I must admit I've not been much of a fan of night dives in the past. But with the proper light, enthusiastic night dive buddies, and some knowledge of what's going on out there, you can really see some amazing fish behavior and nocturnal critters.
The main difference between daytime and nocturnal migrations of schooling fish is their destination. Fish that feed during the day follow pathways over the reef or near the edge, never risking open water. Small plankton-feeding fish such as blue chromis usually stay within easy reach of coral, although they feed in the water column. Nighttime migrations are made up of fish that move out and away from the reef's protection, either to feed in the sand or grass flats or on plankton in the darkness of the open water. Some schools break up into small groups and forage a short distance from the reef. Others move as far away as a mile or so. Predators are not as great a threat to these exposed reef fish at night, although some of the smaller bottom-feeding fish sometimes fall prey to a lurking nocturnal scorpionfish or stingray.
The majority of night fish are carnivores, out for a meal. The available food is different at night - just as there is a nocturnal population of fish, there are nocturnal varieties of invertebrates. Even the plankton changes. Small shrimps and crabs that live in the sandy bottom during the day migrate upward at night to feed on plankton, becoming fair game for the night fish. Other crustaceans, together with worms, starfish, mollusks, and a host of other invertebrates emerge from their hiding places to scour the sea floor for food. The tiny ones are a meal for squirrelfish and cardinalfish; the larger ones fall prey to stingrays and larger predators. The squirrelfish family is well adapted for nocturnal living. Their reddish color makes them almost invisible in the darkness and their huge eyes give them excellent night vision.
Many species of the snapper family are primarily nocturnal, dispersing from the reef area at night to eat crustaceans and small fish. The yellowtail snapper, a boldly curious, opportunistic feeder seems to be active around the clock.
Eels are at home in the nocturnal world, and morays are often seen free-swimming along the reef during a night dive. Bright moonlight apparently affects fish behavior. A triggerfish scours a sandy bottom, excavating invertebrates from the sand while hovering head down and blowing streams of water downward. Dark nights find it asleep in a crevice, but if there is enough moonlight, it goes out to get a midnight snack.
Many corals feed at night and divers can lure light-sensitive plankton into the extended coral polyps with a dive light trained on one coralhead and watch the feeding process. In the water column your light will pick up the odd shapes of small jellyfish, microscopic plankton, and tiny self-propelled organisms twisting and pulsating in the light. Turn the light off and see the bioluminescent animals shining with their chemically generated light, trailing behind your sweeping arm movement.
Basket stars unfold and nocturnal crinoids, having crawled out from their daytime crevices, feed by extending their arms into the current. Sea urchins emerge from their holes and march about the reef borders, grazing on algae and creating bare pathways as they go. Brittle stars and small shrimp and crabs are often seen, as are bristleworms out eating coral polyps or munching on anemones. This is the time to look for active octopuses, lobster, giant crabs, and large annelid worms. Hermit crabs are usually everywhere, scavenging the reef in their clumsy, borrowed shells.
About an hour before dawn, a reversal of the evening transition begins. Schools of foraging goatfish, snappers, grunts, sweepers, squirrelfish, and drums reassemble from the sandy and rock bottom surrounding the reef and prepare to make their mass transit back to daytime resting valleys and crevices. Plankton eaters, some in groups and some one by one, scurry back to bed in coral recesses as the night sleepers awaken.
Samet Bilir writes about technology trends, digital camera reviews, and photography, such as DSLR camera bags 2012 and Nikon P7100. To read more articles from him visit his website at chi-photography.com.