Oogenesis and embryogenesis in a cryptogenic species of calcareous sponge (Calcaronea, Heteropiidae) in the southwestern Atlantic

Calcarea is a diverse group of sponges, in which individuals are generally small and found in cryptic habitats. These characteristics hamper the access to them and, consequently, the knowledge about several aspects of their biology, including reproduction. Little is known about many reproductive aspects of these sponges and broad generalizations are made upon few studied species. Therefore, investigations dealing with the oogenesis and embryogenesis of more species are still necessary. Sycettusa hastifera is an alien species in the southwestern Atlantic. Knowledge about its reproduction would not only help to improve our comprehension of reproduction in Calcaronea but would also help to understand the capacity of this exotic species to colonize. Histological and electron microscopy analyses of individuals sampled at Arraial do Cabo (Rio de Janeiro, Brazil) showed that oogenesis in S. hastifera was long (4–6 months) and nutrients were acquired by association with nurse cells and endocytosis of bacteria, yielding heterogeneous and complex yolk inclusions. Cleavage, inversion, and amphiblastula larvae were similar to what has been observed in other calcaroneans, except for the larval cavity lacking amoeboid cells and the relatively long swimming period of the amphiblastula (on average 12 h, but reaching 72 h). Our results indicate that the vitellogenic mechanism characterized as mixed (an ancestral characteristic of Calcaronea) is strongly influenced by autosynthesis; during cleavage, cell fates could be determined from embryo to larva; and inversion of the embryo (which will become the amphiblastula larva) directly into the choanocyte chamber is an ancestral characteristic of Calcaronea. Despite the absence of amoeboid cells or other structures to nourish the larva, the long swimming period suggests that vitellogenesis is sufficient for the maintenance of the embryo and larva until metamorphosis. This characteristic may facilitate the spread of this cryptogenic species to new regions.