CHECKING OUT THE LOCAL shore dive off the New South Wales coast, Malcolm was a little taken aback by the scene in the shallows.
Off the Dorset coast a few years previously, he had shore-dived off the beach looking for sea hares to photograph. He liked their faces, their “ears”, the fact that their eyes – unlike those of their distant cousins the nudibranchs – are obvious and appear to look at you, perhaps bestowing on them some personality.
He also appreciated the photographic challenge, bringing shape and character to an animal that can look a bit of an amorphous blob. Moving across to Australia, however, he had been surprised by the huge size of the hares there – his immediate thought was of a kind of Godzilla-like, radioactively induced giantism.
More shocking still was the flagrant orgy of sexual activity displayed by these Australian sea hares. In a few clusters around the site, several of the giant snails at a time joined together in a curved chain, draped over bushes of red algae. A big slimy orgy, all of them gorging on algae and all copulating at the same time.
Sixty-two species of sea hare are found throughout tropical and temperate seas, more than half of them in the genus Aplysia. The name sea hare comes partly from the hunched posture – which is basically the internal shell (it is a snail rather than a slug, albeit with a shell that is buried inside its tissues) and organs wrapped pasty-like by the side lobes (the parapodia) of the fleshy body.
In some species these large flaps become wing-like and they can “fly” short hops between patches of algae.
Two more rolls of the same fleshy body (the mantle common to all molluscs) become hares’ ear-like rhinophores and nostril-like oral tentacles ahead of the eyes.
Some of the tropical sea hares, such as Dolabella (see the picture from Lembeh) have a larger shell that expands sideways, so the rear of the body is an expanded wedge-shape.
The UK’s sea hare, Aplysia punctata, is only nudibranch-sized. The Australian species Malcolm had found (either Aplysia extraordinaria or Aplysia gigantea – the classification is still not finalised) is a giant, reaching 60cm.

EVEN THIS MONSTER SLUG is dwarfed by the largest sea hare, California’s Aplysia vaccaria. At up to 99cm long and weighing up to 14kg this is the second-largest snail on Earth, after the giant Papuan Syrinx aruanus.
Sea hares are herbivores, grazing in shallow water, eating almost their own bodyweight per day when they are small and recently settled out of the plankton.
Some species prefer red algae, some like green sea lettuce, and a few even eat seagrasses. Their choice of food is reflected in the colour they become – the same species from different places can take on distinctly different hues.
The smaller and younger hares are rather vulnerable to predators – after all, they are basically chunks of unprotected protein living in shallow waters. So they tend to stay hidden during the day, coming out at night to graze the algae.
As they grow, they concentrate defensive chemicals from the algae they eat. Some of these are used to make a purple ink they can squirt at potential predators in a similar way to their distant cousins the octopodes and squids.
Other chemicals in a different gland produce a white slime that is distasteful to predators.
Larger adult sea hares, as they accumulate defensive chemicals in their skin and tissues, effectively become poisonous enough to become free from predators, so they can forage in daylight.
Sea hares’ lives are short and simple – grazing, gorging and growing fast for the first handful of months and reaching their maximum size rather quickly.
Resources are switched from growth to egg and sperm production, and the hares spend the last two-thirds of their lives mating as much as possible, then wrapping egg strings around raised algae clumps during high tidal periods, to better disperse their millions of plankton-drifting larvae. They live for a year at the most.

IT’S A NUMBERS GAME, and when conditions are right their populations can explode. Walking down a beach in Peru several years ago, I came across the end of the line for a boom generation of sea hares – a mass stranding.
Tens of thousands of grapefruit-sized snails had all died together and washed up on the beaches of the Paracas peninsula, rolling in the waves at the shore. Sometimes die-offs come after storms, but at other times it seems a particularly large generation has simply all come to the end of its pre-programmed lifespan all at once.
These mass strandings can be a bit of a hazard to beach-goers – the fleshy molluscs at the end of their lives have accumulated serious loads of toxins, and dogs walked along beaches have died after eating hares in the surf.

AS FOR THE ORGIES Malcolm witnessed, this is a pretty normal aspect of the life of an adult sea hare. It is hermaphrodite, able to act as male and female at the same time.
The penis sits along the side of the head facing forward, the vagina further back facing backwards, so a sea hare can’t reciprocally mate with another single animal.
A hare usually initiates mating “as a male”, approaching a potential mate from behind after picking up her/his scent downcurrent. The hare follows the odour trail, waving “his” head from side to side to home in on his mate.
Once mating starts, the mated animal then becomes aroused to seek its own mate, and the two crawl off joined together, attracting more slugs as they go.
Chains of up to 30 mating hares have been recorded, and large groups will sometimes close the loop, the first in the chain mating with the last to form a ring, often draped over a clump of algae so that they can all eat at the same time.
Once they become mature, sea hares might spend a quarter to two-thirds of their time copulating. It’s a rather simple life – graze, grow, mate and die.