Whilst looking at some samples from an experiment conducted on mesocosms beside a chalk stream, Candover Brook in Hampshire for Mark Ledger and colleagues, I came across a diatom that I had not seen before and which, at first glance, was out of place. As the images above show, it is a diatom whose cells join together to form chains which, in turn, means that they typically present their sides to the viewer rather than the valve face, which is the way that the writers of identification guides generally assume that we can see. It took some time to track down a couple of cells that were lying face-upwards so that I could try to name the species and some of the few that were lying this way were damaged (see left hand image), perhaps itself a consequence of the naturally strong links between the cells.
Naming the genus was relatively straightforward: the valve shape, fine striae and very narrow axial area (the gap along the median line of the valve face between the two rows of striae) coupled with the tendency to form chains all pointed to Fragilariforma. However, most of the Fragilariforma that I encounter are in soft water, often acid habitats whilst this sample was from a flume beside a chalk stream in southern England. After scratching my head a little more, and sending images to my friend Lydia in Germany, I eventually decided that Fragilariforma nitzschioides was the most likely name for this diatom. Searching through my records, I found only one other record for this species: from the River Itchen (into which Candover Brook drains) in the mid-1990s. That must be more than coincidence. Interestingly, Hoffman et al. (2011) describe the species as “rare” and say that its ecological preferences are “difficult to define”.
The limited records that we have show that this species does not behave in the same way as most other representatives of the genus. The weighted average of pH for the genus is 6.6 (see graph below), but there are plenty of records extending into more acid waters. By contrast, the River Itchen population was recorded at pH 8.1 and the pH in Candover Brook will be very similar. Most of the records for the genus came from relatively soft water, in contrast to the very hard water found in a chalk stream. The scarcity of records of a species that is well described in the literature also suggests that this might be a genuinely rare diatom (see “A “red list” of endangered British diatoms”).
One other peculiarity of this species is the name itself. Fragilariforma was one of a number of genera split away from Fragilaria by Dave Williams and Frank Round in 1986, originally as “Neofragilaria”. Fragilaria nitzschoides, was not formally transferred at the time, presumably because the authors did not have access to the type material. They presented good evidence for this new genus but a few people – notably Horst Lange-Bertalot – have continued to group these species under Fragilaria. This is the situation in Diatomeen im Süsswasser-Benthos von Mitteleuropa but, curiously, for Fragilaria nitzschoides, he created the new combination of “Fragilariforma nitzschoides” purely as a synonym (see p. 268). The good news is that the next version of this book (see “Tales of Hoffman”) does use these new names.
The relationship between Fragilariforma spp and pH (left) and alkalinity (right) in UK rivers, based on the mid-1990s dataset described in “The challenging ecology of a freshwater diatom”. Vertical lines show the boundaries for high (blue), good (green), moderate (orange) and poor (red) status classes based on current UK standards and the arrows show the location of the River Itchen population of Fragilariforma nitzschoides along these gradients.
Hofmann, G., Werum, M. & Lange-Bertalot, H. (2011). Diatomeen im Süßwasser-Benthos von Mitteleuropa. A.R.G. Gantner Verlag K.G., Rugell.
Williams, D.M. & Round, F.R. (1987). Revision of the genus Fragilaria. Diatom Research 2: 267-288.
Williams, D.M. & Round, F.R. (1988). Fragilariforma nom nov., a new generic name for Neofragilaria Williams & Round. Diatom Research 3: 265-267.