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So long, and thanks for all the Fish!
Huge! We don't know their exact numbers but the IUCN conservation group lists them as LC (least concern). They can be found along any coastline in the UK, but also along the coasts of mainland Europe and North Africa.
About 4.5 Gb (4,500,000,000 DNA base pairs). This is about 1.5 times bigger than a human genome (~3 Gb).
Humans last shared a relative with sharks about 420-450 million years ago in the Palaeozoic era. We sometimes think of sharks as 'living fossils' because they look so similar now to their fossils from back then.
I should be sequenced because...: We can reveal the genetic basis behind catshark superpowers including regeneration, electroreception, and fluorescence!
At only 1 metre in length when fully grown, the catshark isn’t exactly ‘Jaws’ but make no mistake this cartilaginous critter is every bit the shark as its bigger cousin the great white (Carcharodon carcharias). With a light and flexible skeleton made primarily of cartilage, a streamlined body protected by tiny tooth-like scales (denticles), and a mouth full of multiple rows of teeth this catshark is a talented predator in its own right. Any prey lucky enough not to give away its location by sight, smell or sound to an average predator might get away except for the extra super-sense catsharks share with larger sharks: electroreception. Even the tiniest of electrical current produced by a beating heart can be enough for special organs, called Ampullae of Lorenzini, to give away the prey’s location.
The catshark is a demersal (bottom-dwelling) species and although it does inhabit the shallows from about 4 metres in depth you would rarely encounter one whilst swimming in the sea as they prefer prawns to people! Like most sharks, catsharks are completely harmless to humans. This is just as well since here in the UK we are quite completely surrounded by catsharks which inhabit every mile of our coastline. The lesser-spotted catshark and its close relative the greater-spotted catshark, are the two most common sharks inhabiting the British Isles and truly are iconic species.
Despite their iconic and abundant status you could be forgiven for not having heard much about them being, as they are, generally shy marine organisms which have also fallen out of favour at the fishmongers. The most common way we encounter catsharks are usually in the form of their egg-cases or ‘Mermaid’s Purses’ which frequently wash up on the beach. Catsharks are an oviparous (egg-laying) group of sharks and deposit their camouflaged, leathery egg-cases on the sea floor. Long tendrils attached to the corner of each egg-case snag seaweeds and prevent the eggs from drifting away from this ideal habitat for the hatchlings which will emerge 5-9 months after laying, depending on the sea temperature.
If you are interested in sharks and want to to try looking for these Mermaid’s Purses the next time you head to the beach check out The Great Eggcase Hunt, a citizen science program run by The Shark Trust.
One reason scientists are interested in catsharks is because they can be maintained in small aquaria in research laboratories just about anywhere in the world. This means we can study their biology in detail. Because of this, catsharks are rapidly becoming what scientists call a ‘model species’. Model species are species which can be used as a representative of their whole group and which are suitable for scientific investigation.
Catsharks growing in an aquarium in the Gillis laboratory at the University of Cambridge can be taken out and put back into their eggcases once they are old enough.
Using the catshark as a model species is a good idea because many other species of shark are either too large (e.g. the whale shark Rhincodon typus) or endangered (e.g. the angel shark Squatina oculata) to study in the lab. Fundamental principles of shark biology can be worked out using the catshark and this might even help protect some of the most endangered species in the future. By voting to sequence the genome of the lesser-spotted catshark, you will help scientists make the most of this new model species.
A newly hatched baby catshark (called a ‘pup’) in an aquarium in the Fraser laboratory at the University of Sheffield ‘breathes’ by sucking water into its mouth and pushing it out past its gills while it rests on the bottom of the tank.
Another reason to study the catshark, besides helping to protect the most endangered species, is because they are a distant relative of humans. Although we don’t have a sequenced genome for the catshark yet we know that they have many of the same genes as people do. By comparing genomes between sharks, humans and other animals scientists can work out what has changed to make them different from one another during evolution. Using the catshark genome in this kind of work, called ‘comparative genomics’, is important for trying to understand the evolutionary origin of our own body parts, like bones, limbs, and even teeth.
Because of fossil evidence, palaeontologists like Dr. Zerina Johanson at the Natural History Museum think that a very long time ago, even before there were sharks, now-extinct jawless (and toothless) fishes first used dentine and enamel (the two hard tissues in your teeth) in a kind of body armor called ‘odontodes’. Then, somehow during evolution, the genetic program for making this body armor was borrowed, or ‘co-opted’, by the mouth and that is how the first teeth were formed. Not many animals still have both teeth and this body armor, but sharks do which makes them very special! They are called ‘denticles’ and they are just like having body armor made of thousands of tiny teeth. Fossils are very important for figuring out when this happened, but we need to use comparative genomics to figure out how it happened.
Speaking of teeth, do you know how many a shark has? During their lifetimes, some sharks can produce more than 30,000 teeth! Although sharks and humans share a lot of biology and have many of the same genes, one major difference between sharks and humans is that sharks regenerate their teeth constantly throughout their whole lives! We have only two sets of teeth in our life – what we call ‘baby teeth’ and ‘adult teeth’. In sharks, the so-called tooth ‘conveyor-belt’ is very handy for making sure there is never any shortage of teeth for chomping down on their prey. This power to regenerate teeth, called ‘polyphyodonty’, is at least as old as the last common ancestor between sharks and humans, but humans have almost completely lost it. If we don’t brush or lose a tooth in an accident or due to old age we have no way of growing it back. Scientists are trying to figure out how sharks are able to do this using the catshark as a model species. By sequencing the catshark genome scientists will be able to work out exactly which genes are responsible for tooth regeneration in sharks.
X-Ray micro-CT scanning at the Natural History Museum’s Imaging and Analysis Centre showing the regenerating teeth and tooth-like scales (denticles) covering this baby catshark’s head.
The lesser-spotted catshark, Scyliorhinus canicula, is a wonderful species and I hope you will be tempted to learn more about it after reading above. If you can’t wait until the web chats and plan to do some diligent googling please beware the trickery of taxonomy (the naming system of living things)…
This species is often also called the ‘small-spotted catshark’ or even the ‘small-spotted dogfish’. Be careful not to confuse it with the spiny dogfish, Squalus acanthias, which also lives nearby, or the greater-spotted catshark (Scyliorhinus stellaris) which is a close relative. When in doubt always use the latin name (in italics above) to look up your favourite species because common names tend to change over time!
Remember to vote for the lesser-spotted catshark here:
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