The morphology of the chemosensory aesthetasc-like setae used during settlement of cypris larvae in the parasitic barnacle Sacculina carcini (Cirripedia: Rhizocephala)

Abstract

Rhizocephalans of the suborder Kentrogonida are parasitic barnacles whose female cypris larvae must locate a suitable host organism (a decapod crustacean), while the male cypris larvae must find a host containing a virgin female. Some of the sensory structures assumed to be involved in this behaviour are the aesthetasc-like setae found on antennules of the cyprids, of which the females possess one and the males two. To verify the aesthetasc nature of these setae we examined their morphology and innervation in the cyprids of the “model” kentrogonid rhizocephalan, Sacculina carcini, using SEM and TEM. No structural differences were found between the two types of aesthetasc-like setae, and their ultrastructure strongly indicates them as being aesthetascs. They have an exceedingly delicate cuticle, 20–50 nm thick, and the lumen contained essentially only very fine ciliary branches of which many were in close contact with the cuticle. No sheath cells were seen in the setal lumen. The ciliary branches could be traced back through the fourth and third antennular segment. The bodies of the sensory cells were not located, but morphological evidence suggests that their inner dendritic segments are located in the second antennular segment, giving rise to cilia that evolve into sheath-wrapped ciliary bundles. All these characteristics support their role as aesthetascs and olfactory organs, while the absence of these traits in the so-called terminal aesthetasc invalidates its previous description as an aesthetasc altogether, although it may still be a chemosensory seta.