Marine Matters – by Phil Coulthard – Operations Manager & Marine Biologist, Dolphin Discovery Centre
As featured in the South Western Times
Over the past few weeks some very bizarre looking animals have been washing up along our beaches. Looking a bit like large, dark blobs of slime, these unusual creatures called Sea Hares have suddenly appeared without any obvious reason. Called Sea Hares, these slimy characters measuring almost 40 cm in length are in fact one of the largest members of the mollusc family and closely related to their beautiful sea slug cousins called nudibranchs. Usually found in pools grazing on algae, their black/brown colour (although some species have a dark mottled green pattern) acts as perfect camouflage, as they slowly crawl over the seabed.
A baffled beach walker found this one (photographed) stranded on Dalyellup beach last week and decided to bring it into the Dolphin Discovery Centre for identification. Realising the animal was still alive, staff at the Centre placed the Sea Hare into an aquarium and within hours it started moving slowly along the bottom to the delight of the Centre visitors. Knowing it was a herbivore, the addition of a few algae encrusted rocks from the Bay soon satisfied it’s hunger and saw our little friend thrive in its new home for over two weeks before the unbelievable happened. It laid eggs!! Millions of eggs in fact, all deposited within long strings that resembled a bird’s nest, perfectly nestled safely in the corner of the aquarium. This was an unexpected turn of events and provided us with the likely explanation as to why so many of these interesting creatures have been washing up on the beach recently and it has everything to do with their love life. They have been congregating along our coast to breed and like many of their mollusc relatives, they die soon after the eggs are laid.
Boasting quite a unique reproductive strategy, Sea Hares generally wait until they are around a year old before travelling to their final destination where they form a long chain to reproduce. As ‘hermaphrodites’, they can simultaneously act as a male for the animal in front and a female for the animal in behind if required maximising reproductive efficiency. You would have to agree that this is quite a party trick! They then deposit long strings of up to 180 million eggs on the sea floor and die soon after. Knowledge of their fate prepared us for the inevitable end to the life of our popular mollusc friend, who proceeded to dance around the tank for two more days.
For those who do find Sea Hares along the beach at this time of the year, please be careful. If they are still alive they are capable of excreting a toxic purple ink that can irritate the eyes and can even kill a dog if eaten. In the water this acts as a smoke screen, enabling them to escape from predators. Similarly, they are most likely at the end of their life following reproduction so there is no point throwing them back into the ocean or taking them home for an aquarium display.