From Dusk 'til Dawn

For some, standing on the diving platform contemplating jumping into the uninviting black water is a terrifying prospect.  For others it is merely a distraction keeping them from a well earned beer. What cannot be denied is that night dives offer a brief glimpse into an entirely different marine environment from that which can be experienced during the day.

As soon as the sun creeps below the horizon the entire landscape of the underwater world begins to change. Once the light begins to fade the speed hunters become the rulers of the reef; with their excellent vision the jacks and trevallies are perfectly adapted to dart through the water and catch unsuspecting smaller fish that, with the reduced vision that dusk brings, no longer have the awareness to escape.

There are several different techniques used by the diurnal creatures to survive the perilous hours of darkness. Schooling is undoubtedly one of the most spectacular; tens, even hundreds of thousands of tiny fish use a simple behavioural algorithm to remain together in an intricate organic pattern which can often be clearly seen in the boat floodlights.

Other methods are better seen from under the water. Some species merely hide in a convenient crevice or hole, whilst others have developed unique tactics to make it through the night. Members of the parrotfish family are well known for producing a clear mucus cocoon inside which they shelter for the night, before consuming it the next morning. The cocoon is thought to have healing properties for the parrotfish and stops its scent attracting predators as well as providing an early warning system if attacked.

Other fish rely on camouflage for protection. During the day, yellow fin goatfish (Mulloidichthys vanicolensis) are a distinctive white and yellow and can often be seen schooling in mid-water. By night the fish hide on the bottom, turning themselves a mottled red and grey to better blend in.  Many creatures that are active at night are also red, this is because it requires less pigment (and therefore less energy) to be coloured red than black.

The combination of the limited moonlight and colour absorption of the water ensures that no red light can penetrate even the shallows, rendering red fish apparently black.
This gives rise to one of the most popular night dive sights. Nicknamed for its swirling locomotion, the Spanish dancer (Hexabranchus sanguineus) is a giant blood-red nudibranch regularly seen at lengths of 40cm. It hides inside the reef by day but can be seen in its full glory at night.

Brittlestars (ophiuroids) and featherstars (crinoids), while occasionally seen moving by day are at their most spectacular when feeding by night. Basket stars (Euryalida) are a personal favourite.  Bearing a striking resemblance to a large leafless shrub, they have a central mouth and arms that can extend up to 70cm. 

They position themselves on exposed parts of the reef by night to maximise the amount of passing zooplankton that they can catch.

No night dive would be complete without the vast array of crustaceans which venture out into the open during the hours of darkness. Certain shrimps can easily be spotted as their eyes appear as bright spots of red, gold or green in torchlight. Crabs are also out and about searching for food, from the bright red Splendid Spooner crabs (Etisus splendidus) to the ingenious Red Sea anemone hermit crab (Dardanus tinctor) which lives in a symbiosis with the anemones it carries on the outside of its shell. The crab benefits from being excellently camouflaged and the anemone gets moved from the safety of the reef during the day, to prime feeding grounds by night. Slipper lobsters (Scyllaridae), with their impressive armour plating, can also be found at night. They are excellent tunnellers and spend their days sheltering under the sand, only venturing out at night to feed and mate.

It is not just the mobile reef life that changes when the sun sets. The coral polyps (covering every inch of living reef) while mainly dormant by day, take advantage of the lack of butterflyfish and bannerfish (which feed on the unprotected open polyps) to extend their tentacles and feed on the passing zooplankton in the water during the night.

The zooplankton itself can display another biological wonder; bioluminescence. Certain species of Dinoflagellates emit light (normally green or gold) when disturbed in the water. Depending on the concentrations present this can cause a twinkling effect in the turbulence caused by a diver’s fin kicks, arm movements or, in extreme cases, even in the column of exhaled bubbles rising to the surface.

The flashlight fish (Photoblepharon steinitzi) displays bioluminescence at a different level.  About 10-15cm long and normally a deepwater fish, it rises to shallower water by night. The bioluminescent organs beneath its eyes contain luminous red bacteria, the light from which is used to communicate and attract prey. I know a dive guide who was able to turn off their torch on a moonless night to see an entire dive site illuminated by thousands of tiny glowing flashlight fish.

Whilst you might not want to make a night dive on every evening of your diving trip, they can be some of the most rewarding and interesting dives you can ever make. Ask your guide at the beginning of your trip about the best night dives on your particular route and delay your evening beer on at least one night.  That way you might encounter some of nature’s marvels which can only be experienced once darkness has fallen.

by Chris GoodaFirst published in BLUE magazine January 2010