Here is a puzzle for anyone who is learning to identify diatoms: how many species are shown in the plate below? All share the same size and outline but they are very different in other respects, including several that we would normally regard as important for separating different species. The left-hand image is an isolated girdle band, so let’s leave that to one side for the moment. What about the two middle valves? Both have a raphe in two parts, that runs along the midline, but the arrangement of their striae is very different. And how do these relate to the pair on the right, which seem to have stout silica bars which traverse the cell?
The answer is that all belong to the same species: Craticula cuspidata. Image b. is the way that it is most often seen (although it is not a particularly common species in the UK). You should be able to see the raphe and fine striae which are more-or-less parallel to one another and perpendicular to the midline of the valve. If you look with a scanning electron microscope, you’ll see that each of the striae is composed of a series of round or elliptical pores, equidistantly spaced so that the striae may appear to be running longitudinally as well as across the valve.
Craticula cuspidatafrom Pitsford Water, January 2019. a. isolated girdle band; b. “normal” valve; c. valve at “heribaudii stage”; d., e.: valves at “craticulae” stage. Scale bar: 10 micrometres (= 1/100thof a millimetre). Photos: Chris Carter.
Although the genus Craticula was described in the 19thcentury by Grunow, it was considered to be part of the genus Naviculafor most of the 20thcentury. We now regard the strictly parallel striae as one of the characteristics of Craticula but, if you think of it within in the broader realm of “Navicula” (basically, boat-shaped diatoms with a central raphe), many of which have radiate striae, then you might be happy to consider valve c. as being related to valve b. In this case, it would have been called “Navicula cuspidata var. heribaudii”. However, in 1979 Anne-Marie Schmid of the University of Salzburg, grew cultures of “normal” Craticula cuspidata in increasing salt concentrations and was able to show this (and the structures seen in images d. and e.) were responses to the stresses that this caused.
Under certain conditions, it seems, the normal process of cell division breaks down so that, rather than producing two daughter cells, each composed of two silica valves, just one “internal valve” is produced so that there are, in effect, three valves for two cells. One of the cells then degenerates leaving a single functional cell albeit with one extra valve. This phenomenon is not confined to Craticula but seems to be better understood for this genus than for others for reasons that I will come to shortly. In this particular case, the internal valve has a similar outline to the parent, but a different arrangement of striae
Images d. and e. show another aspect of the same phenomenon: the formation of a “craticula” (from the Latin for “grid-iron”). Schmid showed that this stage actually happens at lower salt concentrations than the “heribaudii” stage but that it, too, is related to the formation of these “internal valves”. There is a thickening of silica along the central rib, after which transverse “buttresses” grow out and, finally, a silica band is laid down around the edges of the valve. Schmid suggested that the resulting structures were resting stages, noting that she had found such structures in ponds in the Namib Desert that were only wetted for short periods every other year or so. When they dried up, salinity increased very rapidly and these “resting spores” lay in the bottom muds protected by layers of “jelly” (i.e. extracellular polysaccharides). About 11 days after she re-suspended them in distilled water, she observed viable cells gliding around again.
In the early 1990s, it became clear for other reasons that members of this genus were quite different from Naviculaso the original name was resurrected. That leaves us with the unusual situation of a genus that is named after rarely-seen monstrosities. It would be akin to naming Fragilaria “twisty diatoms” because, as we saw in “A twist in the tale …” a different form of stress causes a characteristic reaction in members of that genus. Because Craticula is not a particularly common genus, and because “craticulae” valves are a relatively rare phenomenon within that genus, it is likely that most people have never seen the structure after which it was named.
Mann, D.G. & Stickle, A.J. (1991). The genus Craticula. Diatom Research6: 79-107.
Schmid, A.-M. (1979). Influence of environmental factors on the development of the valve in diatoms. Protoplasma99: 99-115.