Tuesday, May 22, 2012

Onch Slugs (Phylum Mollusca: Family Onchidiidae) of Singapore

Onch slugs (family Onchidiidae) are air-breathing marine slugs mostly found in intertidal areas, and are more closely related to air-breathing land snails and slugs than other sea slugs like nubibranchs, sea hares and headshield slugs etc. They breathe air using a simple lung modified from a section of the mantle cavity. The lung opens through a pore alongside the anus at its back end below the mantle. Some can also breathe through its skin (or mantle) or special breathing structures on their back under water, but generally, they will die if left in sea water for long periods of time. Most onch slugs hide in air pockets among rocks or burrow into the substrate during high tide, and emerge during low tide to feed. It is believed that many species leave a mucus trail as they were feeding, so that they can find their way back "home" before tide rises.

In some parts of Asia, onch slugs are considered a highly nutritious source of food, and are cultivated in farms. They usually have colours and patterns which match their surroundings, allow them to camouflage nicely to avoid predation. They usually feed on algae. Much of our knowledge about the onch slugs species in Singapore were based on a study by Liow Lee Hsiang's honours project, which attempted a phylogenetic analysis on these slugs. The slugs on this page are identified using the description and photos provided in Liow's paper. In addition, slug expert, Dayrat, also helped to identify a number of species when he was here a few years ago.

Peronia verruculata
Peronia verruculata is perhaps the most common onch slug in Singapore. It is commonly found on rocky shores and sea walls, and is able to tolerate heat from the sun. Unlike most other onch slugs, it has retractable branching structures on its back, which it extends when it's under water for gas exchange. In other words, it can breathe better than most other onch slugs underwater, which explains why they can sometimes be found on rocks further out to the sea. The underside is greenish in colour.

Peronina alta
Peronina alta is a burrowing species found on mudflats, and grazes on the surface of the mud. It can be differentiated from other mud-living species by its huge, protruding dorsal central papilla.

Onchidium tumidum
Onchidium tumidum is a smaller species (about 2cm) that is also found on mudflats, occuring together with the previous species sometimes. The broad foot is dirty yellow in colour.

Onchidium cf. steenstrupi
This was identified as Onchidium cf. steenstrupi in Liow's paper. It is normally found on shaded seawall or cliffs where it grazes on algae.

Paraonchidium sp.
This is possibly a Paraonchidium sp.

Platevindex sp.
Platevindex species generally have a thinner foot in contrast with a broad "skirt" (hyponotum). Several species can be found here, but mostly unidentified, and the appearances may vary quite a lot. The above species has not been identified.

Platevindex luteus
Platevindex luteus can be recognised by its knobby back (notum), and bluish hyponotum with whitish spots (if examined closely).

Platevindex coriaceus
Platevindex coriaceus is usually found on trees, and has an orange foot and dark mottling patterns on its yellowish hyponotum, which unfortunately I do not have a photo as yet. Will try to get a photo and update this soon.

Platevindex sp.
This Platevindex species has a black foot instead, and is often found on trees too.

Onch Slug
This is another unknown species with a whitish foot.

Hoping to update this page soon with more photos - not sure what went into me, but during a previous field trip, I only took the topside or underside of many of the specimens, and hence was unable to identify most of them except the easier ones.

  • Liow, L. H. 1997. Singapore Onchidiidae (Mollusca: Pulmonata) - An Attempt at Phylogenetic Analysis Using Anatomical and Allozyme Data. Unpublished manuscript.
  • Ng, P. K. L. & N. Sivasothi (eds.), 1999. A guide to the mangroves of Singapore II: Animal diversity. Singapore Science Centre, 168p.
  • Tan, S. K. & H. P. M. Woo, 2010. A preliminary checklist of the molluscs of Singapore. Raffles Museum of Biodiversity Research, National University of Singapore, Singapore. 78 pp. Uploaded 02 June 2010.
  • Tan, S. K. & R. K. H. Yeo, 2010. The intertidal molluscs of Pulau Semakau: preliminary results of “Project Semakau”. Nature in Singapore, 3: 287–296.

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