Diatoms of North AmericaCleve (1894) considered craticular valves “monstrosities”, while Hustedt (1961) referred to them as “Mutanten” or ‘‘mutierte Schalen”. These different siliceous forms are actually components of a resting spore. During formation of the spore, a normal vegetative cell forms four internal valves. First, two craticulae are formed. Then, inside the craticulae, two heribaudii valves are produced. These resting spores are surrounded by mucilage and can persist for many years in dry sediment.
The internal valves form in response to elevated salt concentrations, which also increase with desiccation (Schmid 1979). The cells are a morphological expression of an alternate metabolism under adverse conditions, in particular, changes in osmoregulation as the result of high salinity. Cells germinate after they are returned to water, after up to eight years of being dry.
In the Taxon Workshop in May 2015, we investigated species of Craticula that have been reported in surveys of US rivers. We worked in groups on a number of questions:
• We asked, for example, “what is C. accomoda according to the type?” For each taxon we examined the the original description and type specimen.
• Do specimens that are in accord with the type occur in North America? Can we verify records in the Academy of Natural Sciences of Drexel University Diatom Herbarium? Many times, we have found that that a common taxon does NOT occur in the US. For example, Encyonema muelleri, was a name commonly applied in surveys, yet it was not verified in herbarium collections.
• Each person spent time learning the morphological variability of their taxon, or taxa, and documented variation with images and measurements.
• We determined if a taxon was being confused with another in surveys by examining past datasets. For example, Navicula biconica Patrick was used for many years (Potapova 2013) to refer to Craticula molestiformis (Hustedt) Mayama.
• In discussions around the conference table, we reported our findings and commented on one another’s results.
• Finally, we entered data into the Content Management System (CMS) for the project and prepared the taxon pages for review.
