How many cephalopod species are there in the UK?

Museum visitors ask some of the most innocent yet challenging (and also some important) questions . Why do animals have tails? How do you know it is dead? Is it real? Does it fart? Some of these questions, have a difficult and convoluted answer and other questions like “which animals fart?” can never be comprehensively answered even with an army of fart recorders sent out to the oceans, deserts, forests and cities of the world. As a general rule, and if some clever sod hasn’t already coined it, let’s call it the Carnall Rule, the simpler the question about biology, the more difficult it is to answer. I’d also add the qualifier, the more difficult it is to answer in a way that’s understood by most people.

I love thinking about some of these questions and I love asking them too. These big questions cause us to take a step back, do a bit of research and more often than not, question the question. So with this questioning spirit in mind let’s try to answer the simple question (ut oh): How many cephalopod species are there in the UK?

It’s a question, I don’t know the answer to, it’s a question I might be asked at some point and it’s a question I’m interested to know the answer to.

In addition to a love for asking questions about biology I’m also really interested in how people explore or learn about the world be it through exploration, visiting museums, reading or being taught. A child today in theory has access to information in a way that I couldn’t have imagined growing up. My formative years and experiences of biology, natural history, evolution and palaeontology was mostly through books. Books we had in the house and books at the library or school. As awful as the content was in 70s and 80s popular wildlife and palaentology books was, my interest was piqued by poring over illustrated dinosaur books, animal encyclopaedias and guides to the seashore. We didn’t have a computer at home, the Internet didn’t exist for most of my childhood (and even then there wasn’t much on it for a while) and I would habitually check the three bookshops in my home city for the random arrival of a new book on palaeontology. This probably seems like an archaic way to access information but for my upbringing and generation this is how it went and I don’t take today’s access to a vast wealth of information at my fingertips for granted.

But this democratic access to information is very much theoretical. Access to the Internet is sadly still a luxury to many and on top of that you have to know or be taught a little bit about how to find, process and question information. My mind was blown when I was lucky enough to go to University to find that the subjects I loved filled entire libraries of books and journals. Used to absorbing just the limited sources I could find in my home city, there was a whole world of information I could never have imagined. I appreciate this is a very privileged position to have been in. One of the things I quickly learned was that all the books and documentaries I’d seen up to that point were just the tip of the iceberg. Naively, I thought that my copy of Dinosaurs an A-Z guide was a comprehensive list of every known dinosaur! I assumed that my Collins guide to the seashore listed every single animal! Not even close. Nowhere near. So as well as access to information, learning how to learn and learning the limits, biases and boundaries of what is currently known is also a key skill I try to develop every day.

So with these thoughts in mind, let’s try to answer the question, How many cephalopod species are there in the UK? Using different sources of information to see both the limits and reliability of information and the complexity of interpreting what this means.

Of course, we need to start off with questioning our question. We’re after the number of cephalopod species in the UK. By cephalopods we’re looking for animals in the class cephalopoda the group that includes octopuses, ‘squid’, cuttlefish, vampire squid, nautiluses and a few others. As well as being a personal favourite, it’s a small enough group to be able to quickly process data about, answering this question about insects or gastropods would be impossible! What do we mean by species? Well that’s one of those simple but difficult questions in biology and about 45 blog posts for another time so we’ll take on face value what our different sources of information say are discrete species without questioning it too much. I will be limiting the search to recent cephalopods and not fossil specimens again, for ease of reference and scrutinising data. Lastly what do we mean by in the UK? Well, I know that there’s only a handful of terrestrial cephalopods so we’re looking for cephalopods around the coasts and in the oceans. Where possible, I’m not going to include areas like the United Kingdom’s Overseas Territories as it’s not in the original spirit of the question and because so many UKOT are islands with a rich fauna they’re likely to represent a species richness that starts to get difficult to interpret within a blog post. For many of the below resources it often wasn’t clear what the ‘UK’ meant or how to delineate so the results will be a bit messy in this aspect, again highlighting how a simple question raises complications off the bat.

With the answers filtered by number of species according to different sources. The sources of information range widely in their authority, age and reliability and some have required a bit of cross referencing to end up with a number.

Collins Complete British Wildlife (1) Collins and other publishers publish a range of wildlife guides intended for amateur use and identifying common species, the edition I have to hand is a 1997 edition so well out of date to boot. Now, as a young Mark Carnall, was once hoodwinked you’d think that the ‘Complete’ British Wildlife guide may be errrr complete. Misleading title aside, the introduction to the book does state that it is not a comprehensive guide and that task would be impossible to squeeze into anything near just one book. The guide lists just one species of cephalopod, the common octopus Octopus vulgaris.

A Handguide to the Sea Coast (2) One of the books from my childhood. A beautifully illustrated little book from 1981 with painted dioramas showing the flora and fauna of the British coast. Only two cephalopod species are featured, the common cuttlefish and the little cuttle (Sepiola atlantica) making it 100% more complete than the complete Collins guide.

The Marine Life Information Network (15) MarLIN is the marine biological association’s portal with information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles. Species profiles can be found at the bottom of this page. MarLIN lists 15 species of cephalopods.

World Register of Marine Species (16) One of the foremost data portals for a list of marine species (as the name suggests). Data is controlled by a global community of taxonomic experts pulling data from a number of global species databases. It’s a fancy list. A huge, authoritative, important but not perfect list. According to WoRMS there are 16 species of cephalopods with a UK distribution. Interestingly, WoRMS doesn’t recognise Sepiolida, the bobtail squid, as a valid order.

