The marvellous creature you see in the picture above is not a slug. It is known as a cowrie! Cowrie snails are aquatic animals in the class Gastropoda, which also includes land snails. They are found in countries bordering the Indian ocean, including the intertidal regions of India, Africa and Southeast Asia.
Cowries begin their lives in clusters of eggs encased in capsules, bound to a solid substrate. Unusually for an invertebrate, the mother cowries sometimes remain with her egg capsules to protect them from parasites. Depending on the size of the cowries, they can live from 2 years to even 10 years!
Once hatched, the shells of cowries are spiral-shaped, similar to their land-dwelling cousins. However, as they mature, they produce the larger egg-shaped outer shells you see in the picture, with slit-shaped openings lined with teeth-like structures. The size of this shell can range from a measly 5 mm to a whopping 19 cm!
While the cowrie is moving around, it extends its mantle to cover the entire surface of the shell, using the mantle to fix the shell and prevent barnacles and algae in the intertidal waters from growing on it. The mantle also secretes a nacreous layer with colourful patterns, and due to the regular cleaning, the shell becomes shiny and colourful! The mantle only retreats back into the shell once the cowrie is disturbed or during low tide, protecting the cowrie from predators and dehydration. When the mantle covers the shell, cowries are often mistaken for sea slugs. We even managed to see this little critter in action!
The colourful patterns of the shells vary distinctively between each species. For example, the ovum cowrie has a vibrant reddish-brown shell with white stripes and bands of gold, while the shell of the tiger cowrie can vary from whitish yellow to light bluish gray, with dark black and brown spots. The cowrie we observed during the trip was Lyncina vitellus or the milk-spotted cowrie, whose yellowish-brown shell decorated by white spots is somewhat reminiscent of milk tea. This species of cowrie is of average size ranging from 7 cm to 10 cm.
Unfortunately, their lustrous shine is often their downfall. Cowrie shells were used commonly as currency in Africa and Southeast Asia, and are still commonly collected today for jewelry and decoration. Along with land reclamation and pollution in Singapore, this has led to the extinction of half the cowrie species in Singapore, the endangerment of the tiger cowrie and the near elimination of the gold-ringed cowrie. Perhaps it is better to admire these creatures from a distance.
References:
Cowries (Cypraeidae) on the Shores of Singapore. (n.d.). Retrieved from http://www.wildsingapore.com/wildfacts/mollusca/gastropoda/cypraeidae/cypraeidae.htm
Milk-spotted cowries (Lycina vitellus) on the Shores of Singapore. (n.d.). Retrieved from http://www.wildsingapore.com/wildfacts/mollusca/gastropoda/cypraeidae/vitellus.htm
Onyx cowries (Erronea onyx) on the Shores of Singapore. (n.d.). Retrieved from http://www.wildsingapore.com/wildfacts/mollusca/gastropoda/cypraeidae/vitellus.htm
Tiger cowries (Cypraea tigris) on the Shores of Singapore. (n.d.). Retrieved from http://www.wildsingapore.com/wildfacts/mollusca/gastropoda/cypraeidae/vitellus.htm
Jan Hogendorn and Marion Johnson (1986). The Shell Money of the Slave Trade. Cambridge: Cambridge University Press. ISBN 9780521541107. Retrieved 29 April 2015.
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