seemed to be epithelial, but followed different paths. In S. hastifera, the chambers were formed by folding the primordial choanoderm towards the external portion of the sponge, whereas in P. magna, invagination of the choanoderm into the spongocoel segregated groups of choanocytes into spherical chambers. The morphogenesis of the heterocoelic AS in these calcareans is likely different from that in Demospongiae (leuconoid AS is established via mesenchymal-to-epithelial morphogenesis) but similar to Homoscleromorpha (AS arises through epithelial morphogenesis). Characterizing the post-embryonic development in these species is the first step toward understanding the mechanisms that regulate the ontogeny and evolution of Porifera's primary synapomorphy: the aquiferous system.