MALDONADO, Manuel; Centro de Estudios Avanzados de Blanes: Ecological and evolutionary roles of the larval stage of the porifera: a new perspective

The poriferan larva is often seen as an evolutionary acquisition of the sessile Porifera, needed for dispersal and selective settlement. This view contradicts the fact that sponge larvae are not known to have sensory organs or a nervous system. Such anatomical constraints should mean that sponge larvae have a limited ability for orientation and selection of suitable settlement sites, compared to those of other invertebrate larvae. In a comprehensive review, I show that sponge larvae use a combination of active and passive mechanisms to maximize selective settlement and minimize juvenile mortality. More importantly, some larval abilities are based on remarkable histological features. Interestingly, sponge larvae are revealed to be architecturally closer than adult sponges to the remaining metazoans. It may mean that the basic multicellular organismal architecture from which diploblasts evolved, that is the putative planktonic archimetazoan, was more similar to a modern poriferan larva than to an adult sponge. Alternatively, it may also mean that other metazoans evolved from a neotenous larva of ancient sponges. A review of some aspects of the larval and adult histology shows that the Porifera posses some features of intriguing evolutionary significance: 1) widespread occurrence of internal fertilization and a notable diversity of gastrulation modes, 2) dispersal through architecturally complex lecithotrophic larval types, in which an ephemeral archenteron and multiciliated and syncytial cells occur, 3) acquisition of direct development by some groups, 4) replacement of choanocyte-based filter-feeding by carnivory in some sponges. This histological information, when discussed in light of recent molecular findings, provides a new evolutionary perspective that leads to a drastic reinterpretation of the evolutionary meaning of the Porifera within the Metazoa.