When paddling on a rocky shore in spring and summer, it’s not unusual to find clumps of pink ‘silly string’. These egg masses are often more obvious than the animals that produce them. But if you’re up for the challenge, then get down on your knees and start to scrutinise the rockpools for sea hares, Aplysia punctata.

What are sea hares?

Related to the sea slugs of the nudibranch order, sea hares are a similar-looking group of herbivorous marine gastropod molluscs. One of the most notable features of the group are their ‘ears’, protruding from the top of the head, which somewhat resemble the long ears of a hare. These are actually a pair of rhinophores – structures that can detect chemicals in water.

What do sea hares look like?

Since they take on the colour of the algae they consume, sea hares blend in with their surroundings so to be almost invisible. What you’re looking for are brownish blobs with varying levels of black or white speckling.

These mad-looking marine molluscs are frequently lumped together with sea slugs and other superficially similar, shell-less animals. Don’t be put off by what seem to be ugly, glutinous masses – I guarantee that they’re fascinating and rather lovely.

How do sea hares reproduce?

Sea hares have a bizarre sex life (though to be fair, not that odd for molluscs). Like many others, they are hermaphrodites, with functional male and female organs in the same beast. This can lead to confusing orgies, where a chain of animals forms, each fertilising their neighbour and laying eggs at the same time, with the animal that started the chain acting like a female and the last to the party a male.

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Why are they called sea hares?

But when sea hares stretch out to feed, they start making some kind of sense. How they get their name is immediately apparent. On top of their ‘head’ are a pair of tentacles called rhinophores, which stick up like bunny ears. Together with another pair of oral tentacles closer to the mouth and a couple of tiny beady eyes inbetween, these form a sensory array used to detect food, mates and predators.

Meanwhile, on their back is a frilly, upside-down skirt made up of wing-like appendages called parapodia. Hidden under these is a soft internal shell, the gill chamber and a structure known as the siphon, which sucks water in and out, allowing the animals to breathe.

How do sea hares defend themselves from attack?

Sea hares slide around rockpools, grazing in a slug-like manner. They use their radula (equivalent to a tongue, but wielded like a file) to scrape away mouthfuls of seaweed. However, these creatures represent a good mouthful of protein to many a potential predator, from crabs and lobsters to anemones and fish. Soft and squishy they may be, but defenceless they are not.

If you were to pick up a sea hare, you might find your hands covered in a spectacular purple substance. Underwater, the cloud of ink they produce must undoubtedly serve as a visual warning, yet it also contains a special kind of molluscan alchemy. The ink is made by one gland, while another produces opaline. This secretion is squirted at an attacker using the exhaling siphon.

When mixed with seawater, opaline becomes a gloopy, sticky compound – imagine getting this all over your gills or mouthparts! Initially, the gunk becomes a super-stimulus to crustacean predators, who think that the sticky soup is their prey, rather than the sea hare itself. Shortly afterwards, secondary effects kick in. The sensory organs with which the crustaceans located
the ‘prey’ become blocked.

Better still, the sea hares’ glandular secretions react with each other to form more nasty compounds, including sodium peroxide and carboxylic acids, which act as extra deterrents. One squirt of this foul chemical cloud is usually enough for a sea hare to slink away unharmed.

Main image: sea hare Aplysia punctata © Getty Images


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Nick BakerTV presenter and naturalist