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?
Back then, freshwater bullhead in England were regarded as European Bullhead (Cottus gobio), however, genetic examination has since confirmed them to be the Common Bullhead (Cottus perifretum), also called the Chabot Bullhead in France; with no evidence of any C. gobio in Britain, native or non-native.
Entire range of Common Bullhead Cottus perifretum, as of 2023, (orange = native, purple = introduced). Map from IUCN Red List, Leaflet, Esri
What changed?
As it turns out, the Cottus genus of bullheads is exceptionally large, with approximately 70 species and growing, but for the longest time, virtually all Cottus bullheads, those from Europe in particular, were all thought to belong to a single species, Cottus gobio, aptly named the European Bullhead; and this persisted for an astounding 250 years. It wasn’t until a revision in 2005, where molecular and morphological analysis of C. gobio across Europe, evidenced it to actually be another 14 different species in their own right. Below are 13 notable species from this study which are valid at the time of this blog’s publication.
5 species rediagnosed:
- Cottus gobio – (full range on figure below)
- C. hispaniolensis – France, Spain (former subspecies of C. gobio)
- C. koshewnikowi – Russia, Estonia, Finland, Sweden (former subspecies of C. gobio)
- C. microstomus – Poland, Lithuania, Slovakia and Ukraine (formerly synonymized with C. gobio)
- C. petiti – Lez basin, France
8 new species:
- C. aturi – Adour basin, France
- C. duranii – upper Dordgone, upper Lot, and upper Loire basins, France
- C. rondeleti – Hérault basin, France
- C. perifretum – Britain, and Scheldt, Rhine, Seine, lower Loire and lower Garonne basins, Netherlands, Germany and France (full range on figure above)
- C. rhenanus – Meuse and lower and middle Rhine drainages, Netherlands and Germany
- C. scaturigo – Endemic to a spring in northeastern Italy
- C. metae – upper Sava, Slovenia and Croatia
- C. transsilvaniae – upper Arges, Romania
To be clear, C. gobio is still a valid species, we just wrongly assumed all these other species were C. gobio too. It ranges from southeastern France to Moldova, and as far North as Germany, but the closest populations to Britain are not much less than 500km away, so if C. gobio ever comes to be identified in Britain, they probably would have gotten here through anthropogenic means.
Entire range of European Bullhead Cottus gobio, as of 2023. Map from IUCN Red List, Leaflet, Esri.
How is it that so many species were mistaken to be C. gobio?
For the sake of reading comprehension, and at the risk of infuriating someone in the field of biogeography, or taxonomy, I need to simplify some explanations here, but if you’re interested, I implore you to investigate the references cited at the bottom of the page.
Paratethys, the ancient mega-lake inhabited by the ancestor species of Cottus gobio. Late Miocene mega-lake regressions in Eurasia, 2021. Figure modified for The Saga of the Paratethys, Eduardo Geraque, da Revista Pesquisa FAPESP.
We’re going to have to turn back time here a little… okay, more than a little. The founding population of C. gobio stemmed from species which inhabited an inland sea, or so-called “mega-lake” called Paratethys (now the Caspian and Black Sea), which formed 34 million years ago. Those species populated Europe in the Pliocene (5.3–2.6 mya) from Paratethys through the Danube river, eventually reaching the northern coast of Europe, including Britain (which was connected by land to mainland Europe at the time), further colonising the rest of western Europe from there. In addition, there was another independent spread into Europe by C. gobio estimated around 1.2 million years ago in the Pleistocene, this time for eastern Europe—and there are two leading theories for how this particular expansion happened. The first is they may have exited what remained of Paratethys (the Black Sea) into the Dniester River which was then connected to the Vistula River, meaning they were able to migrate into various Polish river basins. Alternatively, in the Pliocene, much of Poland was covered by a shallow lake which ranged from Germany to Ukraine, and could have allowed for the spread of C. gobio across Europe. Along their tour of Europe, populations of C. gobio would diverge and adapt to these new ecologies and niches, becoming what we’ve defined as separate species entirely. Today, some C. gobio still persist alongside, or relatively nearby their diverged populations, though it’s likely the most genetically relict C. gobio remain closer to where Paratethys once was. What this means for us Brits all the way on the other side of Europe, is that by the time these specific lineages reached us, they may have already stemmed into a different species we now know as the Common Bullhead (C. perifretum). To answer the question more directly, most bullheads in Europe diverged relatively recently from a single common ancestor, C. gobio, so it was incredibly easy for us to believe they were all the same species without molecular and thorough morphometric analysis.
In taxa where there is so much similarity and overlap, an ingenious way to discern one species from another is to continually compare immediately adjacent populations until boundaries between populations can be detected (meristic, geographic etc.). When those boundaries are shown to be lineages rather than convergent features, then, according to the Evolutionary Species Concept, the lineage is considered a species.
Why are we only just finding this out now?
Herein lies the problem with Google and social media; a large proportion of websites and publications still list British bullhead as C. gobio, not C. perifretum, although popular fish websites like FishBase, IUCN, and even the citizen science app, iNaturalist seem to have recently caught up. There are a few reasons for this, the main one being a rather confusing element to taxonomy. C. gobio, while a species in its own right, is considered a “species complex” which contains all the aforementioned European bullhead species, so when a paper mentions C. gobio, they are could be referring to C. gobio as a single species, or any of the relevant species encapsulated within that complex. In addition, often in post-2005 scientific publications that feature British bullhead, C. perifretum can be synonymized with C. gobio, as it is the only member of the C. gobio complex in Britain; this has led to the false assumption we have both bullhead species in the UK. In fact, claims have already been made on social media about genetic proof of both species being found here in the UK, and these are entirely unsubstantiated. I even came across one person on social media, who vehemently claimed that the visual difference between the bullheads we have in chalk streams and those found in larger rivers amounts to irrefutable proof of the presence of both species. These are known as ecotypical variations (not to be confused with subspecies, which is a tier higher), and we’ve continued to document this single species variance in our bullhead since the 1950s. In fact, the ecologies of freshwater habitats are so influential, these differences go beyond their morphological plasticity, and we know that bullhead from our upland rivers mature slower and live longer than their counterparts from chalk streams, so for the sake of clarity, both are regarded as C. perifretum, not C. gobio.
