Algae World: Sellaphora

All species confirmed to belong to Sellaphora grow in freshwaters and all of them appear to be epipelic, i.e. organisms that live freely in sediments, moving in or on the surface layers of mud or sand in lakes and rivers. No species is known to form stalks or tubes and, although cells often attach firmly to cover-slips or other surfaces in the laboratory, it is unlikely that any species is truly epiphytic or eplilithic; nevertheless, cells do sometimes occur in samples of attached diatoms. The best way to harvest Sellaphora cells is to use modifications of the lens tissue method developed by Eaton & Moss (1967) to strip epipelon from sediment.

The ecological information recorded for individual species is, of course, dependent on (1) the quality of the underlying taxonomy and (2) the consistency and accuracy with which this taxonomy has been applied. Molecular and breeding data have shown that the traditional (pre-1990) taxonomy is 'coarse-grained' and does not accurately reflect diversity within the genus. On the other hand, traditionally-defined species (though not the varieties within them) were easily recognizable and illustrated in all the major floras used to identify freshwater diatoms; they were probably recorded consistently.

Each of the principal, large-celled species recognized until c. 1990 – viz. Sellaphora pupula, S. bacillum, S. americana and S. laevissima (all then recorded as Navicula species) – seem to have wide tolerances, although there are few quantitative data for S. americana. As examples (you may need to wait some time before the data-points are returned), see the European Diatom Database data for:

• Sellaphora pupula response to pH
• Sellaphora pupula response to Total P
• Sellaphora pupula response to conductivity
• Sellaphora bacillum response to pH
• Sellaphora bacillum response to Total P
• Sellaphora laevissima response to pH

It seems very likely to us, from our qualitative surveys of many lakes,representing a wide range of pH and trophic level, that better species definition will be accompanied by a significant narrowing of the ecological range recorded for each species. For example, S. pupula has already been split into several species (S. capitata, S. blackfordensis, S. obesa, S. lanceolata, S. auldreekie and S. pupula sensu stricto) and these are not found in equal proportions everywhere, nor are they found everywhere S. pupula has previously been recorded. The range of 'Sellaphora pupula' recorded in the literature is the sum of the partially overlapping ranges of many different species. Denys (2006) has noted that a refined taxonomy of the S. pupula group may increase the precision of ecological monitoring.

Sellaphora pupula was sometimes split into several varieties and forms by pre-1990 ecologists . Unfortunately, these taxa have not been identified consistently and records of their occurrence cannot usually be relied upon.