Principal Component Analysis of Variation in Stephanodiscus rotula and S. niagarae (Bacillariophyceae)

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Abstract
For valves of the same diameter, type material of Stephanodiscus rotula, observed with light and scanning electron microscopy, has more total spines, more central strutted processes, and narrower mantles than does S. niagarae. However, these characters covary with diameter and with one another within each species. Thus, characters that distinguish between individuals in a species also contribute, in a multivariate context, to differences between species. Principal component analysis resolves variation in these and other characters into two major factors: a general pooled diameter-effect and a factor explaining differences between the two species.