See how giant clams help their symbiotic partner

Special cells in giant mollusks shift the wavelength of light to protect them from ultraviolet radiation and increase the photosynthetic activity of their symbionts, research by King Abdullah University of Science and Technology, originally conceived as a study of photonics, shows. The photoprotective effect allows giant underwater inhabitants to dwell in very shallow tropical waters, where there is enough light for photosynthesis, as well as potentially harmful levels of ultraviolet radiation.

Like corals, giant mollusks are important participants in reef ecosystems and live in symbiosis with the photosynthetic algae Symbiodiniaceae. Mollusks also have special cells, known as iridocytes, which can manipulate light through layers of nano-reflectors in each cell. Earlier work showed that these iridocytes scatter and reflect light to increase the photosynthetic efficacy of Symbiodiniaceae algae.

Now a team of scientists from the Red Sea Research Center and the Photonics Laboratory has discovered yet another way in which iridocytes help the symbiont photosynthesize. Experts studied the morphology and optical characteristics of iridocytes at the maxima of the giant mollusk Tridacna. As a result, scientists found that they absorb ultraviolet radiation, and in addition, re-emit it in the form of photosynthetically useful light with a longer wavelength.

Ram Chandra Subedi, one of the authors of the study, explains that iridocytes contain alternating layers of a high refractive index guanine crystal and a low refractive index cytoplasm. The compression and relaxation of these layers allows the cell to adjust its effect on the light. As a result, guanine palettes not only reflect harmful ultraviolet radiation but also absorb it and emit light at higher wavelengths, which are safe and useful for photosynthesis.

This increases the amount of photosynthetically active radiation available to the algal symbiont, and also helps protect mollusks and algae from ultraviolet radiation.

In addition, this process can explain the colors of the mantle of giant mollusks. Their bright colors are associated not with optical differences in tissue, but with a difference in the distribution or number of symbionts relative to iridocytes in each individual.

Scientists warn that this is just a hypothesis, but, in their opinion, the most reasonable explanation for the different colors of mollusks. Whether color differences are functional implications remains an open question.

Researchers say the project itself was originally based on curiosity. Scientists wanted to see if iridocytes have optical properties that can be useful in photon technology.

Initially, it was not a question of answering the question of biology, but in the end, she explained a lot about this symbiosis and opened up new questions in the field of biological photonics.

Susanne Rossbach, lead author of the research