Abstract
Coastal phytoplankton samples were analyzed from two locations in the tropical Mexican Pacific Ocean. Three solitary, presumably epipsammic diatom species were studied in detail: Aulacodiscus petersii, A. scaber, and a new species of Cerataulus, C. simsae. Most morphological features previously described for A. petersii and A. scaber were also found in this study; however, we found great variation in the external tube of the rimoportulae in terms of size and complexity. Although C. simsae was less common, it also showed significant size and shape variation. The species has slightly undulated frustules in girdle view, with elliptical to circular valves; it also has two opposite, marginal ocelli that are not prominent, but rather almost flush with the valve face. The valves are usually covered by granules (marginal granules have a short peduncle) of different sizes that mask the areolae. A marginal ring of rimoportulae, with no large protrusion to the outside, but rather small “knobs”, was found in valves of the species. Cerataulus simsae is probably closely related to C. californicus. Cerataulus should be revised because its generic circumscription and relationships are unclear.
©2012 by Walter de Gruyter Berlin Boston
