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@ARTICLE{TreeBASE2Ref17272, author = {Greg Rouse}, title = {Bias? What bias? The evolution of downstream larval-feeding in animals.}, year = {2000}, keywords = {}, doi = {10.1046/j.1463-6409.2000.00040.x}, url = {}, pmid = {}, journal = {Zoologica Scripta}, volume = {29}, number = {3}, pages = {213--236}, abstract = {The homology of larval feeding modes has been a major element in recent discussions about animal phylogeny. 'Downstream feeding' is one of the two main larval feeding modes, and is usually equated to an opposed-band system involving ciliary bands called the prototroch and metatroch. Larval feeding in Spiralia 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 in relation to the Spiralia, Trochozoa and particularly polychaete annelids. Cladistic parsimony analyses are performed based on data sets of Rouse & Fauchald (1997) and Rouse (1999), with an additional character based on downstream larval-feeding. Methods adopted to assess the possibility of a bias in transformations towards loss of downstream larval-feeding include: expanded primary homology arguments, character transformations favoring reversals, and polymorphic terminals coded as having downstream larval-feeding. These measures all tend to favor the possibility that downstream larval-feeding will be plesiomorphic, and tend to be lost rather than gained. Nevertheless, all analyses show that downstream larval-feeding is not plesiomorphic, and appears to have evolved multiple times. The results support a conclusion that the prototroch has become associated with feeding a number of times. Hypotheses of metazoan phylogeny that are predicated on the assumption that downstream-feeding larvae are plesiomorphic (e.g. Peterson et al. 1997) should be re-assessed.} }