Revision of the diatom genus Sellaphora:: a first account of the larger species in the British Isles

As a step towards a global monograph of the freshwater diatom genus Sellaphora, we made detailed surveys of Sellaphora diversity in the epipelon of 38 lakes and ponds and three Holocene diatomites in the British Isles, restricting our analysis to those specimens that would be classified in S. americana, S. bacillum, S. pupula or S. laevissima according to the standard European diatom flora of KRAMMER & LANGE-BERTALOT (1986; Susswasserflora von Mitteleuropa, vol. 2/1. - G. Fischer, Stuttgart & New York). From a total of c. 10(4) specimens observed, over 3200 were recorded as digital images and sorted into 54 mostly informally-named phenodemes that can act as operational taxonomic units during future revisions. Of the 54 phenodemes, c. 40 are well-differentiated, while the others seem to intergrade and are in particular need of further study using morphometric, molecular and mating approaches. The limitations of conventional diatom taxonomy by visual comparison are clearly demonstrated. To help standardize identifications, published DNA sequences are assigned to some of the denies as 'molecular barcode'. The phenodemes of S. pupula sensu lato cannot be equated with the infraspecific taxa previously recorded for the British Isles and both these and most other pre-1990 records of species now assigned to Sellaphora must be regarded as having very little value for ecological and biogeographical purposes. Possible non-British records of the phenodemes are discussed but few are totally convincing; of those records most likely to be of the same phenodemes, the majority come from elsewhere in Europe.