TROCHOPHORE CONCEPTS: DOWNSTREAM FEEDING, CILIARY BANDS, AND EVOLUTION OF LARVAE IN POLYCHAETES
Rouse, G. W.
School of Biological Sciences, University of Sydney, N.S.W. 2006 Australia
The idea that a free-swimming 'trochophore' with an opposed-band feeding system is a primitive larval form for the 'Trochozoa' (Mollusca, Sipuncula, Annelida, and Echiura etc.) or 'Lophotrochozoa' has a long tradition. It has also been proposed, and is generally accepted, that in the evolution of marine animals there is a biased transition from feeding larvae to non-feeding larvae. The homology hypotheses of the trochophore, downstream larval feeding and various larval features are combined with a matrix of over 100 other morphological characters developed for the Polychaeta and other 'Trochozoa' by Rouse and Fauchald (1997). The results suggest the feeding trochophore is a rare and homoplastic form; that lecithotrophic larvae are plesiomorphic for the Polychaeta; that various ciliary bands may have evolved several times and that downstream feeding has also evolved several times. There is no sign of any biased transition to non-feeding larvae.