Abstract
Oblea acanthocysta of the diplopsalid group is described as a new species from Omura Bay, West Japan. The motile cells are subspherical, have a large sulcal list at the left margin, and are characterized by the plate formula, Po, X, 3′, 1a, 6″, 3c+t, 6s?, 5‴, 2″″. The species resembles Oblea torta in shape and plate distribution, but is smaller and differs in the shape of plate 1′ and position of plate 1a. Resting cysts of O. acanthocysta are spherical and pale brown in color, possess many hollow acuminate spines, and have a theropylic archeopyle. In the SSU rRNA gene sequences of three cells of O. acanthocysta and two cells of O. torta, no intraspecific base substitutions were detected within either species. Based on phylogenetic analyses, the O. acanthocysta clade is included in the diplopsalids clade together with other diplopsalid species such as O. torta, Diplopsalis lebourae, Diplopsalopsis bomba and Gotoius excentricus. The sequences of O. acanthocysta are different from those of O. torta in 154 base pair substitutions, but O. acanthocysta and O. torta have a very close phylogenetic relationship.
