Trochophore concepts: ciliary bands and the evolution of larvae in spiralian Metazoa

'Trochophore' is a term used in a strict sense for larvae having an opposed -band method of feeding, involving a prototroch and metatroch. Other ciliar y bands such as a telotroch and neurotroch may be present. The trochophore has been proposed to represent the ancestral larval form for a group of met azoan phyla (including all members of the Spiralia). The name trochophore i s also often applied to larvae that do not conform to the above definition. A cladistic analysis of spiralian taxa (with special reference to polychae te annelids), based on a suite of adult and larval characters, is used to a ssess several hypotheses: (1) that the trochophore (in a strict sense) is a plesiomorphic form for the Spiralia; (2) that the strictly defined trochop hore is plesiomorphic for members of the Spiralia such as the Polychaeta. T he homology of each of the various separate ciliary bands of spiralian larv ae, and features such as the apical tuft and protonephridia is also assesse d. The results favour the conclusion that the trochophore, if defined as a feeding larval form using opposed bands, should not be regarded as an ances tral (= plesiomorphic) type for the Spiralia, or any other large taxon such as the Polychaeta or Mollusca. The evidence suggests that the various cili ary bands have differing evolutionary histories, and only the Echiura (poss ibly an annelid group) has members with the classical trochophore. The troc hophore is re-defined as a larval form with a prototroch. This broad defini tion covers a wide variety of larvae, and matches the current usage more ac curately than the restricted term. Features such as the neurotroch, telotro ch and opposed-band feeding show convergence and reversals. The nature of t he metatroch requires further investigation. The presence of a prototroch ( and hence trochophore larvae) is used to identify an apomorphy-based taxon, Trochozoa, that includes the first ancestor to have evolved a prototroch a nd all its descendants. This minimally includes the Annelida (sensu lato), Echiura, Entoprocta, Mollusca and Sipuncula and is a less inclusive taxon t han the Spiralia. (C) 1999 The Linnean Society of London.