Meeting Abstract

S8-2.1  Saturday, Jan. 5  The bearing of larval morphology on decapod phylogeny CLARK, Paul F.*; CUESTA, Jose A.; The Natural History Museum, London; Instituto de Ciencias Marinas de Andalucia p.clark@nhm.ac.uk

Traditionally, brachyuran classifications have been established on the basis of adult morphology, but all phenotypic characters are the end product of the interaction between genotype and environment; including adaptation to diverse life styles such as symbiotic association with other organisms. In comparison all crab zoeal stages are adapted to the same habitat, a planktonic existence, and larval characters are consequently all subject to the same selection pressures. Larvae present a new suite of characters for analysis and differences between zoea may reflect phylogeny-based genotypic relationships. However, reliable data are everything and setotaxy must be founded on high quality observations and interpretation. Brachyuran first stage zoeae of congeneric species appear to have virtually identical setotaxy. This similarity provides a degree of predictability within a taxon. Setal differences (incongruence) within a group are usually indicative of incorrect assignment of taxa. But setal similarity or differences between taxa are not simple indicators of phylogenetic relationships; such inferences should be based on shared derived characters. The phylogenetic interpretation of setal patterns within zoeal development has been problematical. For example on certain body somites and appendage segments the numbers of setae do not increase through successive zoeal moults and are considered to be conservative. In contrast, some somites and segments accumulate setae at each zoeal moult. Successful interpretation of such setal characters may be helpful in analysing phylogenetic relationships between brachyuran taxa.