It was a warm, dark night on the Northern shore of Isla Holbox, Mexico: perfect conditions to view one of nature’s most spectacularly ethereal phenomena. The paradise island attracts visitors from around the world to its pleasant beaches, and is famous for its nearby whale shark breeding grounds. But on this mid-September evening, it was the island’s stunning bioluminescence that took centre stage.
Bioluminescence, the phenomenon of light emission by living organisms, is a marvel to behold. When disturbed by movement in the water, tiny channels in the bodies of phytoplankton allow a small electrical current to pass through them, stimulating a chemical response which produces a faint blue light. The result, on this particular evening, was a beautiful trail of tiny particulate lightbulbs following each movement of my body as I waded through the water.
Mesmerised by the blue brilliance of my tiny glowing followers, I became absorbed in painting the ocean with ghostly light trails. The water became a canvas, and I felt like a child with a sparkler. That was, however, until I spotted a pelican, perched on a rocky groin in the midnight gloom ahead of me. I approached carefully and quietly, though the element of stealth had been taken from me by my little blue friends, and came within spitting distance of the large marine bird. The bioluminescence in the shallow waters had wowed me with its beauty, but the pelican on the groin impressed me in a much more tangible, intelligent way.
I followed the bird’s stare for a few moments, peering into the dark waters around us. Then I saw it: every ten seconds or so, a quick flash of blue in the pan of murky ocean. What we were watching was the bioluminescence stimulated by the sudden explosions of motion of fish. I cracked a smile as I recognised the pelican’s genius - he was catching himself a late-night snack. With each little movement the fish gave away their positions, and the pelican took careful note as he bided his time. Then, once he was sure of his target, he leapt from his perch and scooped a mouthful of water - and hopefully fish - into his mouth like a firefighting seaplane.
The irony of this remarkable hunt is that the bioluminescence of these plankton is designed as a defence mechanism. Dinoflagellates, the type of plankton which exhibit bioluminescence, use their light either to startle their active predators or to ward off potential ones. The bioluminescence of the prey attracts attention to the predator, who is then vulnerable to predation from larger fish below – or in this case, from the pelican above.
The phenomenon of bioluminescence is present in many forms in nature. As a defence mechanism, some species of squid can delay predators with a sticky bioluminescent mucus. Some other marine species use counterillumination, an ingenious form of active camouflage; as sharks look up from the depths below, prey appear as dark silhouettes against the bright sky - therefore, a fish which illuminates its belly becomes almost invisible.
Bioluminescence can be used to catch prey, too. The notoriously menacing angler fish uses a bioluminescent lightbulb attached to a long stalk on its head to attract curious fish in the deep ocean, before snapping them up when they approach close enough. The loosejaw dragon fish uses a rare red bioluminescence as a searchlight, as most marine species are unable to detect red, yellow and violet colours.
Interestingly, aquatic bioluminescence appears to be unique to saltwater environments. The light emitting mechanism involves two types of chemical - luciferin, the protein that produces light by undergoing oxidation, and luciferase, the enzyme which allows the reaction to happen quickly enough to produce light. These chemicals are both synthesised within the bodies of dinoflagellate plankton, but their activation requires chemicals which are only found in saltwater.
But this special reaction is an amazing piece of bioengineering, producing a “cold light” with very little heat generated. This makes it a source of inspiration for human engineers looking to reduce energy wastage - bioluminescent street lamps could soon become a reality, and bioluminescence is already being used to track biological reactions within the human body.
Back in the Gulf of Mexico, I considered the critical role of these tiny creatures in the global marine ecosystem, as the flashing water washed over rough cracks in the rocks. Above me, a clear, dark sky allowed the safe passage of the International Space Station overhead, and I wondered whether, from their heavenly perspective, they could see my little personal light show. Perhaps not. But I’m sure they could see the rest of the pale blue dot in all its infinite complexity and beauty.
