ABSTRACT
Recent molecular and morphological phylogenetic analyses have lent support to the Pancrustacea hypothesis, which argues that the Crustacea is either the sister group to the Hexapoda, or is paraphyletic in relation to the Hexapoda. Developmental evidence has been used to argue for a sister relationship between the Malacostraca and the Hexapoda, while the molecular evidence has been equivocal. These data provide four scenarios that require testing with increased taxon sampling: 1) hexapods and crustaceans are sister groups; 2) crustaceans are paraphyletic with hexapods most closely related to branchiopod crustaceans; 3) crustaceans are paraphyletic with hexapods most closely related to malacostracan crustaceans; or 4) hexapods are more closely related to myriapods, and that the Pancrustacea hypothesis is incorrect. The focus of this chapter is on the monophyly of the Crustacea, relationships within the Malacostraca, and the relationship of the Crustacea and Hexapoda. To this end, we examine the developmental and molecular evidence for the above hypotheses by testing them with new crustacean, specifically malacostracan, 18S and 28S sequence data. Our data suggest that branchiopod crustaceans may be the sister group to the Hexapoda, but they are not resolved on the placement of the Myriapoda. Within the Malacostraca, the Eucarida and Peracarida are found not to be monophyletic groups due to changes in the affinities of mysids, krill, and caridean shrimp.
