Fine structure and ecology of Nanoneis longta in the northeastern Gulf of Mexico with a revised definition of the species

A population of the rarely reported diatom Nanoneis longta was observed over a period of more than two years in the northeastern Gulf of Mexico. This permitted a detailed examination of the fine structure of the valve in scanning and transmission electron microscopy. The population matched the basic features of the original description of the species, but displayed a greater size range. Our use of SEM revealed the presence of a novel structure, the porticus marginalis, along the perimeter of the valve. The species was most abundant below the thermocline, at depths between 25 and 125 m, and from summer to early fall. An emended definition of the species to accommodate the observed size range and the new feature is included.