Fast Fact: Phototransduction is the process by which many light-sensitive proteins, such as opsins, convert information about light into electrical signals for the brain.
Animals do not necessarily need to have eyeballs to be able to see. Eyespots, used by some organism for detecting light (phototaxis), are the simplest type of “eyes” that exist in nature. They are made of pigment molecules and photosensitive opsin proteins. Opsins are found in neurons, and they communicate information about light to the nervous system. But some basic organisms with simple visual systems, like sponges and jellyfish, don’t have nervous systems. And they don’t have opsin proteins either.
Yet sponge larvae are still able to differentiate between different intensities of light, an ability that helps them find optimal locations for survival, such as the darker areas under coral rubble. Since they lack opsin, the rings of light-sensitive cells in sponge larvae are fundamentally different from the eyes of other basic invertebrates. This means that they must have evolved independently. And until recently, the proteins that make these unique sponge eyes work have remained a mystery.
Fast Fact: The first species to develop photosensitivity were aquatic, and only two specific wavelength ranges of electromagnetic radiation, blue and green visible light, can travel through water. This is probably why eyes can only detect such a narrow range of wavelengths on the electromagnetic spectrum.
Researchers at California’s University of the Pacific have discovered that cryptochrome is the molecule responsible for sponge larvae’s responsiveness to light. A research team searching the sponge genome found two genes that code for cryptochrome proteins. When they tested where the genes were being used, they found that the messenger RNA (which contains the blueprint for a protein) of one gene, Aq-Cry2, was produced near the sponge’s simple eye cells. The most important piece of the puzzle was discovered when the research team tested which wavelength of light the proteins absorbed. Sponge larvae respond the most to 450nm (blue) light, the same light that Aq-Cry2 proteins most readily absorb.
It is not clear exactly what role cryptochrome plays in a sponge’s ability to detect light. But given the wavelength common to both sponge larvae behaviour and protein light absorption, researchers think it acts on ciliary rudders (small hairs on the outside of cells that help the sponge move). It is possible that when the ciliary rudders detect blueish light, indicating shadowy areas that would be a good place to settle, they prompt the sponge move to that spot. Another possibility is that cryptochrome plays a role within a signalling pathway, allowing multiple sponges to react in a synchronous way. For example, they might be prompted to swim or remain immobile, in response to the different types of light present during different seasons or times of day.
Electromagnetic Spectrum (click to enlarge)
So an animal does not necessarily need eyeballs and a central nervous system to be able to see. Basic organisms like sponges can detect light in other ways, although both types of sight likely evolved from a common ancestor. And thanks to the research discussed above, scientists are closer to understanding exactly how sponges, individually or in groups, use cryptochrome in order to react and adapt to their environment.
Ecology, Evolution and Marine Biology (Oakley Evolution Lab, University of California Santa Barbara)
Sponge Larvae Could Be Guided by Cryptochrome (The Journal of Experimental Biology)