Achnanthidium subhudsonis from the Afon Seiont, North Wales; the three images on the left are raphe valves, the two on the right are rapheless valves. Scale bar: 10 micrometres (1/100th of a millimetre).
I have had a couple of emails in recent months asking for help naming a diatom which, in both cases, after showing the images to colleagues, was identified as Achnanthidium subhudsonis (Hustedt) H. Kobasayi 2006. This species was originally described by Hustedt from samples collected in tropical Africa but there seem to be a growing number of records in temperate regions, leading some to describe it as an “invasive species”. I am sceptical of many statements concerning the distribution of diatoms (see “A “red list” of endangered British diatoms?”) but, in this case, I think there might be some justification. This is because the original suggestion that A. subhudsonis is spreading in Europe was based on careful observations over a decade by Michel Coste. He has the benefit of examining samples from a large number of French rivers regularly over a long period of time, so has a large dataset on which to base his proposal. His evidence does suggest that A. subhudsonis was quite rare in the early 1990s but subsequently was recorded from more sites and in increasingly large numbers. I am less convinced by some more recent papers that have made similar assertions about A. subhudsonis spreading, as they do not present enough evidence to show that it wasn’t present in the past. One paper, for example, claimed: “The relative abundance of A. subhudsonis reached 20% in the Chiusella and Ticino rivers, highlighting the invasive behaviour of the species”. No it doesn’t: it just means that one in five of an unknown number of diatoms belonged to a species that hasn’t been recorded from this site before. I go no further at this stage than claiming A. subhudsonis as a “new record” for Britain and leaving open the possibility that it has been here for some time but has been overlooked.
The first record of A. subhudsonis in Europe was made in 1991, a significant date in our story as this was also the year that the volume of the standard diatom identification guide covering the family to which A. subhudsonis belongs was published. This group includes many small diatoms that are difficult to tell apart and, consequently, there is a risk that some potential records of A. subhudsonis will have been recorded under a different name. Consequently, it is dangerous to infer that A. subhudsonis is now more widespread simply because we do not have strong enough evidence for its absence. Unless we have regular records from the same site over at least 20 years the best we can do is make a map of its present distribution and speculate.
So far, the records we have for the UK are from the River Dart in Devon and the Afon Seiont, in North Wales, but there may be other records lurking out there that we have overlooked. One interesting feature of these records is that they are more northerly than the previous records, which are mostly from southern and central Europe. The ecology, based on published observations seems vague, but there is a suggestion that it likes low levels of pollution but is often associated with agricultural areas where there may be elevated nitrogen. That means that there are many potential habitats for A. subhudsonis in the UK so it will be interesting to see how many more records are out there, waiting to be discovered.
Coste, M. & Ector, L. (2000). Diatomées invasives exotiques ou rares en France: principales observations effectuées au cours des dernières décennies. Systematic and Geography of Plants 70: 373-400.
Falasco, E. & Bona, F. (2013). Recent findings regarding non-native or poorly known diatom taxa in north-western Italian rivers. Journal of Limnology 72: 35-51.
Krammer, K. & Lange-Bertalot, H. (1991). Susswasserflora von Mitteleuropa 2: Bacillariophyceae. 4 Teil: Achnanthaceae. Kritische Ergänzungen zu Achnanthes s.l., Navicula s.str., Gomphonema. Spektrum Akademischer Verlag, Heidelberg.