Evolution of morphology, ontogeny and life cycles within the Crustacea Thecostraca

Abstract

We use a previously published phylogenetic analysis of the Thecostraca to trace character evolution in the major lineages of the taxon. The phylogeny was based on both molecular (6,244 sites from 18S rna, 28S rna and H3 genes) and 41 larval morphological characters with broad taxon sampling across the Facetotecta (7 spp.), Ascothoracida (5 spp.), and Cirripedia (3 acrothoracican, 25 rhizocephalan and 39 thoracican spp.). Morphological apomorphies are identifi ed in larval morphology for almost all major branches within the Thecostraca. Characters from the cypris larva provide a long suite of apomorphies for the Cirripedia and reinforce the concept that this larva was a prerequisite to the tremendous success of that taxon. The evolution of parasitism, obligatory in three major taxa, is discussed. We conclude that the last common ancestor to the Cirripedia was most likely a suspension feeder, and the advanced metamorphosis and endoparasitism known from the Rhizocephala and strongly indicated for the Facetotecta are the result of convergent evolution. We also discuss reproductive systems, which range from separate sexes, over hermaphrodites combined with a separate male sex (androdioecy), to pure hermaphroditism. It is concluded, as envisaged by Darwin, that the Thecostraca provide excellent opportunities for studying the evolution of a wide range of complex life history traits which can now be better analyzed and understood in a robust phylogenetic framework.