Onch Slugfest

Onch slugs (family Onchidiidae) are a group of shell-less gastropod molluscs in the same class as the more familiar snails and slugs. However, they are mostly found in coastal habitats or in the inter-tidal area.

Like snails, onch slugs have a pair of simple eyes on stalks that are only good for detecting light. Most of them look like shell-less snails or a bumpy brownish lump - the word "onch" literally means lump. They are not the prettiest of organisms, but their humble appearance grants them excellent camouflage. It definitely follows the wise adage that if you do not want to be eaten, do not look like food!

One unique characteristic of onchs is that they breath air using a simple lung, unlike marine slugs such as nudibranchs that get oxygen via feathery gills. Hence, onchs are considered pulmonates (from the Latin word pulmonarius, meaning of the lung). In this aspect, onchs are more similar to land snails and slugs.

Since onchs need to breath air, they are usually found above the water level on rocks, tree trunks or even man-made surfaces and in the inter-tidal area during low tide where they feed on algae and organic detritus. They scrape bits of algae off the surface with a rough rasp-like mouthpart called a radula.

Coupling in onchs is a strange affair. These slugs are hermaphrodites bearing both male and female organs. When they get together, they jab each other with hard and sharp love darts in a courtship ritual prior to mating where sperm is exchanged.

A soft-bodied animal living out of water will often face 2 major problems that is dangerous to its health - drying out and predators. Onch slugs are extremely well suited to their habitats and combat desiccation and predators with both structural and behavioural adaptations.

A thick mucus is secreted, covering the skin of the animal to prevent desiccation. In addition, the mucus is also thought to be foul tasting, and any predator attempting to swallow an onch will get a mouthful of a yucky gooey lump. In some species, the mucus leaves a trail for the onch to follow back to their own home.

Some onch can also burrow into the substrate to avoid heat, strong currents and predators. This one below burrowed into the mud within seconds upon sensing danger. Couple excellent camouflage, multi-purpose mucus and a nifty burrowing behaviour, onchs have a lot going for them where survival is concerned.

Unfortunately, on the whole, these charming creatures remain a poorly studied group and little is known about the biology of onch slugs in Singapore. Hopefully, more work can be done in this area to demystify these amazing little slugs.

Further reading:

1. McFaruume, I. D. 1980. Trail-following and trail-searching behavior in homing of the intertidal gastropod mollusc, Onchidium verruculatum. Marine and Freshwater Behaviour and Physiology 7(1): 95-108.
2. Ng, P. K. L. and Sivasothi. N. 1991. Mangrove slugs (Onchidiidae). A Guide to the Mangroves of Singapore Volume 2. Singapore Science Centre.