Rhodopsin is a photoreceptive molecule composed of a Vitamin A derivative (retinal) and an opsin. Opsins are proteins that complement the retinal, and are “tuned” to certain wavelengths of light, and this means that they respond most vigorously to a certain wavelength but usually respond to wavelengths close to the critical wavelength. Opsins can be identified as being tuned to “green” but this is misleading as that opsin will have a peak response to wavelengths in the green range, but respond to light with longer and shorter wavelengths, too.
All metazoan opsins are related, making them homologous. They probably arose from a single opsin and have mutated or diverged.
Rhodopsins are the principal molecule that responds to light in animals, and creates the impulses that lead to sight, and most animals that have vision do so with a form or rhodopsin.
But, Vitamin A and an opsin are not the only way to receive and respond to light. There are other molecules that respond too, and some are important to vision in many animals.
The cryptochromes are biochemical molecules that respond best to blue light and are quite ancient. These probably arose from DNA repair molecules called photolyases, or at least the cryptochromes are close cousins to the photolyases. The DNA found in early and ancient life was easily damaged by ultraviolet light, and there would have been a lot of UV because there was no oxygen to make ozone, and few other molecules in the atmosphere. The DNA would found a way to be repaired, which would have been through the photolyases. These chemicals probably evolved into cryptochromes. Cryptochromes are derived from flavins or Vitamin B2 , and can convert the energy of light into chemical energy. This would have been a valuable asset to an early cell , and hence, this chemical would have been used in by almost all cells in some way.
Cryptochromes are responsive to blue light, and when these chemicals first arose, there was likely a lot of blue and ultraviolet light. Cryptochromes are able to use that energy to repair DNA. Cryptochromes are very common and present in almost all (if not all) plants, animals, and even many forms of bacteria. Most of the time, these molecules are used to set circadian rhythms or to measure light and dark. But, not always.
Cryptochromes are used by many corals especially in the southwestern Pacific to measure the length of moonlight and the color of the moonlight. This permits these corals to synchronize with each other to spawn on virtually the same night in early spring every year.
Why would corals want to spawn at the same time each year? Predators like to eat the spawn (essentially eggs) of almost any animals including coral, but with so many eggs it is impossible for the predators to eat all of them The volume of release is simply too big. Furthermore, if the male and female spawn are released at the same time into the water, they stand a much greater chance of finding one another to spread the species. As it turns out, this is the largest mass sex act in nature. All in one night! Cryptochromes are important in other ways, and we will visit them again.