Diatom identification in the face of changing species concepts and evidence of phenotypic plasticity

Although it is often suggested that diatom wall morphology is faithfully replicated at each cell division, it is also well known that the average cell size of a diatom population usually decreases as cells proliferate. Comparisons between the two valves of a single frustule may also reveal morphological differences, indicating that valve ontogenetic processes are susceptible to modification. This paper will discuss the different factors affecting valve morphology in diatoms and some of the implications for ecological and palaeo-ecological studies using diatoms. It will also consider some of the problems of evaluating variation between clonal populations, and the influence of findings from molecular biology and reproductive studies on the interpretation of morphology and species concepts.