Sea hares’ sticky defence uncovered

March 28, 2013
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Sea hare 'inking'Sea hares release more than just purple ink when threatened

The sea hare, a soft-bodied marine creature, uses a sticky secretion to fool hungry predators, say scientists.

The slow-moving animals are known for defending themselves by squirting an off-putting mixture of purple ink and a white substance called opaline.

However, exactly how this sticky opaline is used to deter predators was previously unknown.

Now scientists have shown the substance coats predators’ antennae, deactivating their chemical senses.

Researchers suggested that with their sense of smell blocked predators lose their appetite and spend a long time cleaning themselves of the sticky coating, allowing the sea hares to escape.

The team from Georgia State University, Atlanta, US said that their study is the first time “sensory inactivation” as a defence against predators has been shown in an experiment.

Details of their findings are published in the Journal of Experimental Biology.

Research team member Dr Charles Derby described the finding as “significant”.

“It is the first demonstration involving not only the chemical senses, but to our knowledge, for any sensory system,” he told BBC Nature.



Video clip: sea hare uses its ink secretion defence to block a spiny lobster's sense of smell (Video courtesy of Paul M Johnson)

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Watch an “inking” sea hare uses its sticky opaline defence to block a spiny lobster’s sense of smell

Ink squirting

Wild sea hares have a variety of defensive adaptations, including chemicals found in their skin and skin mucus as well as their ability to squirt ink secretion onto predators.

“Inking is a last line of defence,” explained Dr Derby, “[It is] only produced when the sea hare is taken into the mouth of a fish or only after being bitten by a lobster.”

The purple ink and sticky white opaline squirted during “inking” are produced in separate glands, and sea hares can release them separately or together.

To simulate how opaline affects predators’ chemical senses in the wild, the research team used an extract taken from sea hares’ glands and painted it onto the antennules – the first pair of antennae and olfactory organs – of spiny lobsters in water tanks.

With the predators’ antennules coated with the sticky substance, Dr Derby and his colleagues presented the lobsters with the appetising odour of “shrimp juice”.

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The strange lives of sea hares

Sea hares are gastropod molluscs. They appear to have no shells, but in fact do have small, completely internal ones.

Aplysia is the name of the sea hare genus, which contains many different species found around the world.

Other than their chemical defences, sea hares’ large size puts off many predators: some species grow up to 70cm in length.

The creatures are called sea hares because of the resemblance of some species to a sitting hare.

They then measured the electrical activity in the chemosensory and motor neurons in their antennules which detect odours and are responsible for sending signals from the brain to the muscles.

Both types of neuron are activated by food odours and are essential for the animal’s motivation and ability to feed.

The team found that the spiny lobsters’ responses to tasty smells were significantly reduced when their antennae were blocked compared to when they were clean.

The finding that sticky opaline physically limits predators’ reception of food odours represents one of at least three ways sea hares use ink secretion as a defence.

Dr Derby’s previous studies have also shown high concentrations of amino acids in ink can be appetising to some animals, effectively acting as a distraction.

“A lobster, when it bites a sea hare and gets a whiff of the ink, will drop the sea hare and attend to the ink secretion,” he explained.

And off-putting chemical compounds found in the ink such as aplysioviolin, which gives it its purple colour, also help drive away attackers.

“Sea hares have many potential predators, each with feeding habits [and] sensory systems… So, some chemicals may work on some predators and not on others,” said Dr Derby, explaining the animals’ multiple defences.

“A combination of mechanisms acting simultaneously may be more effective than any one alone.”

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