A NEW MOTILE, MULTICELLULAR STAGE INVOLVED IN HOST INVASION BY PARASITIC BARNACLES (RHIZOCEPHALA)

RHIZOCEPHALANS are barnacles (Cirripedia), but are extremely specializ ed for parasitic life on decapod crustaceans. A cypris larva settles a nd develops into a new instar, the kentrogon, which inoculates the hos t with the parasite. The very early primordial parasite has;been argue d to consist solely of embryonic stem cells or even eggs(1), hut the t rue nature of this unknown stage has remained a puzzle for more than a century(2). We present data from in vitro experiments on the rhizocep halan Loxothylacus panopaei documenting that, unlike previous postulat ions, the recently injected parasite is not naked embryonic cells, but has the form of a motile, vermiform body, enclosed in an acellular sh eath, After a period of maturation the vermiform body splits up into a number of naked and independently moving cells, which in our in vitro experiments disperse by amoeboid movements. This suggests that, in vi vo, the cells disperse in the haemolymph of the host crab, where each has the potential to develop into an adult parasite, although in most cases only one will succeed.