It can get extremely confusing, especially as what defines a species complex is variable in what it encompasses. As this is unclear, authors misunderstand and believe the wrong species to be present in a given location, and it just cascades across the board, sometimes, people just aren’t aware of the Cottus revision. What I find most interesting, however, is that authorities in England deliberately continue to list our C. perifretum as C. gobio, because they already have protections in place under the name C. gobio. With their limited habitat range and private populations restricted to small sections of river basins; unable to genetically mix—it makes each population distinct from the next. This further troubles conservation efforts, as different groups in each river, can’t be treated as a single population which freely mixes, so I understand the reluctance to acknowledge the revised nomenclature, given the endless uphill-battle already faced in the protection of our rivers and their inhabitants.
Whilst some may not be satisfied by these changes, or argue that more research is needed, this is not always practical, and in several genus or family level revisions, it acts as a foundation or stepping stone for further research to be made.
“Sometimes, lines need to be drawn, be it only temporarily, to make the accumulating data usable for handling real world’s issues. In the case of sculpins, whose populations are under stress in most of Europe and which are listed in a number of local, national and international legal instruments, the morphological and molecular observations and the conclusions which can be reached at the moment need to be made available in a pragmatic way to those concerned by or responsible for management and conservation of natural resource” —Freyhof, J. et al. (2005)
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. 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.
The southern branch of Cottus perifretum from the River Ash, Surrey. Notice the “prickles” on the body © themainman13, iNaturalist, 2024
I’ve seen bizarre claims online that the second dorsal of C. perifretum runs along the whole of the peduncle, almost touching the caudal (tail) fin, whereas it doesn’t in C. gobio. It actually doesn’t do this in either species, and I have found nothing in literature which has cited this observation. I suspect the original person making this claim just saw a picture of C. perifretum with its second dorsal fin folded down, so it appeared longer.
The northern branch of Cottus perifretum from the River Wharfe, Yorkshire © max-pics, iNaturalist, 2024
I’ve made no mention of meristics, morphology and markings, as unsurprisingly, there is far too much overlap for this to be informative, and will further muddy this mud pie; though it is available on Freyhof, J. et al. (2005).
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!
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.
References
Cox, I.G. and Harvey, J.P. (2003) Monitoring the bullhead. Peterborough: Life in UK Rivers.
Crisp DT, Mann RHK & McCormack JC (1975).The populations of fish in the River Tees system on the Moor House National Nature Reserve, Westmoreland. Journal of Fish Biology 7, 573–593.
Englbrecht, C.C., Freyhof, J., Nolte, A., Rassmann, K., Schliewen, U. and Tautz, D. 2000. Phylogeography of the bullhead Cottus gobio (Pisces: Teleostei: Cottidae) suggests a pre‐Pleistocene origin of the major central European populations. Molecular Ecology 9(6): 709-722.
Fast, K., Aguilar, A., Nolte, A.W. and Sandel, M.W. 2017. Complete mitochondrial genomes for Cottus asper, Cottus perifretum, and Cottus rhenanus (Perciformes, Cottidae). Mitochondrial DNA Part B 2(2): 666-668.
Ford, M. 2024. Cottus gobio. The IUCN Red List of Threatened Species 2024. e: T259224371A135085385. Accessed on 23 November 2024.
Ford, M. 2024. Cottus perifretum. The IUCN Red List of Threatened Species 2024. e: T135511A137244297. Accessed on 23 November 2024.
Freyhof, J., Kottelat, M. and Nolte, A. 2005. Taxonomic diversity of European Cottus with description of eight new species (Teleostei: Cottidae).
Geraque, E. (2021) The Saga of the Paratethys, Revista Pesquisa Fapesp. Available at: https://revistapesquisa.fapesp.br/en/the-saga-of-the-paratethys/.
Mann RHK (1971).The populations, growth and production of fish in four small streams in southern England. Journal of Animal Ecology, 40, 155–190.
McLeish, J., Briers, R.A., Dodd, J.A. and Rueckert, S. 2020. First genetic evidence that invasive bullhead (Cottus L. 1758) in Scotland is of English origin and the difficulty of resolving the European Cottus species taxonomy. Journal of Fish Biology 96(3): 617-630.
McLeish, J. Non-native Bullhead in Scotland: Molecular and Morphological Identification and Parasite Links with Native Fauna. 2018. Edinburgh Napier University.
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Nolte, A.W., Freyhof, J., Stemshorn, K.C. and Tautz, D. 2005. An invasive lineage of sculpins, Cottus sp.(Pisces, Teleostei) in the Rhine with new habitat adaptations has originated from hybridization between old phylogeographic groups. Proceedings of the Royal Society B: Biological Sciences 272(1579): 2379-2387.
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Van Liefferinge, C., Dillen, A., Vught, I., Auwerx, J. and Meire, P. 2019. Microhabitat suitability as a tool to improve the success rate of a translocation or reintroduction, case‐study of the bullhead (Cottus perifretum) in Flanders, Belgium. Restoration Ecology 27(3): 504-512.
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