MORPHOLOGY OF VEGETATIVE AND RESTING SPORE VALVES OF STEPHANOPYXIS NIPPONICA

Abstract

The marine diatom Stephanopyxis nipponica Gran & Yendo forms three types of valves, a vegetative and two resting spore valves, which are classified based on their overall morphology. The siliceous fine structure of these valves is described here for the first time using scanning electron microscopy. Although the two resting valves composing a resting spore possess different shapes, both have a similar fine structure. Furthermore, they are distinguished from vegetative valves in the following ways: (1) The external processes and their internal rimoportulae are clustered at the center of the resting spore valves, whereas the slender, tubular linking processes and their internal rimoportulae form a ring on the vegetative valves; (2) Resting spore valves do not possess an acceptant process for linking cells while vegetative valves always have such processes; (3) Scattered rimoportulae are present in resting spore valves, but absent from vegetative valves; (4) The cribral pores of resting spore valves form clusters corresponding to the areolae but those of vegetative valves are evenly dispersed over the basal siliceous layer. The occurrence of three different valve forms in S. nipponica indicates that one should take heterovalvy into account when classifying species within centric genera such as Stephanopyxis Ehrenberg.