ICES 2015 Report on Cephalopod biology and fisheries in Europe (17) This 2015 report by the International Council for the Exploration of the Sea lists just 17 species of European species, however, the report is focused on the species most commercially exploited as well as species often caught and identified as bycatch or fished in smaller operations.

International Union for Conservation of Nature Redlist of Threatened Species (28) Not really meant for looking up distribution data per se but the ability is built into the portal. The IUCN redlist is “the most comprehensive, objective global approach for evaluating the conservation status of plant and animal species”. The IUCN lists 28 species but some of the more common species such as the common octopus, veined squid and European squid aren’t listed. Fun fact, 17 UK species are listed as least concern, 9 are data deficient and 2 are vulnerable.

Dictionary of UK Species (42) The Natural History Museum London hosts the dictionary of UK species a portal with over 250 checklists of species. After a bit of playing around with the search interfaces, there seems to only be one checklist with coverage of cephalopods, which is the Ulster Museum and Marine Conservation Society Marine Directory. The portal is designed more for looking up a specific name rather than all species within a given group. I couldn’t get the taxonomic group search on this page to work and the checklist itself is only searchable by phylum. Doing it the old-fashioned way, I looked up the original document. The directory lists 43 species of cephalopods in the “British Isles and surrounding seas”.

The Global Biodiversity Information Facility (49) This resource is one of the most amazing portals for biological data, although far from comprehensive it is perhaps the largest portal for biodiversity information pulling together information from biological records, literature, larger museum collections, human and machine observation. With over 850 million records it’s currently our best resource for understanding big biodiversity data. However, as it’s geared towards georeferenced data it’s not too simple to search for species recorded from the UK and its surrounding waters. GBIF lists a super whopping 76 cephalopod species from the UK however there are some errant fossil species that come up in the search even with fossils filtered out of the search. Correcting for fossil taxa, GBIF lists 49 species from the UK.

Discrepancies in Data

These are the information sources I could easily pull together and analyse in an evening but as can be seen, excluding the two popular books, there’s a wide discrepancy between these authoritative, scientific sources. With the highest number almost three times that of the lowest. Without comparing the lists entry by entry there were also a few notable differences such as the IUCN missing some of the most common species and higher level taxonomic differences. Across the resources there were a number of unique species occurrences, species which were only found in one resource and not another. There were 11 in the Ulster Museum list, 9 for the IUCN, 1 in the ICES report and 12 on GBIF. In total, 71 different species of cephalopod are listed across these six resources.  

So why does our total number come out almost 5 times as many as our lowest estimate (excluding the two guides) and 1.5 times higher than our highest estimate? Can we reliably say that we know anything about the number of cephalopod species in the UK at all? How can networks of marine biologists, museums, research institutes, universities and taxonomic experts get it so wrong? Well this is where learning to learn as I mentioned above come in. Each of these resources has its own biases and differences in recording even basic information like numbers of cephalopod species which is not necessarily explicit from the portals themselves.

As I mentioned above, defining the UK was a bit of a fudge and no doubt there are some non-UK or near-UK species which have crept into some of these data sets depending on how regions, especially marine areas are defined. Then there’s subtle differences in how species data will have been recorded and depending on the data set, it may be more or less open to scrutiny. The IUCN for example is biased towards species which are likely to be threatened or vulnerable, hence the omission of common cephalopod species. MarLIN isn’t a comprehensive database but an information resource of some of the most common species. WoRMS lists many species but not all taxa are linked to distribution data. The ICES report is a list of species of commercial interest and GBIF is a comprehensive resource that lists every single known occurrence in the region, including species which are rarely seen or occur within the geographical region covered. Some of the resources will have ignored one-off occurrences  such as Cryptoteuthis brevibracchiata known from a single specimen. Others may have filtered out species that aren’t considered ‘native’ and that can be very hard to define for marine species.

Another difference between these resources is their transparency. WoRMS and the IUCN may be authoritative but aren’t necessarily easy to trace exactly where data has come from, especially distribution data. Although GBIF had the highest number of unique species, suggesting that it might have the most errors, with a lot of legwork, in theory, each and every entry can be traced back to a museum specimen or data set and at least in some cases verified.

The differences between the species totals for cephalopods is clearer to unpick here because cephalopods are a comparatively small group of species, thought to be less than a thousand living species. You can imagine that for larger groups like insects, crustaceans and gastropods undertaking a similar exercise would take a lot longer than an evening and would end up with hundreds or thousands of dubious species records. These problems also have implications for interpreting big data sets and why some are wary of relying on automated analyses without knowing how to scrutinise the source data. This is the wonder and complexity of biodiversity and biodiversity data and why meta-analyses are sometimes the best way to get around dealing with messy specific data sets. I also find it humbling that with all our intelligence and technology and tools the natural world still defies our attempts to catalogue, classify and understand it, sometimes at a very basic level.

I hope this little exercise has also demonstrated how important it is to always question the data set. Just because it comes from a consortium of experts, or is called a ‘complete’ resource, or has millions of data points doesn’t mean it should be inherently trusted or that data isn’t skewed. This doesn’t mean that scientists are out to lie or deceive either but some expertise is needed to explain or interpret complex information. Some of these resources do have their caveats and biases tucked away on an ‘about’ page somewhere but others could be more upfront with the limitations in data quality and reliability.

So how many species of cephalopod do we have in the UK? Like I said at the beginning, sometimes the simple questions are the hard ones to answer. It’s definitely more than 1 and probably a bit less than 70 but of course, this doesn’t include the species still to be encountered and described…