Giant Onchidium from South Africa

May 8, 2001
From: Valda Fraser

Dear Bill
An amazing animal! I found it crawling right out of the water on a rock in the intertidal zone. I was so worried about it that I popped it into the pool. This was not the right thing to do because it immediately crawled out again. I am sending you two photos - the anterior [upper photo]and posterior [loer photos] ends.

Locality: Pumula, south coast KwaZulu-Natal, SOUTH AFRICA, intertidal zone
Date: April 2001
Size: 10 cm

Fraser, V., 2001 (May 8) Giant Onchidium from South Africa. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Valda,
This is a Pulmonate Slug. While many live in mangrove environments, this very large species, Onchidium verruculatum Cuvier, 1803, is common on rocky shores in many parts of the Indian Ocean. It hides in crevices at high tide emerging at low tide to graze over rocks. Studies show that it follows a mucous trail home after each feeding 'expedition'. I remember, in northwestern India, where I saw these animals alive for the first time, wondering what these large warty dorids were doing so high on the shore. As you can see in the bottom right photo, they have remarkable gill-like branching papillae over the posterior half of their backs, and it is only on picking them up that their pulmonate anatomy becomes obvious.

McFarlane, I.D. (1981) In the intertidal homing gastropod Onchidium verruculatum (Cuv) the outward and homeward trails have a different information content. J. exp. mar. Biol. Ecol, 51: 207-218

Best wishes,
Bill Rudman

Rudman, W.B., 2001 (May 8). Comment on Giant Onchidium from South Africa by Valda Fraser. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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