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Trochophores etc.

Greg Rouse gregr at bio.usyd.edu.au
Mon May 1 21:29:30 EST 2000


There is a review of this issue in a paper I did last year.

Trochophore concepts: ciliary bands and the evolution of larvae in 
spiralian Metazoa.
Biological Journal of the Linnean Society 66(4):411-464.

ABSTRACT: 'Trochophore' is a term used in a strict sense for larvae having 
an opposed-band method of feeding, involving a prototroch and metatroch. 
Other ciliary 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 metazoan phyla (including all members of the Spiralia). The 
name trochophore is also often applied to larvae that do not conform to the 
above definition. A cladistic analysis of spiralian taxa (with special 
reference to polychaete annelids), based on a suite of adult and larval 
characters, is used to assess several hypotheses: (1) That the trochophore 
(in a strict sense) is a plesiomorphic form for the Spiralia; (2) That the 
strictly defined trochophore is plesiomorphic for members of the Spiralia 
such as the Polychaeta. The homology of each of the various separate 
ciliary bands of spiralian larvae, and features such as the apical tuft and 
protonephridia is also assessed.  The results favour the conclusion that the 
trochophore, if defined as a feeding larval form using opposed bands, 
should not be regarded as an ancestral (= plesiomorphic) type for the 
Spiralia, or any other large taxon such as the Polychaeta or Mollusca. The 
trochophore is re-defined as a larval form with a prototroch and 
plesiomorphic features such as an apical plate and protonephridia. This 
broad definition covers a wide variety of larvae, and matches the current 
usage more accurately than the restricted term. Features such as the 
neurotroch, telotroch and opposed-band feeding show convergence and 
reversals. The nature of the metatroch requires further investigation. The 
presence of a prototroch (and hence a trochophore larvae) is used to 
identify an apomorphy-based taxon, Trochozoa, that includes the first 
ancestor to have evolved a prototroch and all its descendants. This 
minimally includes the Annelida (sensu lato), Echiura, Entoprocta, Mollusca 
and Sipuncula and is a less inclusive taxon than the Spiralia.  


As for how to subdivide the various forms of trochophores among 
polychaetes, consensus is yet to be reached.

greg


*********************************
Greg Rouse
School of Biological Sciences A08
University of Sydney
N.S.W. 2006
Australia

gregr at bio.usyd.edu.au
Tel.     (02) 9351 5571
Fax     (02) 9351 4119
International: 61 2 replaces 02
http://wallace.bio.usyd.edu.au/papers/gregr/


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