Most Sellaphora species exhibit a reduction in average cell size during the life cycle, as do most other diatoms. This is caused by the unique construction of the diatom cell wall, and the fact that the formation of new wall elements (valves and girdle bands) takes place within the existing cell wall. For example, new valves (produced immediately after cytokinesis) are formed beneath the girdle of the parent cell. In most diatoms, the epivalve (inherited from the parent cell) is slightly larger than the hypovalve (formed at the previous cell division). Although each decrement is small, over many cell divisions (and hence many days, months or years), the decrease in cell size becomes noticeable and may amount to 50% (as in the two examples shown in the graph) or more of the original size.
In elongate diatoms the shape of the hypovalve usually differs very slightly from the shape of the epivalve, because the girdle bands are somewhat flexible (though not plastic or stretchable) and the girdle as a whole tends to 'round off' under pressure from the cells within during formation of the new valves. Width alters far less than length and may even increase slightly. In addition, the shape of the cells almost always simplifies, any undulations and polar features tending to be lost as the cells get smaller.
Length-width plots can be a simple but effective way to investigate the nature of variation in Sellaphora species. For example, Sellaphora capitata and S. blackfordensis are very similar diatoms, both having linear valves with slightly 'capitate' poles, and they overlap considerably with respect to length and width ranges as single variables. However, as the graph shows, there is no overlap between them in bivariate plots. The data for the graph were obtained from Blackford Pond, Edinburgh, where the species occur sympatrically. They do not interbreed and are not in fact sister species, despite their morphological and apparent ecological similarity.