Thursday, May 17, 2012

Sap-sucking Slugs (Phylum Mollusca: Clade Sacoglossa) of Singapore

Sap-sucking slugs (order Sacoglossa) are marine slugs that feed on algae. As such, they are usually found with the algae they feed on, and their populations usually boom with algal blooms, and decline when the algae recedes. To feed, sap-sucking slugs use a special feeding structures to pierce algal cells and suck out the content. Many species are able to retain the algae's chloroplast in their body, using them for photosynthesis! It is hence no surprise that some naturalists call them "solar-powered slugs", since they can make food from the sun like plants and algae! Here are some of the sap-sucking slugs that I have seen in local waters.

Ornate Leaf Slug (Elysia ornata)
The Ornate Leaf Slug (Elysia ornata) is perhaps one of the most commonly seen sap-sucking slugs in Singapore. Looking just like leaves fallen into the water, they can be rather hard to spot for people new to shore exploration. The black and yellow margin on the edge of the mantle, however, gives it away. These slugs are especially common when the Hairy Green Seaweed (Bryopsis spp.) is blooming, since that's what they feed on.

Bang Tawa's Leaf Slug (Elysia bangtawaensis)
The Bang Tawa's Leaf Slug (Elysia bangtawaensis) is sometimes seen in our mangroves. This slug is named after the Bang Tawa village in Thailand. At times, many of these slugs can be found in the same tide pool in the mangrove.

Verrucose Leaf Slug (Elysia cf. verrucosa)
This slug was tentatively identified as a Verrucose Leaf Slug (Elysia cf. verrucosa) by scientists, as while it has most of the features of the Verrucose Leaf Slug, it lacks some of them. More work probably need to be done to determine whether it is a variation of the above mentioned slug, or some thing new to science altogether.

D'Aguilar's Sap-sucking Slug (Placida daguilarensis)
The D'Aguilar's Sap-sucking Slug (Placida daguilarensis) was originally described from Cape D'Aguilar in Hong Kong, and hence its common name. This slug is sometimes found among the Hairy Green Seaweed (Bryopsis spp.), and appears to be seasonally abundant. The many hair-like structures (cerata) on its back camouflages the slug with the algae, and together with its small size usually not more than 1 cm make this slug very hard to spot unless you look up close.

Oriental Leafy Slug (Polybranchia orientalis)
The Oriental Leafy Slug (Polybranchia orientalis) is sometimes seen on our shores too, and is also seasonal. It blends in well with the surrounding during an algal bloom with its many leaf-like cerata. Interestingly, it can drop its cerata to distract predators when disturbed.

Jewel Sap-sucking Slug (Thuridilla ratna)
The Jewel Sap-sucking Slug (Thuridilla ratna) is sometimes seen around coral reefs. Ratna means "jewel" or "gem" in Sanskrit. There has been lots of confusion over the identity of this species. Rudman and Gosliner previously believed that Thuridilla ratna, T. gracilis, and T. bayeri are synonyms, and named them T. gracilis since it was described earlier. DNA studies in 2007, however, have shown that T. bayeri should be a different species. Since the description for T. gracilis can fit both T. bayeri and T. ratna, Jenson (who reviewed the Sacoglossans in Singapore) decided that it is more appropriate to revert back to using T. bayeri and T. ratna for the two species, and identified the species recorded in Singapore as T. ratna.

Ocellated Sap-sucking Slug (Plakobranchus ocellatus)
The Ocellated Sap-sucking Slug (Plakobranchus ocellatus) is rarely seen on our shores, but possibly because it can burrow into the soft substrate. At this moment this genus is considered to be monospecific (only has one species in this genus). There are many variations though, and it is possible that another species may be uncovered from the "variations".

Pawel's Avrainvillea Slug (Costasiella paweli
Three species of Avrainvillea Slugs (Costasiella spp.) have been recorded in Singapore. The above is a Pawel's Avrainvillea Slug (Costasiella paweli), named after the scientist who found the initial specimen. These slugs are very tiny, not more than a few millimetres, and are found on the Fan Seaweed (Avrainvillea spp.).

Rabbit Avrainvillea Slug (Costasiella usagi) and Kuroshima's Avrainvillea Slug (Costasiella kuroshimae)
The slug on the right is a Rabbit Avrainvillea Slug (Costasiella usagi), with the distinctive white lines on its cerata. "Usagi" means "rabbit" in Japanese. On the left looks like a Kuroshima's Avrainvillea Slug (Costasiella kuroshimae), named after the location in Japan where it was described from. The twirling structures on the top right are egg ribbons.

Volvatella vigourouxi
The Volvatella Slug (Volvatella vigourouxi), unlike most other shell-less sap-sucking slugs, has a thin orange shell marked with darker spiral bands. The body of the slug is orange as well. This slug is believed to feed on Caulerpa seaweed.

  • Gosliner, T. M., D. W. Behrens & Á. Valdés 2008. Indo-Pacific nudibranchs and sea slugs: a field guide to the World’s most diverse fauna. Sea Challengers/California Academy of Sciences: Gig Harbor/San Francisco, 426 pp.
  • Jensen, K. R., 1996. Phylogenetic systematics and classification of the Sacoglossa (Mollusca, Gastropoda, Opisthobranchia). Philosophical Transactions of the Royal Society B, 351: 91–122.
  • Jensen, K. R., 2009. Sacoglossa (Mollusca: Gastropoda: Opisthobranchia) from Singapore. Raffles Bulletin of Zoology, Supplement 22: 207–223.
  • Ng, P. K. L. & N. Sivasothi (eds.), 1999. A guide to the mangroves of Singapore II: Animal diversity. Singapore Science Centre, 168p. 
  • Rudman, W. B. 2012. Sea Slug Forum.  Retrieved May 10, 2012, from
  • 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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