Marine biofluorescence: why corals glow

Corals are one of the many animals in our ocean that exhibit biofluorescence. Biofluorescence is the phenomenon where the animal absorbs a high-energy light such as violet or blue light, and gives off a lower-energy light such as green or red light due to loss of heat energy.

But why do corals exhibit biofluorescence?

1. Protect zooxanthellae

Zooxanthellae are unicellular algae that live symbiotically inside corals. They are photosynthetic and provide the coral with food in exchange for shelter. One reason why corals exhibit biofluorescence is to protect this symbiotic zooxanthellae from the sun’s harmful UV rays. By absorbing the high-energy UV rays and converting them to a lower-energy light, it serves as a form of sunscreen for the zooxanthellae.

Close-up of translucent coral polyps, showing the symbiotic algae living inside. Image source: Smithsonian Ocen

2. Attract prey

Some marine animals can see beyond the visible light spectrum (i.e. ultra-violet spectrum or infra-red spectrum). This is possibly one reason why corals often fluoresce around their mouth and tentacles – to use biofluorescence to attract prey.

Brighter glow = less healthy

Scientists have recently discovered that they can use the level of glow to monitor the health status of corals. They found that unhealthy corals (those affected by warming ocean temperatures and ocean acidification) tend to glow brighter. The loss of zooxanthellae as a response mechanism to stressful environmental conditions means that the coral’s underlying fluorescence is no longer shielded, so the corals appear brighter.

With this ability to measure the level of glow, scientists can better detect early signs of stress in corals before level of coral bleaching becomes serious.

Here at the S.E.A. Aquarium, you can observe biofluorescence at our Night Reef Habitat where the corals are illuminated by blue light, which allows us to see their fluorescence.
corals biofluorescence

Want to learn more about why marine animals glow? Visit Glowing Ocean at S.E.A. Aquarium from now till 6 Jan 2019.


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