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The Shore-dwelling Shell-less Snail – Sea Slug

Dec 2019
Author : Green Power

When the tide leaves the shore, boisterous fiddler crabs, dancing mudskippers, and foraging wading birds throng the mudflats. Among the bustling species there are some moving bulges on the land. Guess what? There are onch slugs, rocking the tideland carnival!

Onch slug (Onchidium spp.) belongs to the class of Gastropoda within the phylum Mollusca. By contracting its abdominal muscles, the stomach-footed animal crawls forward like a dancer waving a dress for a waltz.

Take a closer look at its body; the brown and lumpy body that resembles land slugs could perfectly blend with its surroundings. Not even sharp-eyed top predators could easily tell whether it is a piece of food, or a lump of mud!

As most slugs do, onch slug possesses both male and female reproductive organs. Instead of breeding like a one-man band, the hermaphroditic creature tends to mate with another individual. The increased genetic diversity means the immune system and adaptability are improved; that is how the onch slugs stand and multiply in the washing waves.

Slug from the Sea

Thriving for generations in the ocean, the ancestors of onch slugs breathed dissolved oxygen by their fins in the water. As the landscape changed over time, they developed their lungs to adapt to the environment on land. Filled with micro-blood vessels, the lungs of onch slugs function almost the same as ours, allowing the species to breathe in oxygen and settle themselves in the intertidal zone.

Today, the land-living onch slugs scrape moss from rocks for food. By closing their breathing pores, onch slugs safeguard themselves from being washed away by the ferocious flow.

By far, onch slugs were the main gastropod species with the most unsolved mysteries. Despite their common features, onch slugs share relatively few DNA similarities with lung-breathing snails. Scientists are still struggling to decipher the evolution of the shell-less species – because of the scarce fossil evidence.

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Could you spot the onch slug?
©Henry Lui
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Stomach foot observed on a belly-up onch slug. They move at a snail’s pace by contracting muscles on the stomach.
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The muddy trail left by an excreting onch slug.
©Henry Lui

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