In the present paper we redescribe the ciliate
Chattonidium setense Villeneuve 1937 combining morphological observations (live, stained, scanning, and transmission electron microscope) with behavioral notes and molecular data. Ultrastructural analysis revealed remarkable similarities between
Chattonidium and representative members of the class Heterotrichea in cortical structure and cytoplasmic organization. The most similar genus for these aspects appears to be
Condylostoma. To verify this relatedness, 18S rRNA genes from
Chattonidium and from one
Condylostoma species were sequenced. Phylogenetic analysis indicates
Chattonidium belongs to the class Heterotrichea defined according to the modern taxonomy, and confirms its relatedness with
Condylostoma already hypothesized by Villeneuve-Brachon (1940). The presence of the aboral cavity complex, a unique feature never described in other ciliates, and its peculiar organization revealed by ultrastructural analysis fully justify, in our opinion, the maintenance of
Chattonidium in the separate family Chattonidiidae, established by Villeneuve-Brachon in 1940.
(Edited by Masanori OTSUKA, M.J.A.)
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