Bioluminescence, the biological production of light, is a breath-taking event to behold. Brilliant squid, elegant jellies and glowing fish use this strategy to attract mates, detect prey, camouflage themselves or deter predators. Extraordinary adaptations to transmit light are well characterized in open ocean species but are not well described in marine molluscs in other habitats. Although the intertidal, thumb nail-sized clusterwink snail, Hinea brasiliana, is known to be among these luminous creatures, the mechanism behind its light production was a mystery. Dimitri Deheyn and Nerida Wilson at the Scripps Institution of Oceanography (UCSD) set out to unravel these secrets, exposing an incredible, unique tactic behind the glow of this snail.
Deheyn and Wilson caught the tiny snails on the beaches of eastern Australia, where they cluster in groups among rocks. The duo transported their catch to the laboratory to quantify the snail's emitted light. First, they stimulated the snails to produce light using chemicals and, within seconds, the clusterwink emitted a radiant blue–green light from an area of its body incapable of extending beyond the shell. Next, they placed snails with other organisms from their habitat to find out what provoked the molluscs' light display. They found that the snails produced their highest light levels when they encountered species that they contacted frequently. To determine whether the clusterwink's light show could be mechanically stimulated, the team then placed the snails with actively swimming amphipods, providing ample opportunity for physical interaction. Although rare as a trigger of bioluminescence in other species, physical contact did trigger intense light displays from the snails.
As the source of light cannot extend beyond the snail's opaque, pigmented and calcified shell, the authors then characterized how the shell transmitted and diffused light. They found that despite its hardiness, the shell transmits most wavelengths of light, with the exception of the blue–green peak of the snail's bioluminescence. However, when blue–green light is shone into the shell (mimicking natural bioluminescence), it scatters to parts of the shell not directly exposed to the source, allowing regions of the shell that are not directly illuminated to glow. Deheyn and Wilson also found that the wavelength of the light is not altered by the pigment or opacity of the shell, in contrast to other bioluminescent organisms that do alter the light's colour. The shell also transmits light incredibly effectively for its thickness: 8 times higher than an equivalent commercially available diffuser. Although high transmittance is usually associated with low diffusion in materials, the shell's biomaterial retains a remarkable capacity to diffuse light. In fact, the blue–green glow projects over the entire shell, an area 10 times larger than that of commercial diffusers. The authors think that this remarkable property co-evolved with H. brasiliana's luminous capacity as the shells of closely related non-bioluminescent species do not diffuse or transmit light.
This is the first account of a calcified structure that diffuses and enlarges biologically produced light, perhaps providing inspiration for the next generation of light-transmitting materials and optical devices. However, from the clusterwink's perspective, their shells provide an exceptional defence tactic: producing an intense and enlarged flash of light to deter predators while offering shelter to the resident within.
