If you’ve spent a lot of time at the beach recently, you might have seen some interesting creatures.  We’ve been thinking about mermaids.

The notion of a mermaid – a creature with the torso of a human and the tail of a fish – is pretty ridiculous from a biological standpoint.  But imagine, for a second, that you came across a mermaid while surveying a coastal area.  How would you classify it?  What would you tell people about your discovery?

Biological classification is a tricky science.  Species that look similar may actually only be loosely related to each other, or their similarities could be a result of convergent evolution.  What class would you put mermaids in – fish or mammals?  Or something different entirely?

When encountering a new species, biologists look for similarities to other species that go beyond visual similarities. One question here might be how the mermaids breathe.  If they use gills, they might be more closely related to fish, but if they breathe air, they could be more mammalian.

In most myths, mermaids live underwater but can breathe air.  Does this mean they’re amphibians?  Maybe, but not necessarily – crabs aren’t amphibians, and they can breathe both air and water through their gills as long as their gills are damp.

Maybe mermaids don’t breathe underwater at all.  Dolphins, seals, whales and sea snakes all breathe air, but they can all hold their breath for a very long time.  Some seals can hold their breath for two hours, and this pseudoscorpion can hold its breath for 17 days! So it’s quite possible to be a sea-dwelling creature without needing to breathe water.

If you were really keen to classify this mermaid, the best option would be to take a DNA sample (you would presumably need to ask the mermaid’s permission to do this).  From there, you could see which species the mermaid had the most DNA in common with.  There might be a case of horizontal gene transfer, where DNA from one species has been transferred into another. DNA analysis is a modern tool for biological classification that has allowed scientists to re-classify a number of species.

Of course, this is unlikely to appear in any scientific journal anytime soon, but it’s a useful thought experiment for biologists.

But here’s a deep-sea visualisation that examines some real – and really bizarre – ocean creatures.

If you’re interested in animals, classification and adaptation, you might be interested in our Animal Behaviour degree.