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.

There are hundreds currently 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 are 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.

We have taken expert advice and are monitoring water conditions and the numbers in the pool. Sea hares have been naturally moving out of the Baths with tides and currents. We are also preparing to manage their removal sensitively, as well as the egg masses,  to protect both the animals and the environment.

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 are 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.

Following advice from marine experts, we plan to begin draining and cleaning Merewether Ocean Baths on Tuesday 13 May, with the aim to reopen for swimming later that day. We’ll share updates on this page and on our Facebook.

Sea hares are harmless to humans. However, they can be toxic to dogs if ingested, so please keep pets away from the water.