The British Bullhead; a case of mistaken identity?
*Please note, this is the short version of the article, which excludes discussion around the Cottus revision, their migration history, and the reasons why they are surrounded with misinformation. The full version was published here, on CE Fish Essentials*
The northern branch of Cottus perifretum from the River Wharfe, Yorkshire © max-pics, iNaturalist, 2024
For many years, the classification of the freshwater bullhead has been a topic of much confusion. So what species do we have in Britain, and how did they get here?
Growing up along the river Avon, before I’d ever considered fishing with rod and line, as all young children should, I explored! I can recall many fond memories of walking along the margins with a little dip net, catching Three-Spined Stickleback, Minnows, and tadpoles; turning up stones in chalk streams, clutching Stone Loach and prickly Bullhead by hand; and even the using the age-old jam-jar-tied-to-string-with-a-chunk-of-bread-inside technique, an effective way of catching tens of Dace at once—and a lesson in patience.
The underwater world was of great mystery to me, not knowing what was underneath sparked an intense curiosity, and when I did encounter some of its piscine treasures, I didn’t immediately know what they were. It’s a time of my life that I greatly miss, everything was a discovery and completely novel. These days, I’m familiar with all of our freshwater fish inhabitants, and roughly 200 British marine ones too, so a considerable amount of that mystery now seems uncovered. That being said, there are always new things to be discovered (taxonomists are forever changing our perspective), but how many would expect one of our staple, native species would be revealed to be another species entirely?
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| Entire range of Common Bullhead Cottus perifretum, as of 2023, (orange = native, purple = introduced). Map from IUCN Red List, Leaflet, Esri. |
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| Entire range of European Bullhead Cottus gobio, as of 2023. Map from IUCN Red List, Leaflet, Esri. |
What’s the best way to identify them?
So no doubt the question on everyone’s mind, is how do you tell these two species apart? To put it bluntly, if it took ichthyologists over two centuries before they were able to distinguish between them, it’s a safe assessment you’re not going to be able to do this in the field, at least not without inflicting stress or physical harm on the fish, and without genetic sampling. Like a lot of fishes, bullheads vary in appearance, even within their own species, so it’s easy to believe two individuals of the same species are in fact different. For the UK readers especially, it’s not relevant anyway, as the only verified freshwater bullhead we have here, is our native Common Bullhead (though, they are invasive in Scotland). If we did have true C. gobio here, they’re yet to be seen, simply due to how far removed they became from Paratethys [the ancient mega-lake which they invaded the rest of Europe from. More about this on the original article]. Not to mention the brackish/freshwater highways for any further populations would be cut off by the expansion of the North Sea and the English Channel. Nevertheless, they can be differentiated by all other western European Cottus species by:
- C. perifretum – The anterior (front) two thirds of their body, at least, is covered in subtle “prickles” in specimens under 50mm standard length (tip of head to tail fin base), or under the pectoral fins in individuals North of Leeds. These prickles are modified scales, and are well-developed in juveniles and subadults, but are less noticeable in mature males.
- C. gobio – The last anal fin ray is connected to the body by a membrane which is half the length of the aforementioned fin ray. The prickling is entirely absent, or restricted to just a few prickles under the pectoral fins.
*Please note, the above applies only to specimens West of the River Rhine.
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| The southern branch of Cottus perifretum from the River Ash, Surrey. Notice the "prickles" on the body © themainman13, iNaturalist, 2024 |
Is it possible there are two species of Cottus in Britain?
It’s not all bad news for species hunters wishing for two species of freshwater bullheads over here, C. perifretum is actually split into two branches:
- One from southern England and the River Scheldt (northern France, western Belgium).
- The second from northern England (no further South than the River Wharfe)
The northern branch has a slightly more elongated body, and the prickles on their body are largely restricted to under their pectoral fins, so…
“It cannot be excluded, that more than one species occurs in Great Britain” —Freyhof, J. et al. (2005)
Therefore, if (big if) an additional species of freshwater bullhead exists in the UK, it would most likely be identified as a new species, as (or within) this northern branch of C. perifretum, rather than being rediagnosed as C. gobio. Studies of bullhead from Europe could do with larger sample sizes for the British specimens, though a lot can already be inferred from pre-existing data.
The genus of Cottus is far from settled, with new species being described, reverted to old nomenclature, or reclassified to other genera, but this is normal for a genus of over 70 species. In this ever-changing world of ichthyology, never say never—tomorrow we might even find there were seven species of barbel, right under our noses!
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A special thank you to Jörg Freyhof for taking the time to review this article, and additional gratitude to Maurice Kottelat and Chris Englezou for their assistance in providing additional references.
