Male cypris metamorphosis and a new male larval form, the trichogon, in the parasitic barnacle sacculina carcini (crustacea: cirripedia: rhizocephala)

Abstract

Virgin externae of the parasitic barnacle Sacculina carcini Thompson were exposed to settlement of male cyprids. The events from settlement around the mantle aperture to arrival of the male cypris cells into the receptacle of the externa were studied by transmission and scanning electron microscopy. A small, hitherto unknown larva, the trichogon, escapes from the cyprid within ca. 20 min after settlement. A thin cuticle armed with long spines is preformed beneath the carapace of the male cyprid, and after metamorphosis this cuticle encloses the free trichogon. The trichogon is up to 220 $\mu$m long, unsegmented, has a variable amoeboid shape and a very simple structure. It includes parts of the cypris epidermis and two other types of cypris cells, but it has no appendages, muscles, sense organs or nervous tissue. The trichogon migrates through the mantle cavity of the externa and arrives at the entrance to the receptacle duct within 2 h after settlement. During the ensuing migration through the receptacle duct, the trichogon loses its spine-armed cuticle. Once inside the receptacle, the trichogon cells and the female cells of the receptacle are in direct contact, with no intervening cuticle. The implanted trichogon is regarded as a very specialized dwarf male. The formation of the trichogon from male cyprids, and of the kentrogon from female cyprids has many similarities, and the trichogon and the kentrogon are regarded as homologous instars. A trichogon is present in the Sacculinidae, the Lernaeodiscidae and most probably also in the Peltogastridae; i.e. in the same families where a kentrogon has been demonstrated to accomplish invasion of the decapod host.