After the spring full moon, corals at the Great Barrier Reef spawn by the many thousands. Last year, scientists found how corals are able to sense moonlight; cryptochromes allow corals to detect even the faintest light source!

It is widely known that coral reefs spawn in synchrony every year, just after the July/August (Caribbean reefs) and November (Great Barrier Reef) full moons. Scientists believe this is regulated by lunar cycles, as corals always spawn several days after such full moons. But how exactly do corals sense moonlight and respond to it?

Figure 1, right: The coral Acropora millepora is able to detect light sources by production of light-sensitive proteins called cryptochromes (photo by Michael de Regt).  

Light-detecting proteins

Recently, scientists (O. Levy et al, 2007) found that two photo-sensitive proteins, called cryptochrome 1 and 2 (CRY1 and CRY2), are responsible for the ability of corals to detect light. These proteins are very senstive to blue light, which is emitted by the sun, but especially by the moon. They discovered that these proteins were mainly produced by the coral Acropora millepora (fig.1) during daytime, and only in small amounts during the night.                      

The scientists subjected the corals to a light-dark cycle, and a continuous dark period. Figure 2 shows that A. millepora responds to light by producing a lot of cryptochrome 1 protein. This goes down after several hours, and up again just before dawn. When the corals were kept in the dark continuously, protein production was erratic. This shows that corals have adapted to natural day/night cycles, as they need both light and dark to produce cryptochromes in a steady rhythm.         

Figure 2: cry1 gene expression (which leads to CRY1 protein production) of A. millepora under light-dark cycles (blue line) and dark cycles (red line). The scientists took samples from the coral tissue at different time-points during the day and night. CRY1 expression increased during the day. At night, production was low. The same was true for CRY2 expression, which was also much higher during the day compared to night time. The red line shows that without a day/night cycle, a steady protein expression pattern is absent (which means the corals simply do not know whether it is day or night) (O. Levy et al, Science, 2007)

These results show that corals are able to sense (sun)light, and respond to this by increasing the production of CRY1 and CRY2 cryptochromes. Next, they determined whether these corals were able to sense moonlight. It was found that CRY2 protein expression is increased during full moon, which shows that corals really are able to sense moonlight and to respond to it (fig.3).

Figure 3: cry2 gene expression (which leads to CRY2 protein production) during the new and full moons in August/September 2005. CRY2 protein expression is doubled at midnight, during full moon. At 18:00 in the evening, there was no difference in cry2 expression during new and full moon days. This might be because the sun has not set at this time (O. Levy et al, Science, 2007).

They also investigated in which part of the coral tissue these proteins were expressed. Figure 4 shows that cryptochrome 1 and 2 are expressed in the ectoderm (ec, which is the outer tissue layer) of both A. millepora adult corals and planula larvae. This is a very usual expression pattern for light-sensitive proteins, as this part of the coral tissue is hit by light first. The scientists believe that these proteins tell corals to protect themselves against UV-radiation, and they also seem to tell them when to spawn every year. It is known that the moon cycles correspond with water currents, and it seems that corals prefer to spawn their sperm and eggs when currents are low.

Figure 4: cry1 and cry2 gene expression in coral tissue. C: Control to show the specificity of the test. D: CRY2 is expressed in the ectoderm layer of the A. millepora larva tissue (brown staining). E: CRY1 protein localization in the ectoderm layer of adult A. millepora corals. F: CRY2 expression.  ec, ectoderm; en, endoderm; mes, mesoglea. Scale bars 50 mm. Red arrowheads mark expression of the antibody in (D) to (F); the rectangle shows one cell in the ectoderm layer (O. Levy et al, Science, 2007).

Why do cryptochromes exist?

Biologists believe that cryptochromes exist to regulate Circadian rhythms (the responses of organisms to day/night cycles). Many plants and animals are able to adapt themselves to various kinds of cycles, and  these results show that corals can do this as well. This proves that Circadian rhythms evolved millions of years ago. Corals already show clear responses to sun- and moonlight, possibly allowing them to protect and reproduce themselves.

Cryptochromes are also found in insects and mammals. The two cryptochromes found in mammals play an important role in the generation and maintenance of Circadian rhythms. A recent study suggests that cryptochromes allow migratory birds and fruitflies to navigate by sensing magnetic fields. Other theories suggest this ability lies dormant in all mammals. The fact that corals are already able to produce these light-sensitive proteins, again underscores how complex these animals really are.

References:

O. Levy, L. Appelbaum, W. Leggat, Y. Gothlif, D. C. Hayward, D. J. Miller, O. Hoegh-Guldberg, Light-Responsive Cryptochromes from a Simple Multicellular Animal, the Coral Acropora millepora (2007) Science 318 (5849): 467–470

Roenneberg, T.; Foster R.G. (1997). "Twilight times: light and the circadian system". Photochem. Photobiol. 66: 549–561

Foster, R.G. (1998). "Shedding light on the biological clock". Neuron 20: 829–832.

Heyers, Dominik; Martina Manns, Harald Luksch, Onur Güntürkün, Henrik Mouritsen (2007). "A visual pathway links brain structures active during magnetic compass orientation in migratory birds". PLoS ONE 2 (9)