Inbuilt lenses transmit sunlight to symbiotic algae
SLURPING OYSTERS from their shells may be a rare indulgence for humans, but these bivalve molluscs and their relatives, such as clams and mussels, slurp for a living. Most are filter feeders, ingesting microscopic organisms as well as debris from their aquatic environments. A handful of mollusc species, however, have formed symbiotic partnerships with algae that live within their tissues. These individuals augment their diet with sugars that their microscopic partners produce through photosynthesis.
Some species, such as giant clams, must open their shells in order to give the algae the sunlight they need. But smaller relatives called heart cockles are able to nourish their internal algae farms without popping their lids. A recent preprint claims to have worked out how. These heart cockles are able to channel sunlight through their shells using the biological equivalent of fibre optics—the first time such an innovation has been seen in nature.
Fibre-optic cables are engineered to carry light over long distances with minimal loss. They do this by in effect trapping the light inside a material with a higher refractive index than the cladding that surrounds it. Every time the light meets the boundary between the core and the cladding, the difference in refractive indices means light reflects off the boundary rather than passing through.
Dakota McCoy, an evolutionary biologist at the University of Chicago, and her colleagues believe that something similar is going on in heart cockles. Their shells possess transparent windows formed of fibrous crystals of aragonite (a form of calcium carbonate), surrounded by a “cladding” made of aragonite and chitin, a material also found in arthropod exoskeletons.
After taking detailed microscopic images of heart-cockle shells, Dr McCoy and her team determined that the opaque aragonite crystals and chitin surrounding the transparent windows have a lower refractive index than the windows themselves, ensuring that incoming light reaches the hungry symbionts within. That, says Oded Raz of the Eindhoven Technical University of Technology, who was not involved in the research, makes it a form of fibre optic.
The team also measured the wavelengths of light that the windows transmit to the algae using slices of the cockles’ shells in laboratory experiments. Though the windows allow photosynthetically useful wavelengths of light to enter, they also filter out ultraviolet (UV) radiation that might harm the cockle and its symbionts. This UV protection mechanism appears to have the edge on those used by coral, which house the same types of algae and exploit them in similar ways.
Why such abilities evolved in heart cockles remains unclear. It is possible, says Dr McCoy, that their aragonite windows could serve as primitive eyes for viewing the world outside their shells. This evolutionary innovation may also be protective: owing to their small size, heart cockles are more vulnerable to predation while open than giant clams are. Whatever the cause, these spineless technological masterminds seem to have it all worked out. (Oct 28th 2024)
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