Just one species of land snail and a few species of freshwater snail glow in the dark.
Bioluminescence is one of nature’s neatest tricks: animals that glow in the dark. It’s such a neat trick that nature keeps inventing it. Like the eye, the shape of the crab, or opposable thumbs, glowing has evolved in several different species at different times. This is known as convergent evolution, where species evolve similar forms not because they share an ancestor with that characteristic, but because the species are subject to the same environmental pressures. They just end up developing along similar pathways because those pathways lead in a useful direction.
The compounds most often behind bioluminescence are called, delightfully, luciferin and luciferase (both from Lucifer, the light-bringer). Together, plus a little oxygen to kick off the reaction, they give off light energy. There are many different types of luciferin, which tells us that animals have evolved to produce it dozens of times independently throughout history. It’s relatively common in the deep ocean for obvious reasons: fish, squid, jellyfish, brittle stars, and so on. On land, it’s mostly the province of fireflies and glow worms. And, for some reason, one species of snail.
Quantula striata is found in southeast Asia and Fiji, and it glows. The problem is, we don’t know exactly why it glows. The snail doesn’t glow to attract prey, because it doesn’t catch prey. It doesn’t glow to distract predators, because it just hides in its shell when that happens… and the snail’s eggs glow as well, which is not traditionally something that you want to do to hide from predators. It doesn’t use the light to see, because the light’s output is too dim to really illuminate anything.
This leaves two possibilities: it doesn’t have a function and is a genetic hitchhiker; or, the quantula striata flashes are some kind of animal communication. What exactly they’re saying… we don’t know that either.
In contrast to this puzzle of a luminescent snail, the freshwater snails that glow are an open book. Take latia nerotoides, a New Zealand snail that shoots out a glowing slime when disturbed by a predator. It’s like squid ink, but instead of disguise it’s a shining decoy. I quite like the idea of a snail with a built-in anti-missile flare.
(End note: it’s important to distinguish between bioluminescent organisms – who make their own light-producing chemicals – and biofluorescent organisms – which absorb light and re-emit it at a different wavelength. The platypus fits in the latter category. It glows under UV light.)