It would seem a bit foolish to put a big cap on the end of a torch, right in the path of the beam of light. But that’s exactly what one type of sea snail does. Hinea brasiliana has a pair of organs that give off a bright glow. Many sea animals have this ability but the snail’s living torches are permanently hidden. They sit underneath the hard shell and can’t be pushed outside. And yet, the light not only finds a way through, it also somehow diffuses throughout the shell so that the entire structure starts to glow.
Although hundreds of sea creatures can produce their own light (bioluminescence), H.brasiliana is one of the few snails to have this ability. Under natural light, its shell is a yellowy brown colour but once it turns on its living lights, it becomes suffused with soft pulses of blue-green light. The glow comes from patches of cells near the snail’s mucus glands, which sit behind the front opening of the shell. To Dimitri Deheyn, who discovered the snail’s ability, it’s “like a slow strobe on an erratic pacemaker or battery”.
Deheyn found that the shell is incredibly selective. When he shone white light into an empty shell, the structure scattered only blue-green wavelengths (the same ones given off by snail’s shining cells); all the other colours passed straight through. The scattering is so efficient that even when Deheyn used a fairly tight beam of light, every part of the shell started to glow, even those that would otherwise have been in darkness.
Humans also use diffusers to scatter light, particularly during photography or filming where they help to get rid of shadows and harsh lighting effects. But the snail’s shell outclasses high-end commercial diffusers. It not only spreads the blue-green light over a greater area, but it also allows more of it through. It’s an incredible ability that allows the snail to convert pinpricks into beacons.
For the moment, Deheyn doesn’t know how this happens. He thinks that the shell might have a unique arrangement of crystals, or perhaps a special protein that helps to scatter the light. Understanding how this works is the next big challenge, and the results could allow us to design better optical equipment. It wouldn’t be the first time that a sea snail has inspired human technology. Earlier this year, I covered a species whose three-layer shell is one of the most effective animal shields ever discovered, and a possible inspiration for the next-generation of body armour.
It’s a bit clearer why the snail turns its shell into a lamp. Deheyn found that they flash in response to predators. Slow-moving animals like other snails triggered some mild flashing. But true signs of trouble, such as a bump from a roving shrimp, worm or scientist, made them retreat into their shells and give off rapid, intense flashes, up to 10,000 brighter than their standard glows.
It might seem odd to draw attention to yourself in the face of danger, but there’s more to the snail’s technique than that. During low tide, the snails are often found in nooks and crevices, huddling in tight groups, a trait that has earned them the affectionate nickname of “clusterwinks”. It’s possible that by flashing in unison, they could put off any roving predators, either by dazzling them or by making the hunter more conspicuous to its own predators (much like a burglar alarm draws attention to thieves).
The fact that snail’s luminescence can be seen through its shell is a bonus. As Deheyn says, “The good thing is that the snail is already protected by its shell, which offers a physical defence, but not a repulsive mechanism.” The flashes could provide the latter, allowing the snail to deter enemies, while staying inside its already secure fortress.
The snail could also be using its light to communicate with other snails. Deheyn says that he has done experiments to test this idea, but they neither supported nor refuted it. He’ll have to look more into the snail’s behaviour and its life cycle to get better answers – for example, it’s possible that some land snails flash to get together for mating.
The final puzzle for Deheyn to solve is how the shell’s unique properties evolved. H.brasiliana has several close relatives that have shells but aren’t luminous, which suggests that the ability to shine evolved after the snail already had a shell. This bioluminescence must have evolved in tight step with the shell’s diffusive powers, especially since it only scatters the specific blue-green colour that the snail gives off. Deheyn is now looking at the rest of H.brasiliana’s family tree to unpick how the origin of these abilities.
Note: Sadly, there isn’t any video to accompany this story. Deheyn says that it’s hard and expensive to find video equipment that can record colour under very low light conditions. He’s working on it though, and should have something ready for the follow-up paper.
Reference: Proc Roy Soc B: http://dx.doi.org/10.1098/rspb.2010.220
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