Abstract
The systematic position of the little-deformed parasitic copepod family Nicothoidae to the much deformed family Choniostomatidae has been disputed for many years. The new genus Diexanthema gen. nov., based on Diexanthema desistoma sp. nov. and Diexanthema bathydiaita sp. nov. discovered on two deep-sea asellote isopod species, clearly links the family Nicothoidae (via male Diexanthema) to the family Choniostomatidae (via female Diexanthema). Based upon the extreme sexual dimorphism of this new genus, it is suggested that the family Nicothoidae be dissolved and its members placed within the family Choniostomatidae. It is concluded that the differences seen between these two parasitic groups is probably due more to host adaptations than reflecting any phylogenetic trend that is evolutionarily significant. It is felt that the deep-sea environment may serve as a refugium for archaic copepod forms which may possibly offer additional clues and insights into the phylogeny of the parasitic copepoda. This investigation suggests that the life history and parasite/host relationships of Diexanthema desistoma are quite complicated and well geared to the life history of its isopod host, Chelator insignis.
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