The specimen was unusual as it was found at the low-tide mark, with it normally being found on steep rock faces in eight to 15 metres of water.
This particular slug was also lighter than the species’ usual chocolate brown.
Pictured: The Atagema gibba from the side. (Nick Jouault)
There have only been four previous records of the species in the British Isles: two off Cornwall, and two off Sark.
Mr Jouault said that this particular species was not invasive – it was just rare.
This was unlike another of Mr Jouault’s recent finds, a colourful slug called Berghia coerulescens, which has moved up from the Mediterranean as sea temperatures have risen.
Pictured: Where the sea slug was discovered last week. (Nick Jouault)
The Société Jersiaise Marine Biology section member discovered this species last August – the first sighting in the British Isles. In the last year, he has also found the small grey-coloured Pruvotfolia pselliotes, which is another invasive species.