The Hare of the Sea

Keeping with the coastal summer theme of recent species of the months, we bring you: Sea Hares! This month’s focal species is actually a genus; several species of Sea Hare are commonly found along the southern coastline of Australia. Sea Hares are slug-like organisms that live in seagrass beds or rocky reefs and can be found in the intertidal zone, grazing on algae.

Sea hares have four tentacles, with the front pair (oral tentacles) resembling the nostrils of a mammal and a pair of rolled up tentacles (rhinophores) just behind their eye spots which resemble the ears of hares, hence their name. They have been known to take up the pigments and sometimes toxins found in the algae they graze on and so vary in colour, providing excellent camouflage. They have no obvious shell as it is internal, like a cuttlefish. Sometimes when scared they pull back their mantle to reveal a white shell. They have wing-like frills used for swimming.

Sea Hares grow and mature rapidly, within a year or two. They are hermaphrodites and have been known to breed in a chain. Their spaghetti-looking eggs are quite eye catching being florescent yellow or orange masses which are generally found in summer. Sea Hares die after laying their eggs.

There are several genera and hundreds of species with the most common I have seen along the south east SA and western Victoria coast being;

  • Aplysia parvula – a small sea hare, growing to a maximum of 10 cm in length. It’s background colour varies with the colour of algae it feeds on. The main identifying feature is the black trim along the wings (parapodial flaps) and the black tips of its tentacles.
  • Aplysia gigantea – the largest known sea hare in Australia and one of the largest in the world, reaching up to 60cm in length. It has a brown mottled body.

Two species of Sea Hare commonly found on the southern Australian coast: (Left) Aplysia parvula) and (right) Aplysia gigantea. Photos: Jess Bourchier.

Sea Hares are often found stranded on sandy patches of rocky reefs at low tide, looking much like a blob of jelly. They are reasonably safe to pick up and return to the water, and their features are easiest to see in the water. However, it is best to wash your hands straight away, even if that means rubbing them together in sea water, as some secrete toxic slime or dye (known to be bright purple). Their toxicity is variable and depends on what the individual Sea Hare has eaten. Both the toxic slime and toxic dye are a defense mechanism which have resulted in Sea Hares having no predators.

Sea Hares are one of my favourite intertidal creatures and if you’ve ever been on a reef ramble with me you’ll know how excited I get when I see them. If you stumble across any this summer I would love to see your photos or answer any questions by contacting me here.

Jess Bourchier