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March 2020

Fishy remains

What are these? They were found washed up on the beach.

Answer

Swim bladder of a Porcupinefish. Photo: Harriet Preece.

An eroded toothplate of a Porcupinefish. Photos: Lyn Dryden.

Isolated jaws of a Porcupinefish, showing the toothplates.

A Threebar Porcupinefish (Dicotylichthys punctulatus) on display at the Queensland Museum.

High tides bring a treasure trove of curious specimens to the beach, especially after storms. These two objects were recently sent to the museum for identification by two different people, but both objects are actually part of the same species of animal: a Porcupinefish (a member of the family Diodontidae). These bizarre fish are famous for both their highly toxic flesh, and their defensive strategy of inflating themselves with water, ensuring that potential predators are unable to swallow them. They can also use this tactic to wedge themselves into reef crevices. In this position they are anchored in place by the spines the cover their bodies, preventing would-be predators from dislodging them.

The first object is a swim-bladder. Most bony fish have a gas-filled swim-bladder to control their buoyancy, and also to provide stability while swimming. Porcupinefish have a particularly large and durable swim-bladder which also appears to be resistant to decay, as it is still intact and inflated some time after the death of the fish.

The second object is a toothplate. Porcupinefish have powerful jaws, and their toothplates are used to crush molluscs, crabs and sea urchins. The toothplate is very hard, so it tends to persist in the environment long after the fish has died. This specimen looks like it has tumbled around in the surf, becoming rounded, rather like a pebble or a thick fragment of a sea shell.

A porcupinefish has previously featured in our Question of the Month, in a rather tragic encounter with a Numbfish.

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