Underwater chemical warfare in sea hares

Written by Thomas Hesselberg

Wednesday, 19 December 2007

The sea hare (Aplysia californica) squirt a ink and opaline cloud when threatened. Photo from Wikepedia and taken by Genny Anderson.
The battle between prey and predators is a bloody and savage one that has been ongoing for 1000 of millennia. In some cases the weapons are obvious like the mighty claws and sharp incisors in the lions and the impenetrable protection of the tortoises. However, the battle is also fought with more sophisticated weapons. The sea hares are underwater slugs. Unlike other gastropods they are not protected by a shell, but employ a host of chemical defences including deterrent compounds in the skin. However, the most striking defence mechanism in sea hares is the release of a purple cloud of ink and opaline. Like the ink from octopuses it reduces the visibility and increases the sea hares chances of making an unseen get away. Interestingly, the chemicals in the cloud also interact with the sensory system of predators and act surprisingly enough as feeding stimulants. This is not as stupid as it sounds though. In this way the sea hare lures the predator into believing that the cloud contains food, which attracts the predator towards the cloud and not towards the sea hare.

Researchers from the Georgia State University in the US investigated the effect of the cloud from the sea hare Aplysia californica on the behaviour and sensory system of one of its most common predators, the California spiny lobster Panulirus interruptus. They noticed that ink and the opaline had a low pH-value and made the water more acidic and set up to investigate the role of this. They did this by making behavioural tests in the laboratory on spiny lobsterns exposed to sea hare clouds at different pH-values of the water. In addition they made electrophysiological assays on the olfactory system in the lobsters during exposure to the chemicals in the cloud. The results showed that lobsters were significantly more attracted to the individual compounds at a low pH-value, primarily because the chemoreceptor neurons were more sensitive at low pH-values.

Thus the sea hare directly manipulates the sensory system of the spiny lobster by using acid to make the ink and opaline defensive cloud even more attractive.

Source
Shabani, S., Yaldiz, S., Vu, L. and Derby, C. D. (2007). Acidity enhances the effectiveness of active chemical defensive secretions of sea hares, Aplysia californica, against spiny lobsters, Panulirus interruptus.

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