ИСТИНА

Tantulocarida is a group of the smallest parasitic crustaceans recognized as a separate class only in 1983. They have a remarkably complex life cycle including both free-swimming and ectoparasitic stages, while reproduction involves both sexual forms and parthenogenesis. The complex life cycle, the minute size of all stages and the difficulties in sampling live specimens all combine to impede comprehensive studies of these crustaceans. Due to the parasitic mode of life tantulocaridans present few morphological characters that could be used to indicate their relationship among Crustacea. Therefore molecular datasets would be an obvious alternative, but until now DNA sequences of tantulocaridan species have not been available. A more precise hypothesis is that the Tantulocarida are closely related to the Thecostraca, a class including Ascothoraciada, Cirripedia and Facetotecta. Possible synapomorphy for such a relation is the position of male and female gonopores. Complete 18S rDNA sequences of two species of the Tantulocarida from the White Sea were obtained for the first time and used for estimating the relationship of the class with other Crustacea. Tantulocarida were proved to be very close relatives of the class Thecostraca. Moreover, with lower confidence the Tantulocarida are also indicated as nested within the Thecostraca, being sistergroup to the Cirripedia. The cement gland of the tantulus larva and the cirripede cyprid might be homologous structures, but similarities in host infection and root systems between the Tantulocarida and the Rhizocephala are likely to be homoplasies evolved by convergent evolution into advanced parasitism. The precise position of the Tantulocarida must be pursued by a more extensive database of genetic markers.