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

The homology of larval forms, and particularly their feeding methods, has b een a major element in some recent discussions about animal phylogeny. "Dow nstream feeding" is one of two main larval-feeding modes and is usually equ ated 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 involvin g the prototroch. It is often argued that the presence of planktotrophic la rvae 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 p arsimony methodology, in relation to Spiralia, Trochozoa, and with particul ar 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 r eversals, and polymorphic terminals coded as having downstream larval feedi ng. 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 p hylogeny predicated on the assumption that downstream-feeding larvae are pl esiomorphic are re-assessed.