I came across a few strangely-shaped diatoms whilst examining a sample from Llyn Padarn in Snowdonia a couple of weeks ago. The girdle (side) view has a characteristic “spoon” shape whilst the outline in valve view (i.e. seen from above) is lanceolate with expanded (“capitate”) ends. I had previously seen this only as illustrations in continental Floras and once in a sample from Corsica.
Valves of Achnanthidium catenatum from Llyn Padarn, Snowdonia, September 2013; left hand view: two girdle (side) views; right hand view: valve view. Scale bar: 10 micrometres (= 1/100th of a millimetre).
To the best of my knowledge, this is the first record of the species in the UK. I helped to compile a checklist of freshwater algae about 10 years ago and found no records then. A lot of samples have been analysed since this checklist was published, but I have seen no reports of this species being recorded. There are, however, two problems with knowing that this really is the first record for the UK. The first is that there are no formal structures for validating new records, such as exist for higher plants. If a species is found in a part of the country where it had not previously been recorded, then there are specialists, appointed by the Botanical Society of the British Isles, and similar bodies, who can check the identity of the specimen. The algal world still lacks this level of organisation. I can send my images or slides to colleagues who can confirm my hunch, but this is all very informal. More significantly, no single individual holds a truly definitive list of UK algal records.
The second problem with knowing that this is the first record of Achnanthidium catenatum is that I have not, actually, seen this species alive. My sample from Llyn Padwan contains a few dead shells from which we can infer that there is probably a population of A. catenatum present. However, as this lake lies in a popular tourist area, we cannot wholly exclude the possibility that these simply washed off the boot of a visitor from continental Europe. This picks up the issue I discussed in A Christmas Turkey .. that reliance on the dead shells is dangerous. A useful New Year’s resolution might be to visit Llyn Padwan, or find a colleague who can visit it for me, and see if I can find some living populations of A. catenatum. Only then will we be sure that this really is a new addition to the UK algal flora.