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.

Life Saving 123: Marine Animal Defenses at Pulau Semakau

The rich habitat along the sea shore provides lots of feeding opportunities for animals. However, this comes at a price. Compared to animals on land or in the sea, inhabitants here are vulnerable to predators coming both from the sea and land. Consequently, many have evolved anti-predator defences so as not to end up at someone else's dinner table. Examples of these could be found during our exploratory walk at Pulau Semakau today.

The most basic anti-predator defence has got to be fleeing. If you can't catch me, you can't eat me. Those that do that exceedingly well all eluded my capture on camera. The focus today will thus be on those that could not get away fast enough.

If you cannot get away fast enough, then better hope that your enemies cannot see you.

The hairy crab (Pilumnus sp.) is covered with long hair that breaks up the crab's outline and binds sediments to help it blend with its surroundings. If that does not work, it will dash into the closest crevice for cover.

Taking camouflage one step further, cuttlefish and squids such as this big-finned reef squid (Sepioteuthis sp.) can change their colour to match their surroundings. Another trick they may use is to release a cloud of ink to confuse predators while they make their escape.

If you are as slow as a snail, more extreme tactics will have to be used. A turban snail (Turbo sp.) has a hard shell which it can retract fully into to protect its soft body.

On its foot is a trap-door-like operculum that seals up the

opening. The thick and tough operculum has a rounded surface and protects the snail from the prying pincers of crabs and other predators.

The shape and colour of the operculum resemble cat's eyes and is sometimes made into jewelry.

Sometimes, best defence is offense.

The flower crab (Portunus pelagicus) packs a nasty pinch from its sharp pincers. When threatened, the aggressive crab faces its aggressor head on, pincers ready. On top of that, it is heavily armoured with a shell packed with numerous spikes to discourage predators from swallowing it.

The hell's fire sea anemone (Actinodendron sp.) defends itself from any animal that dares to touch it with a painful sting. This comes from stinging cells tipped with venom and they look like little harpoons under the microscope.

If you are a small shrimp with minimal defensive powers, then it is best to find a powerful ally.

Apparently the stinging cells of sea anemones are so effective that the anemone shrimp (Periclimenes brevicarpalis) have found a way to take advantage of them by living among them for protection. But as if free protection is not enough, this species has been observed feeding on the tentacles of its host anemone!

Today's exploratory walk at Pulau Semakau also saw new records of 3 sea cucumber species for the island's shore. Even though they look helpless, sea cucumbers are able to protect themselves too. Some contain toxins or taste downright foul to predators, others try to confuse enemies by vomiting their guts and other organs out while some produce sticky threads to bind attackers in a gooey mess. Very neat.

From left: Ball sea cucumber (Phyllophorus sp.), Holothuria notabilis, and brown sandfish sea cucumber (Bohadschia vitiensis).

At the end of this trip, I still wonder how people squat over ankle-high water without getting their backsides wet. Must acquire this veritable skill.

Further reading:

1. Bruce, A. J. and Svoboda, A. 1983. Observations upon some pontoniine shrimps from Aqaba, Jordan. Zoologische Verhandelingen 205: 3-44.

Pulau Hantu Recce Trip

The maiden post on the first field trip on the first day of the year is also the first time for me on Pulau Hantu.

After we arrived, a heavy downpour sent the intrepid explorers scuttling for shelter.

We eventually headed out along the northern coast though the small mangrove patch within the lagoon where it was mostly soft and sandy.

Sponges as well as algae dotted the area and we found quite a few golf ball sponges (Cinachyrella australiensis) along a stretch. These multi-celled animals have small dimples all over giving them a golf ball-like appearance.

The golf ball sponge produce spike-like spicules that act like the skeleton of the sponge.

Sponges are filter feeders that obtaining food by trapping bits of particles in the water. A larger body that is supported by spicules will therefore enable the sponge to trap more food.

The yellow internal skeleton can be seen radiating out from the centre of the sponge. Some of these needle-sharp spicules stick out of the surface and can cause great discomfort if they pierce and break in human skin when handled.

Passing the rock walls, many nerite snails (family Neritidae) were seen on and along the rocky surface facing the sea. These gastropods feed on algae which they scrape off using their rasp-like radula, an action which is similar to licking ice-cream!

Further out, a seasonal Sargassum bloom blanketed most of the reef and we had to thread gingerly to avoid stepping on unseen organisms. These algal blooms supposedly tend to occur between December to February which coincides with heavy rain and temperature changes. Over at the northern shore, algal blooms involving a different group of algae (dinoflagellates) have been blamed for the mass death of fish and other marine organisms a few days before this.

A tiger-tailed sea horse (Hippocampus comes) was found hiding among the coral reef. The sea horse is in fact a rather odd-looking fish. It is a rather weak swimmer but it makes up for it by having a prehensile tail that can grasp seaweed or rocks to prevent itself from being swept away by strong currents.

This grumpy looking red egg crab (Atergatis integerrimus) was sitting in a tide pool but moved away as we approached.

Red egg crabs are reef dwellers and can grow to a width of 10 cm.

Although they look very attractive, the bright colours actually serve to warn predators that it is poisonous.

The toxins they contain are not broken down even after cooking and they should not be eaten.

Lying in a tidal pool among the Sargassum was a Bohol nudibranch (Discodoris boholiensis). This sponge-eating mollusc has the ability to break off non-vital parts of itself when attacked while the main body makes its escape. The lost parts will then regenerate later.

One of the last animals of the day literally warmed the cockles of my heart. A heart cockle (Corculum cardissa) spotted by Peiya.

This mollusc has a shell made up of two parts - one convex side and a slightly flatter side. They have been reported to host photosynthetic algae in its body in a relationship known as symbiosis. The cockle benefits from food produced by its tenant, while the algae is sheltered and protected from predators.

Soon, it was time to go and we had to leave our heart-shaped cockle behind. Cool weather, great company and lots to discover. I could really enjoy this!

Thanks to Ron for the organism ID.

Further reading:

1. Farmer, M. A., Fitt, W. K. and R. K. Trench. 2001. Morphology of the Symbiosis Between Corculum cardissa (Mollusca: Bivalvia) and Symbiodinium corculorum (Dinophyceae). The Biological Bulletin 200: 336-343.