ИСТИНА

The general importance of ontogenetic data has been acknowledged since Haeckel as well as recent evo-devo achievements. However, apart from a few studies current taxonomy and phylogenetics generally remain almost “development-free” and principally focuses on the adult stages. Here we present the remarkable discovery of a new abundant species of brittle star within an intensively studied prior area, which has remained unrecognized, and confused with close relatives, precisely because ontogenetic heterochronic shifts in several important characters were previously underestimated. Based on the study of the numerous materials obtained by various Japanese expeditions and the type specimens from worldwide collections a complex of the three common species of the deep-sea ophiuroid genus Ophiacantha was revised. The undescribed species was ontogenetically differentiated from closely related Ophiacantha rhachophora H. L. Clark, 1911 and retained juvenile features in the adult morphology. Another species Ophiacantha clypeata Kyte, 1977 was shown to be just a juvenile stage of the North Pacific species Ophiacantha trachybactra H. L. Clark, 1911. Detailed morphology of both adult and juvenile specimens was examined by scanning electron microscopy, and a special grinding method was utilized for the first time to show internal structures of ophiuroids. The distinct bathymetric zonation of the three species was confirmed statistically and correlated with ontogenetic modifications. O. rhachophora predominantly inhabits the shallowest waters at 150–250 m, the undescribed species lives in intermediate depths of 300–500 m, and O. trachybactra in the deepest waters at 1100–1250 m. Keywords: taxonomy, phylogenetics, ontogeny, evolutionary developmental biology, deep sea ophiuroids.