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Phylum Mollusca > Class Gastropoda > sea slugs > Family Onchidiidae
Big pimply onch slug
awaiting identification*
Family Onchidiidae
updated Jun 2020
if you learn only 3 things about them ...
Onchs are very well camouflaged. Don't step on them!
Onchs graze on alge on the rock, see the bare patches on the rock?
They are slippery and hard to handle, so please don't touch them.

Where seen? This medium to large onch slug is commonly seen, sometimes in large numbers, on many of our natural rocky shores. Usually on large rocks and boulders, also on small stones on rocky shores, sometimes also on sea walls. The slug is often covered with sand (so they probably burrow into sand to hide?). These slugs are usually well hidden on a warm and sunny day. But on cool days or early in the morning or at dusk, you might see lots of them crawling about. They can move quite fast!

Changi, Dec 10
Features: 4-5cm. Body hard, broad and rather flat, sometimes with a raised hump along the centre of the body. Skin with many irregular bumps and pimples. Generally beige, brown to grey. Sometimes with spots in darker shades. The eyes are held on short stalks that stick out from under the tough pimply body. Most other snails have eyes at the base of tentacles. The underside of the body may have a bluish or greenish tinge, the narrower foot is beige.

Slippery slug: Avoid touching it as it is very slimy and if you try to pick it up, it generally slips out of your hands to bounce away among the rocks. The poor slug might get hurt and it may not be able to climb back up to where it can find food and safety.

On the underside, a broad foot.

Raffles Lighthouse, Jul 06

One pair of eyes on stalks.
The mouth is on the underside.
Raffles Lighthouse, Jul 06

Pooping from the other end.
Tanah Merah, Dec 10
What does it eat? Like other onchs, it grazes on algae growing on the rocks. As it feeds, it often leaves a bare patch on the rock and a trail of 'processed algae' behind.

Raffles Lighthouse, Jul 06

A pair of tentacles.

Leaving behind a grazed patch, and
a trail of 'processed algae'.
Big pimply onch slugs (Family Onchidiidae)

*Species are difficult to positively identify without close examination.
On this website, they are grouped by external features for convenience of display.

Big pimply onch slugs on Singapore shores
On wildsingapore flickr

Other sightings on Singapore shores

Tuas, Jun 15
Photo shared by Loh Kok Sheng on facebook.

Sentosa, Jul 16
Photo shared by Marcus Ng on facebook.

Pulau Tekukor, Jan 10
Photo shared by James Koh on his flickr.

Pulau Semakau East, Jan 16
Photo shared by Loh Kok Sheng on facebook.

Terumbu Pempang Darat, Jun 10
Photo shared by James Koh on his blog.

Terumbu Pempang Tengah, Apr 13
Photo shared by Loh Kok Sheng on flickr.

Beting Bemban Besar, Jun 21

Photo shared by Vincent Choo on facebook.

Pulau Pawai, Dec 09

Pulau Sudong, Dec 09

Pulau Biola, Dec 09
Photo shared by Ivan Kwan on his flickr.

Terumbu Berkas, Jan 10

Pulau Senang, Jun 10

Pulau Salu, Aug 10
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