Pleurobranchus membranaceus
(Montagu, 1815)

Family: Pleurobranchidae


Western Mediterranean, Atlantic coast of France, British Isles.


West coast of Scotland. Lower photo showing typical swimming behaviour (See message below) June 1996 at Loch Fyne on the west coast. An individual 70 mm long, swimming in open water at 15 metres. Photos: Jim Anderson.

Feeds on simple and compound ascidians. Grows to 12cm in length.

P. membranaceus swims upside down with an asynchronous flapping of the foot. The mantle hangs down and is not used in swimming, while the sides of the foot extend out as large thin flaps, much larger than when the animal is crawling. During swimming the foot on one side performs a swimming stroke by sending a wave from the front to rear and then while it is recovering for the next propulsive stroke, the other side of the foot undergoes its propulsive stroke. This makes swimming a very ungainly unstable movement, the animal rolling from side to side as it swims along. Animals do not appear to swim in any direction.

• Thompson, T.E. & Slinn, D.J. (1959) On the biology of the opisthobranch Pleurobranchus membranaceus. Journal of the Marine Biological Association, United Kingdom, 38: 507-524.

Authorship details
Rudman, W.B., 2001 (April 19) Pleurobranchus membranaceus (Montagu, 1815). [In] Sea Slug Forum. Australian Museum, Sydney. Available from

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