Where is my mind? How sponges and placozoans may have lost neural cell types

Abstract

Recent phylogenomic evidence suggests that ctenophores may be the sister group to the rest of animals. This phylogenetic arrangement opens the possibility that sponges and placozoans could have lost neural cell types or that the ctenophore nervous system evolved independently. We critically review evidence to date that has been put forth in support of independent evolution of neural cell types in ctenophores. We observe a reluctance in the literature to consider a lost nervous system in sponges and placozoans and suggest that this may be due to historical bias and the commonly misconstrued concept of animal complexity. In support of the idea of loss (or modification beyond recognition), we provide hypothetical scenarios to show how sponges and placozoans may have benefitted from the loss and/or modification of their neural cell types.

Keywords: Ctenophora; Placozoa; Porifera; animal evolution; loss; nervous system.

Figure 1.

Evolutionary relationships of the five main branches of animals based on recent phylogenetic analyses [–6]. (Online version in colour.)

Figure 2.

Hypothetical scenarios for the loss of neural cell types in sponges. (a) A nerve-net type nervous system in ancestral sponge may have provided a physical obstruction to maximal filtering efficiency. Losing this nerve net may have been selectively advantageous. (b) The metabolic cost of neural mechanisms is high. The loss of neural cell types may have provided a selective advantage by allowing for the reallocation of resources to filtering activities. (c) Ancient endoparasitic lifestyle of Porifera and/or Placozoa. It is possible that the stem ancestors of either poriferans or placozoans lived as endoparasites, and that some event led to them reverting to a free-living lifestyle. As parasitism can be associated with loss of many cell types including neurons [59], this hypothetical scenario might explain why it is not possible to represent neural cell types in poriferans and placozoans. (Online version in colour.)