divemaster masha ran me through the briefing. Fluo, she told me, is a – relatively – new form of diving, so not as yet widespread.
Biofluorescence occurs when light is absorbed and then re-emitted at a different wavelength, making the object or critter in the middle of the process appear to glow. It’s different from bioluminescence, in which an organism creates and emits light through a chemical reaction, because a light must be shone on the subject for it to glow.
For us to see biofluorescence on a night-dive, we have to use blue-light torches (which results in the best underwater fluorescence but is also harmless to the animals) and slot yellow plastic filters over our masks (to eliminate the blue light).
Used together, they reveal the full effect of the fluorescence, showing the reef in an intense, magical new light.
For many biofluorescent fish, this trippy deep-sea disco is just how they see the world. Simon explained that, just as we use yellow filters over our masks to see these flamboyant colours, “many biofluorescent fish use the same trick – they have yellow filters in their own lenses and corneas.
“Their fluorescent emissions are invisible to us, and most predatory fish, but strobe their presence to other fish that share this adaptation.
“It’s like invisible ink – a secret way to communicate without detection.”
Biofluorescence occurs in more than 250 fish species, as well as hard corals, jellyfish, mantis shrimps, sea turtles and other groups, Simon told me. It’s particularly common in cryptic species.
“Many coral-reef fish species such as angelfish have vivid colour patterns,” he said. “In contrast cryptics, such as lizardfish and scorpionfish, appear well-camouflaged. A recent survey found that 87% of these cryptic fish are biofluorescent, compared to only 9% of other fish.
“They’ve got the best of both worlds – hidden from predators, but gorgeous to each other.”
There’s still much we don’t know about biofluorescence and its role in the ocean. However, researchers have discovered that frogfish use fluorescent lures. “Wrasse use biofluorescent displays during courtship,” says Simon. “Triplefins use their red biofluorescent eyes to spotlight their copepod prey, whose eyes reflect the light but can’t see it themselves.”
If my afternoon dive had been coloured in carefully with crayons, was my fluoro night-dive going to look as if the reef had been scrawled on with fluorescent pens? That’s what I was hoping for.