Tonna perdix (Linnaeus, 1758)
Partridge tun, 160mm

Tonna perdix is moderately common in the Marshalls, but it most often seen as empty or broken shells. The living animals are nocturnal, emerging from hiding at night to hunt for sea cucumbers. They can be found on both lagoon and seaward reefs, and seem most common in the manmade reef quarries along the intertidal reef flat on Kwajalein Island's seaward reef.

One night I caught one in the act of attacking a Stichopus sea cucumber. Unfortunately, my camera was stuck at the wrong focal distance, so the photo below is a bit unfocused. However, it does show the large mouth emerging from the anterior end of the tun shell. Just before this photo, it had expanded its mouth around about half of the sea cucumber, starting at one end. The sea cucumber was frantically working its tube feet and stretching the free portion of its body in an attempt to get away. Suddenly, two strips of the cucumber's thick outer skin, one strip on each side, detached from the cucumber's body. The portion of the cucumber that had been in the tun's mouth then sprang out like a contracting rubber band that had been stretched, which is the moment the photo below was taken. As soon as the cucumber was out of the tun's mouth, it released its tube feet from the bottom and was pushed away from the predator by a surge in the water. The prey escaped, and presumably was able to regenerate its lost skin, which is shown in the following photo below.

Here are the two flaps of thick skin that the sea cucumber sacrificed in order to slip out of the mouth of the attacking tun shell. The tun was left only with these in its mouth, and apparently they are not very edible; the tun dropped them and wandererd off to look for easier prey. Note that some sea cucumber tube feet were also sacrificed and are still holding onto the substrate from the lower flap.

Created 10 January 2011

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