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This paper formed part of the “Diatoms are going molecular: implications for identification, taxonomy and phylogeny” session, contributed as a commentary on the significance of morphology to modern systematics as the molecular age develops. Molecular sequence data for diatoms have been available since the late 1980s (Medlin et al. 1988) but only recently made easy enough to acquire in sufficient quantities that their relevance is seen as self evident, relating to problems posed by “identification, taxonomy and phylogeny”. Seen from a general perspective, a recent summary suggests the following: “Revision of species taxonomy in diatoms increasingly requires a rigorous, multidisciplinary approach incorporating crossing experiments and molecular genetic analysis, as well as careful morphological analysis” (Vanormelingen et al. 2007). One might imagine that this prescription suggests what is required to discover species taxa. Yet, surprisingly, it contains no such thing. It simply suggests: Revision of species taxonomy requires lots of data. Is that true? Whatever the answer to that question might be (and it is an empirical one), data sources are not equivalent to a method(s) of discovery no matter how self-evident it might appear. Now fashionable, ‘cryptic species’ – those hidden from view – are being discovered everywhere, largely as a result of molecular data. ‘Cryptic clades’ (higher taxa) are also becoming fashionable: “Several major molecular clades are cryptic, with no or few morphological or life history traits that can be convincingly argued to be synapomorphies” (Mann in Adl et al. 2005: 431, note 18). Rather than examine the notion in detail, we first present some basic ideas, hidden by the passage of time, concerning ‘cryptic species’ and their meaning. That “revision of species taxonomy” is understood (mistakenly in our view) as resolved (or possibly resolved) by gaining many varied datasets, it leaves the question of higher taxa (‘Cryptic clades’) unanswered, a topic we briefly discuss using the genus Cistula Cleve as an example.

Some commentary on molecules and morphology, species and higher taxa in diatoms, with a note on the relationships of the genus Cistula Cleve