Tales from the Aquarium: Squid Dissection

I’ve recently had the opportunity to do some squid dissections for an audience of fascinated/horrified children and adults. Although we did do a squid dissection lab in my first or second year of University I’d long since forgotten the details, so it was quite fun to relearn these and share them with the public.

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Firstly, squids are molluscs in the class Cephalopoda, joining octopus, nautilus, and cuttlefish. They have eight arms, and two tentacles. Typically the arms have suckers all over, while the tentacles only have suckers at the ends, but generally if you try to demonstrate that to an audience you will inevitably have a mutant squid with suckers all over its tentacles too. The tentacles can be fired forward to grab prey and retracted to bring the prey back to its mouth, which is a keratinous beak (virtually identical to a bird’s beak, superficially anyway). This beak is surrounded by a ring of very strong muscles as it is used to crush the shells of crabs, bivalves, etc.

There is also a muscular tube called the siphon, which is used for rapid movement by propelling water through it, and for squirting ink in a defence mechanism when threatened. Squid ink is not toxic/poisonous/venomous, but purely a visual distraction. At the opposite end of the squid are two fins which are used for directionality when swimming.

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The main body of the squid is the mantle, and by cutting this you open the mantle cavity and expose the internal organs. Rings of mantle tissue are what calamari is made of (and I bet I’ve put some people off that for life doing this presentation). Once inside you can sex the squid based on the presence or absence of male gonads. Females are often also carrying thousands of eggs. There is a silvery organ which is the ink sac, and if you puncture it you can usually get some black ink out. There was never very much in any of the squids I dissected, presumably because they had used this defence mechanism when caught.

Squid also have a stomach and three hearts – one for each gill and one for the rest of the internal organs.

By making a small incision at the top of the dorsal side (back) of the mantle, you can reveal the gladius or ‘pen’ of the squid and pull this out. As squid are molluscs, this isn’t a ‘spine’ but a vestige of the shell which has been mostly lost over evolutionary time. It is mainly comprised of a material called chitin.

While the main aim of doing the squid dissection demonstration was obviously public education, these squid would have been cut up anyway as they are what is fed to the sharks. Before feeding them to the sharks the aquarists remove all hard parts as these can’t be digested and might cause upset stomachs (not ideal in a tank). By now in the dissection I have removed the beak and the pen, leaving only two small hard parts in the body – the lenses of the eyes.

Squid have large eyes which are excellent for seeing underwater and in low light conditions. Part of this is a globular spherical lens inside of the eye, which is moved (rather than changed shape, as in vertebrates) to focus.

Cutting open the eyes was the most perilous part of the dissection as they frequently squirted over audience members or myself – getting squid eye in my eye was not a professional highlight!

With the lenses removed the dissection was done, and I would return the poor mutilated squid to the aquarists to be supper for sharks later that day.

I really enjoyed getting (gloved) hands on for this activity and it went down pretty well with most audiences, although my favourite part was a small child who told me very calmly, “I don’t think I’d like to do your job. It’s quite gross. I’d like a nice job.”

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Squid splash…

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