Cypris Larvae inPolysaccus MediterraneusandMycetomorpha Vancouverensis: Their Importance in Analyzing the Phylogeny and Sexual Evolution of Parasitic Barnacles (Crustacea: Cirripedia: Rhizocephala)

Abstract

We used SEM to investigate the cypris larvae in two species of parasitic barnacles, Polysaccus mediterraneus (Polysaccidae) from the eastern Mediterranean and Mycetomorpha vancouverensis (Mycetomorphidae) from the northern Pacific. Both species belong to the rhizocephalan suborder Akentrogonida and have morphologies of the adult parasite that greatly differ from that seen in the "classical" suborder Kentrogonida. In contrast, both P. mediterraneus and M. vancouverensis have cypris larvae that are remarkably similar to those of the Kentrogonida. For both species, similarities include the presence of aesthetascs on the third and fourth antennular segments and a spinous process distally on the attachment disc. For P. mediterraneus additional similarities to the kentrogonid cyprids are terminal pores in the lattice organs, frontal horn gland pores, frontal filaments, and four terminal setae on the fourth antennular segment. Cyprids of Mycetomorpha have, like most other Akentrogonida, either lost or specialized (terminal setae) these structures. Compared to the stereotyped life cycle found in the Kentrogonida, species of the Akentrogonida have a diversity of reproductive systems and advanced modes of metamorphosis, and this is reflected in specializations of their cyprids. The larval morphologies of P. mediterraneus and M. vancouverensis suggest that these two species have a sexual system and life cycle much closer to that seen in the Kentrogonida. Our results show that adult and larval morphologies are not closely correlated in the Rhizocephala, and provide important information for tracing how the Akentrogonida originated and radiated from within a paraphyletic Kentrogonida.