Cypris metamorphosis, injection and earliest internal development of the Rhizocephalan Loxothylacus panopaei (Gissler). Crustacea : Cirripedia : Rhizocephala : Sacculinidae

Rhizocephala is a group of crustaceans that exclusively parasitizes other c rustaceans. It is taxonomically placed within the class Cirripedia, the bar nacles, with which it shares a unique larval type, the cyprid. The main obj ective of the cyprid is to find and irreversibly attach to a suitable subst ratum and initiate metamorphosis. In the presumed sister group to Rhizoceph ala, the true barnacles or Thoracica, metamorphosis leads to a juvenile fil ter-feeding version of the adult organism. In Rhizocephala the female cypri d settles on the integument of a crustacean and undergoes metamorphosis int o a kentrogon that possesses a hollow cuticular-tube structure, the stylet, which penetrates the integument of the host and acts as a guide tube for t he prospective internal parasite. The first, hitherto unknown endoparasitic stage of a rhizocephalan, the vermigon, was recently discovered (Glenner a nd Hoeg [1995] Nature 377:147-150) and its migration through the hemolymph of the host, as well as its internal development, was described in Glenner et al. ([2000] Mar Biol 136:249-257). The present article provides detailed information on kentrogon and vermigon formation, the injection process, an d the succeeding developmental stages up to the stage of the earliest primo rdium reported from the literature. The anlage of the ovary is traced back to the free-swimming cypris stage and it is implied that the mesoderm and e ctoderm of the endoparasite are already differentiated in the cyprid. (C) 2 001 Wiley-Liss, Inc.