Its eyes are tubular eyes, which are common in deep-sea creatures and consist of a multi-layer retina and a large lens that allows it to detect the maximum amount of light in one direction.

“Does anyone know what this is? Have seen a number of these washed up this week,” The Sutherland Shire resident posted on Facebook, and she wasn’t the only one who was intrigued.

A beachgoer spotted the creature on a Sydney beach and wondered what it could be

Another local claimed to have seen them while walking her dog along the sand at Cronulla’s Darook Beach.

“They are everywhere,” she said. “I was also wondering what they are.”

Mystery creature explained: ‘A local Sydney species’

Many people were perplexed by the slug-like creature, which some compared to an “alien.” However, Yahoo News Australia can confirm that this is not the case.

According to Harry Masefield, Aquarist at Sea Life Sydney Aquarium, the creature is a sea hare. They’re also known as sea slugs, he says.

Aquarist Harry Masefield from the Sea Life Sydney Aquarium said the creature was a sea hare (similar to the one pictured), also known as a sea slug

“This one doesn’t look too happy, likely climate change,” he said, noting recent variable weather in Sydney has probably altered its living conditions.

Sea hares are typically dark purple or green in color and can grow to be as large as a football. They emit a purple dye, similar to that of a squid, and can be extremely dangerous to pets.

The sea animal has a one-year life cycle, according to Dr Lisa Kirkendale, head of aquatic zoology at the Western Australian Museum, and when it dies, it washes up on the shore.

Another woman said she’d seen them when she was walking her dog along the sand at Darook Beach in Cronulla. Source: Sutherland Shire Council

According to Sea Life Sydney Aquarium aquarist Laetitia Hannan, there are many sea hare species, but this one is most likely Aplysia sydneyensis, a local Sydney species.

“We actually have these guys in our rock pool for people to look at and feel as they are super interesting to touch,” she said.

“They camouflage themselves as rocks to stay safe from other animals eating them.”

Meanwhile, black sea hares are frequently spotted off Australia’s west coast, and hundreds wash up on the beach every March.

Dog owners are being warned about the sea hares, or sea slugs, which can be extremely toxic to dogs. Source: Facebook

A warning for dog owners

Some Facebook commenters correctly identified the photo as a sea hare, but warned dog owners to “be careful” due to their toxic state.

According to Professor Culum Brown of Macquarie University, sea hares get their toxicity from the algae they graze on.

“They are only harmful if you eat them, and even then only some of the time,” he told Yahoo News Australia.

“People don’t munch on them but occasionally a dog might try should it find one washed up on the beach. If the individual hare has been eating particular types of algae it may cause harm to the dog.”