What you cannot see can hurt you. Therefore, throughout nature, when the sun goes down, the cover of darkness drives many animals to seek the security of shelter for protection and sleep. To be effective, most predators need to maximize the frequency with which they encounter their prey and exploit the situations when they are vulnerable and night does not offer that opportu nity. But for others, sundown is their wake-up call.

Throughout the oceans of the world, darkness transforms the marine environment. Most fish species cease their activities and others, who during the day relied on camouflage and hiding for protection, emerge from hiding.

In the clear waters of the tropics, where light levels differ dramatically between day and night, many changes are very conspicuous. The parrotfish wraps itself in a mucous cocoon and corals blossom in a feathery coating of polyps.

The changes that occur in the marine environment in Canadian waters are not as dramatic, but are still evident to the careful observer. And many divers are lured to the nocturnal oceans to witness this change of players in nature's eternal game. As the light of day fades and turns to night there is a time that we refer to as twilight or, as marine biologists might put it, the crepuscular period. This interim period is a time of frenzied activity for many creatures. Diurnal animals must find shelter, nocturnal crea tures are emerging and crepuscular species are taking advantage of the chaos to snare confused prey in the dimming light.

Along the Pacific coast, within a half hour of the sun going down, many of the large predatory fish such as the ling cod as well as many rockfish species can be found settled in for the night, under rocks or tucked into cracks. Most of the mid-water schooling fish have retreated off shore or found shelter, but some species of perch will come closer to the bottom and still remain active for a few hours after sundown. In fact during recent dives at a popular dive site we have encountered perch and illumi nated them with our lights only to have seals, usually a diurnal animal, come down for a quick snack. The seals have now learned to occasionally follow divers, so we have to be very careful where we shine our lights.

Most of the fish that are encountered on a night dive are big-eyed fish such as the sculpins, whose eyes are designed to absorb the maximum amount of light. Small members of the sculpin family such as the sailfin sculpin, that would be easy prey during the day due to its large and conspicuous dorsal fin, is a frequent nocturnal sighting.

The most obvious change that we see, both in the tropics and in Canadian waters, is with the invertebrate and crustacean community which is always much more active at night. Crabs that are, for the most part, buried in the sand during the day emerge from their sandy lairs after dark. Shrimps and large prawns are much more evident at night and a few species of animals that are rarely seen during the day, such as the large scale worms come out of hiding in search of food.

All this activity does not go unnoticed. The octopus, an active nocturnal predator, can be found prowling the sandy bottom in search of crabs. Small cousins of the octopus such as the stubby squid will emerge at night and, during certain times of the year, market squid can also be sighted.

Some invertebrates, not usually seen in the day, make their debut after the sun goes down. In some areas basket stars hide beneath the rocks during the day but will emerge at night to spread their web of arms to catch prey. Sessile invertebrates such as the small cup corals, whose bright orange polyps are often retracted during the day, and gorgonian corals are more likely to have their polyps open at night.

In addition to these creatures, night diving also gives us the opportunity to experience bioluminescence. Some creatures can emit light from specialized cells called photophores while others give off light thanks to light-emitting bacteria that surrounds them or grows under their skins. When the lights are off, waving your hand through the water can create an underwater fireworks display and your buddy may be outlined in an eerie glow of greens and blues as he swims through the water.

Many divers say they see more on a night dive than on the same dive during the day. This is usually due to their concentrated attention on a much smaller area. During the day, if the visibility is good, we have many distractions from our peripheral vision but at night those are gone and we see only what is in the small area illuminated by our lights.

The nocturnal marine environment is full of wonder and night diving is a fascinating and exciting experience, but it should never be undertaken without the proper training and equipment. A powerful main light and a backup light are a necessity. Other equipment might include a glow stick or battery operated marker light just so you can keep in visual contact with your buddy.