Bias? What bias? The evolution of downstream larval-feeding in animals

The homology of larval-feeding modes has been a major element in recent dis cussions about animal phylogeny. 'Downstream-feeding' is one of the two mai n larval-feeding modes, and is usually equated to an opposed-band system in volving ciliary bands called the prototroch and metatroch. Larval-feeding i n Spiralia is reviewed here and the homology hypothesis of downstream larva l-feeding is expanded, encompassing any feeding involving the prototroch. I t is often argued that the presence of planktotrophic larvae using downstre am-feeding is plesiomorphic among spiralian animals, and that there is a bi as in transformations, such that feeding larvae tend to be lost rather than gained. These hypotheses are assessed in relation to the Spiralia, Trochoz oa and particularly polychaete annelids. Cladistic parsimony analyses are p erformed based on datasets of Rouse & Fauchald (1997) and Rouse (1999), wit h an additional character based on downstream larval-feeding. Methods adopt ed to assess the possibility of a bias in transformations towards loss of d ownstream larval-feeding include: expanded primary homology arguments, char acter transformations favouring reversals and polymorphic terminals coded a s having downstream larval-feeding. These measures all tend to favour the p ossibility that downstream larval-feeding will be plesiomorphic, and tends to be lost rather than gained. Nevertheless, all analyses show that downstr eam larval-feeding is not plesiomorphic, and appears to have evolved multip le times. The results support a conclusion that the prototroch has become a ssociated with feeding a number of times. Hypotheses of metazoan phylogeny that are predicated on the assumption that downstream-feeding larvae are pl esiomorphic (e.g. Peterson et al. 1997) should be re-assessed.