The epitome of hand waving? Larval feeding and hypotheses of metazoan phylogeny.

The homology of larval forms, and particularly their feeding methods, has been a major element in some recent discussions about animal phylogeny. "Downstream feeding" is one of two main larval-feeding modes and is usually equated to an opposed-band system with ciliary bands called the prototroch and metatroch. Feeding in larvae is reviewed here and the homology hypothesis of downstream larval feeding is expanded, encompassing any feeding involving the prototroch. It is often argued that the presence of planktotrophic larvae using downstream feeding is plesiomorphic among spiralian animals, and that there is a bias in transformations, such that feeding larvae tend to be lost rather than gained. These hypotheses are assessed using cladistic parsimony methodology, in relation to Spiralia, Trochozoa, and with particular reference to polychaete annelids. Methods adopted for the possibility of a bias in transformations toward loss of downstream larval feeding include: expanded primary homology arguments, character reconstructions favoring reversals, and polymorphic terminals coded as having downstream larval feeding. Nevertheless, all analyses show that downstream larval feeding appears to have evolved multiple times from a lecithotrophic condition. The results support a conclusion that the prototroch was primarily locomotory, and has become associated with feeding a number of times. Hypotheses of metazoan phylogeny predicated on the assumption that downstream-feeding larvae are plesiomorphic are re-assessed.