A genuinely rare diatom genus?

About ten months ago I speculated on the practicalities of producing a “red list” of British diatoms (see “A red list of endangered British diatoms”).   A couple of weeks ago, I reported on progress towards this goal (see “Why do you look for the living amongst the dead?”) and, today, I can show the first evidence for a genuinely rare freshwater diatom in Great Britain.   Great Britain, alas, rather than the United Kingdom, as we struggled to get the Irish grid references to plot on our graphs, but it is a step in the right direction.

In my post last year, I commented that one of the problems we faced was the taxonomic uncertainty, particularly as there have been so many new species described, often as a result of “splitting” taxa that older books regarded as a single entity.   One of the criteria my intern, Susannah, and I used was a stable taxonomic concept, in order to ensure that older records could be merged with our more recent records.   This does not have to be at the level of species and, indeed, the first subject of our scrutiny was a genus, Tetracyclus. It fits our bill perfectly as it is quite recognizable, despite not being very common, so we can be fairly sure that the chance of an identification error is low.


The left hand map shows all hectads (10 km2) squares from which we have at least one freshwater diatom sample in Great Britain; the right hand map shows those hectads with records of Tetracyclus spp.

As you can see from the map, there are a cluster of locations in the Scottish highlands, three in the Cheviots in Northumberland and a few in Snowdonia. Then there is a record in Pembrokeshire and another in Wiltshire.   That is 23 hectads throughout the UK for all three species, none of which, individually, is found in more than 15 hectads, and therefore each qualifies as “nationally rare” according to JNCC guidelines.   There may be a few more records out there but, at the same time, there is also a risk of false negatives – a single valve, remember, does not necessarily mean that a viable population of the organism is present at the site.   The Wiltshire record worries me, and I need to check that there has not been a transcription error as the record was transferred from notebook to database, and to ensure that the grid reference was recorded correctly.

The literature offers little help when it comes to understanding the habitat of Tetracyclus: West and Fritsch tell us that Tetracyclus lacustris “… prefers hilly districts and is often found in the plankton of mountain lakes” whilst T. rupestris “… occurs on dripping rocks in mountain areas.” Other writers also hint that it might be partly sub-aerial in distribution, which may explain why the records in our dataset only ever record it in small quantities.   Is it possible that our samples, which are mostly from submerged rocks in streams and lake littoral zones, are just picking up a few stray cells that have been washed away from their preferred habitat?   We can interpret each dot, perhaps, as indicating that the species was “present in the vicinity”.

We should also point out that one of the Snowdonian locations on the map is Llyn Perfeddau, the lake from which the first Tetracyclus lacustris specimens were described, back in 1843 by John Ralfs.   Allan Pentecost re-visited Llyn Perfeddau in 2014 but was unable to find it in his samples, which adds to the mystique that this genus and species exert.

So are these three Tetracyclus species the first bona fide candidates for a UK (or GB) red list of diatoms?   Each fulfils the criterion of “nationally rare”, being found in less than 15 hectads, albeit with the provisos set out above. However, rarity alone is not sufficient to place a species on a red list; we also need to demonstrate that it is vulnerable or endangered.   This implies knowledge of trends over time, not just patterns in space.   The default under such circumstances is to consider whether the locations where it is found are fragmented and are, themselves, threatened or vulnerable.   The restricted distribution in low nutrient waters in mountainous and northern areas does suggest that changes in land management or climate change could affect the small isolated populations that we do have, so a designation of “vulnerable” is probably appropriate.   Though I doubt that WWF will be replacing their panda logo with a diatom any time soon.


Pentecost A. (2014). In search of the Welsh Tetracyclus. The Phycologist 88: 42-43.


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