Synonymy of Dinoflagellates with basket-like endoskeletons: Monaster, Amphilothus and Achradina (Amphilothales, Dinophyceae)

Abstract

The basket-like skeleton-bearing dinoflagellates have been divided into three genera: the little-known Monaster as having a skeleton with longitudinal ribs in the epitheca, Amphilothus with a dense and geometric skeletal tessellation in the epitheca, and Achradina with characteristics intermediate between the other two genera. Cells of different sizes and morphologies corresponding to these genera co-occurred within the same samples from the Mediterranean Sea and South Atlantic Ocean. During cell division, a bigger daughter cell kept the endoskeleton, while the smaller daughter cell formed a new skeleton, beginning with the formation of the hypotheca. The different degree of maturation of the skeleton explained the great intraspecific variability in skeletal morphology. These forms share a distinctive crest-like apical structure with several internal radiating slits. These evidences indicate that the genus Monaster corresponds to immature cells with incomplete developed endoskeleton in the epitheca; Amphilothus corresponds to the mature cells with more developed epithecal endoskeleton, while Achradina shows intermediate morphologies and it is the most common form in the plankton samples. The genera Achradina, Amphilothus and Monaster correspond to a single species, with the name Monaster rete having priority.