ON THE REEF AT NIGHT.
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
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.
Lobsters, Shrimps and many other reef residents are only
seen out on the reef at night.
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,”
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
only problem with diving at night s that you never
really know what's behind you!
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.
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.
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!
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
to attract the females
deep-sea fishes have characteristic patterns of luminous organs
that aid in recognition in the zone where no light can
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.
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.
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!