It's a free-living, free-loving, colourful swinger called the sea hare.

A Mercury reader photographed these molluscs, believed to be sea hares, on Little Austinmer Beach this week. She said there were at least 200 of them there, ranging in size from 2cm to about 15cm.

University of Wollongong marine biology lecturer Marian Wong said the sea hare was similar to other gastropods such as land snails and slugs. But in evolutionary changes over the years they had shed their shells, developed bright colours which warn off predators, and found a remarkable way to breed - the underwater love chain.

"They're quite unique amongst the molluscs as they've actually lost the shell . . . and they've adopted this more free-living lifestyle," Dr Wong said.

They can have both male and female sex organs, and use both at the same time with multiple partners. So they form chains of simultaneous sex, which can produce thousands of eggs.

"They can reproduce as either males or females. There are no restrictions on who they can mate with," Dr Wong said.

Sea hares are herbivores, closely related to the smaller and carnivorous nudibranch.

Dr Wong said the specimens found on Little Austinmer Beach probably were washed out of their habitat by the heavy swell over the past week.

But in bad news for beachside foragers, there does not appear to be any world cuisine which values the sea hare, or the nudibranch, as a delicacy.

"Most are so small that it would be pointless to try and eat them," Dr Wong said.