Citizen scientist and Atlas of Life chair Libby Hepburn said the critters were in fact Aplysia Juliana, more commonly known as sea hares.

"They are delightful creatures and we occasionally get a mass of them together if the conditions are right," she said.

"Obviously they are in Coila Lake just now, and that's why there's so many!"

Ms Hepburn said the creatures typically liked estuaries and lakes, particularly as the water started to warm up in summer.

"They tend to like still or slow moving water and that tends to get quite warm in the summer when the sun hits it," she said.

"Coila Lake must be a particularly good place so there are lots and lots of them which is fantastic."

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Sea hares can range in size from species growing less than two centimetres, to large species which can reach more than 70 centimetres in length.

Ms Hepburn said the attractive looking creatures only tended to live for one to two years.

"They do interesting things when they mate, and form long chains in big groups," she said.

"Now the water is getting warmer they'll be looking to mate and people should see long strings of eggs in the water."

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Ms Hepburn urged anyone who spotted a sea slug to submit their photos on the iNaturalist website, a global citizen science database which helps scientists find and use biodiversity data.

"There are currently no records of these species in Coila Lake on iNaturalist," she said.

To find out more about the sea slugs of the South Coast, visit the Atlas of Life website.