Picking a bone with cuttlefish

 

sepia_officinalis-f

Common cuttlefish (Image by Magnefl via Wikimedia Commons, CC-BY-SA 3.0)

After ethical concerns excluded all vertebrates from the UK school experiments, the only animals to observe are invertebrates e.g. stick insects and snails.

We buy garden snails and students sketch them and feed them various foodstuffs. Apparently, snails like cucumber or bread and don’t like tomato or carrot. We also abandoned giving them sugar as one of the tutors swears that “snails crazy on a sugar rush”.

The same tutor had collected snails and decided to keep them until needed for experiments. She keeps them in an aquarium with moss on the floor and they are so happy that they have laid eggs. Snails need calcium to maintain their shells. My colleagues had a discussion about the best source of calcium – crashed egg shells or ‘cuttlefish bones’.

cuttlefish-cuttlebone2

The upper side of cuttlebone – not a bone but a shell. It’s comparable to a length and width of your hand.  (Image by Mariko GODA via Wikimedia Commons, CC-BY-SA 3.0)

Cuttlefish bones? I was surprised. Cuttlefish is a relative of octopus, a mollusk, it shouldn’t have bones. However, all mollusks used to have a shell made of calcium carbonate, it just been lost in snail evolution.  But they have a remnant inside their body called ‘cuttlebone’. I was even more surprised to learn that cuttlebone is not a useless atavism but its chambers are filled with gas and used for buoyancy.

See also: Wikipedia article about cuttlebone contains nice images made by industrial micro-computed tomography.

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Nautilus. The ancestor of cuttlefish looked like it, notice the tentacles. (Image by  J. Baecker via Wikimedia Commons)

Literature:  Rexfort, A.; Mutterlose, J. (2006). “Stable isotope records from Sepia officinalis—a key to understanding the ecology of belemnites?”. Earth and Planetary Science Letters247 (3–4): 212–212.

 

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