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DIVING ON THE REEF AT NIGHT.

The last rays of the setting sun are fading as our boat pulls away from the dock and heads out into Montagu bay. In a few minutes we are approaching Atholl Island and can just make out the jutting bridge of the LCT wreck at its eastern end. This is our destination, for we have come to observe the fascinating nightlife of the wreck and the shallow but beautiful reefs surrounding it.

Blackbar Soldierfish The Blackbar Soldierfish only ventures from the safety of cracks and caves in the coral at night. 

The divers, all members of the Bahamas Underwater Club, have a deep interest in Marine Biology, and many of them devote much of their underwater time to photographing the life of the reef. 

Although we drop our anchor in only 15 feet of water, its inky blackness gives the impression of much greater depth. When we have donned our equipment, we check our powerful underwater flashlights, and rolling backwards off the boat, enter the dark and alien world of the night reef upside down in a cloud of bubbles.

I feel a degree of concern at such a time. I know there are sharks down here. I know also that they can detect me in the darkness and that I cannot see them. Fortunately I also know that they are extremely unlikely to attack me. I soon forget the sharks and other fish with big teeth as my eyes adjust to the darkness and my orientation returns. I have entered a magic world.

After a few minutes finning gently along the edge of the reef we switch off our flashlights and wait quietly. Our eyes adjust to the darkness and soon tiny pinpoints of light become apparent. We are seeing bioluminescence - light actually produced by animals of the reef. As we wait, more and more lights flash on until we can see thousands. The lights are so small and so numerous that the reef seems like a city at night viewed from a plane.

Spidercrabs, Lobsters, Shrimps and many other reef residents are only seen out on the reef at night.

Spider Crab

To find out which animals are producing which lights is a difficult undertaking. As soon as a flashlight is trained on the source of the light, the light goes out. Many of the light-producing animals are very small and are often hidden in corals, sponges and algae.

The divers are now observing an eerie phenomenon: even small movements of the arms and legs are causing swirling flashes of light to be emitted. They are caused primarily by a microscopic single-celled animal. This tiny protozoan is called Noctiluca, which actually means “night light.” During the Second World War the Japanese used dried preparations of Noctiluca to read maps when they were on jungle patrols. The dried powder needed only to be rubbed between moist hands to give off sufficient light for this purpose, yet not enough light to alert their enemies.

These are the same organisms that cause the bioluminescence in the bow wave of ships. F.T. Bullen, in ‘The cruise of the Cachalot,” writes:On the way, we one night en­countered that strange phenomenon, a milk sea. It was a lovely night, with scarcely any wind, the stars trying to make up for the absence of the moon by shining with intense brightness. The water had been more phosphorescent than usual, so that every little fish left a track of light behind him, greatly disproportionate to his size. As the night wore on, the sea grew brighter and brighter, until by midnight we appeared to be floating on an ocean of lambent flames.  Every little wave that broke against the ship’s side sent up shower of diamond-like spray, wonderfully beautiful to see, while passing school of porpoises fairly set the sea blazing, as they leaped up and gamboled in its glowing waters. ... in that shining flood the blackness of the ship stood out in startling contrast, and when we looked over the side our faces were strangely lit up by the brilliant glow.”

Noctiluca is not the only animal capable of light production, although it is certainly one of the most spectacular. Many animal groups have representatives capable of producing biological light. The list is very long and in­cludes Jellyfish, comb jellies, sponges, worms, shrimps, crabs, molluscs, brittle stars and fish.

The only problem with diving at night s that you never really know what's behind you!

Mako Shark

Many of these animals possess light-producing organs that are surprisingly similar in their structure to the mammalian eye. For instance, the shrimp, Meganyctiphanes has an almost spherical organ with a transparent cornea at the front. Behind this is a lens to focus the light, and behind this again is a layer of light-producing cells backed up by a reflecting layer to increase the intensity of the light.

Marine animals have four main methods of producing light, the simplest of which involves a light-­producing substance, Luciferin, which in the presence of an enzyme luciferase, is then oxidised to produce light. 

Much thought has gone into the biological significance of light production in animals and it is thought that there are numerous reasons for this. These include recognition, defence and reproduction. One marine worm, Odontoscyllis only luminesces during spawning an hour after sunset on the second, third and fourth days after a full moon! 

There is in the Bahamas a fascinating little marine worm that puts on a light display for anyone who sits on a dock in shallow water on dark nights. This worm swims around in circles, periodically emitting a cloud of luminous particles to attract the females

Many deep-sea fishes have characteristic patterns of luminous organs that aid in recognition in the zone where no light can penetrate.

Squirrelfish

The Squirrelfish

On the night reef we see fish roaming freely that normally are only seen deep within caves and under the coral. The bright red Blackbar Soldier fish and the Squirrelfish are out of their daytime dens! Others have retreated into the reef for safety. The parrotfish have hidden themselves away in mucus cocoons they secrete for protection. We also see big Crawfish or Spiny Lobsters wandering about the sandy seabed around the coral looking for food.  

Our dive is now ending; we swim back to the boat reluctantly after our unique experience. We have seen crawfish, crabs and shrimps marching openly about the reef, coral polyps expanded as they never are during the day, many species of fish that are normally hidden deep in crevices. And above all, we have been witnesses to the nightly pyrotechnics of the living reef.  

One need not be able to scuba dive to see all this. A shallow reef suitable for snorkeling has everything that the deeper reefs have. Not only is a night dive a rewarding experience - an insight into one of nature’s hidden faces - but it is also something of an adventure. One never knows just what may be lurking in the darkness, but it’s fun finding out!