Invasive vermigon stage in the parasitic barnacles Loxothylacus texanus and L-panopaei (Sacculinidae): closing of the rhizocephalan life-cycle

The parasitic barnacles, Rhizocephala, are unique in Crustacea by having an entirely endo-parasitic phase inserted into their lifecycle. A cypris larva, remarkably similar to the cypris of conventional acorn and goose barnacles (Thoracica), settles on the crustacean host and develops an infective stage, the kentrogon, underneath the exuviae of the cypris. The kentrogon penetrates the integument of the host by a hollow cuticle structure, the stylet, and injects the parasitic material into the hemocoelic fluid of the host. Although advanced stages of the internal development have been found and described several times, the nature of the originally injected parasitic material has remained obscure for decades. Recently, however, it was shown that the parasitic material was injected by the kentrogon in the form of a motile, multi-cellular and vermiform body. The present study demonstrates that the vermiform stage is an instar which forms the only and direct link between the kentrogon and the maturing internal parasite. The vermiform instar, or vermigon, is at all times clothed in a cuticle, contains several types of cells, including epidermis and the anlage of the later ovary, and stays intact while growing into the internal parasites with rootlets.