Sea Hare Event

Sea hares are soft bodied marine molluscs most commonly found on intertidal rocky shores, in pools, and in seagrass beds. They are herbivorous and feed primarily on algae. Sea hares can release a dark purple ink as a defensive response when disturbed. Their spawning behaviour is seasonal and often occurs in late summer/autumn. , Sometimes resulting in large reasonably short lived aggregations. This year the sea hares are present in very large numbers.

Based on recent observations, the majority of individuals present appear to be Aplysia juliana, a commonly encountered species in shallow coastal environments. A smaller number of larger individuals have also been noted, including the one seen swimming on our recent Facebook video, which has been identified as Aplysia sydneyensis.

Sea hares can vary in size, but the ones observed in the Baths are typically 15–30 cm long — much larger than a bluebottle.

While exact numbers were hard to determine, it was estimated at that at the peak there were thousands present. This is considered a large aggregation and is rare to see in a public swimming facility like this. 

Yes. Those long, spaghetti-like strands and orange clumps were sea hare egg masses. As part of their natural life cycle, sea hares lay eggs in large numbers, often attaching them to surfaces in the water.

Spawning is the process of laying eggs.

Any individual sea hare can act as either a male, female, or simultaneously as both. When a sea hare acts as both, they are known to be simultaneous hermaphrodites (possessing both male and female reproductive organs).

During spawning events, sea hares can gather in groups to reproduce. If simultaneous hermaphrodites, sea hares will act as both a sperm donor and receiver, a reproductive strategy that increases the chances of successful fertilisation.

The result is a large number of egg masses. These spaghetti-like egg masses are extremely delicate and, once hatched, become known as planktonic veliger larvae. These microscopic larvae drift in the ocean as part of the plankton and, under the right conditions, can settle and grow into adult sea hares, continuing the species’ life cycle.

After spawning, the adult sea hares naturally die. This is a normal and expected part of their short but fascinating lifecycle.

Following advice from marine experts we decided to manually relocate the sea hares and the egg masses to their natural environment, where they are best placed to continue their natural lifecycle. On Monday 12 May, our Environment & Sustainability Team carefully managed their relocation back into the sea. 

Prior to this manual relocation we had observed that some sea hares were making use of the higher tides and moving out of the Baths. When the baths were drained on Tuesday 13 May the remaining sea hares were returned to the surrounding rock pools and ocean.

Some may be present in nearby rock pools, especially after high tides. They are commonly found in seagrass beds and shallow rocky areas.

While sea hare spawning is natural, seeing such a large number in a concentrated location like the Ocean Baths is unusual and ecologically significant.

The Baths were closed to avoid disturbing the sea hares or damaging their delicate bodies and eggs. Cleaning or swimming during this time could harm them, as they are present in very large numbers and are moving about on the floors and walls of the baths. Once it was determined that the spawning event was complete, we proceeded with a plan to get get the Baths ready for swimming again.

Following advice from marine experts, we drained and cleaned Merewether Ocean Baths on Tuesday 13 May, and opened them for swimming the same day. 

Sea hares are harmless to humans. However, they can be toxic to dogs if ingested, so please be careful with your pets.

Sea hares naturally die after spawning. This is a process playing out along the coast (not just in the Baths!) and could see many dead sea hares washed up on shores in the coming days and weeks.