Rush Hour on the Reef: The Best Approach to Snorkeling at Dusk
Article by Joel Simon

Coral reefs are amazing places. Any person who has seen a reef, first hand or even in photographs or television documentaries is immediately struck by the astounding diversity and density of life. In fact, tropical reefs are distinguished by supporting more species of animal life, in greater abundance and closer proximity to each other than any other natural habitat on earth. Yet, for most snorkelers, many of the reef’s most obvious creatures go unnoticed. Why?

Without reference to observational skills, it is simply that the reef, as a community, never sleeps. Most of it’s inhabitants do, however, just not at the same time. So if you’ve never snorkeled at night, you’re missing half the fish, a plethora of nocturnal invertebrates, and at least half the fun. We will treat night snorkeling in a separate article, but it’s in the fading illumination of dusk that we as snorkelers can view the reef in an entirely new light.

In many ways the reef can be compared to an urban zone. Not just a town, or even a substantial city, the reef is more like a major metropolitan center bustling with activity, day and night. In such a city, there are huge populations of people employed during "normal" business hours. At the end of the day, these folks go home (or at least to locations other than their work place) and a new set of people and activities take over. Although these groups are ultimately distinct, anyone who has experienced "rush hour" traffic will understand that the transitional hours between day and night are often the most congested, chaotic, and in some ways, competitive.

Coral reefs also have their "rush hour", the so-called crepuscular twilight periods of dusk and dawn, when diurnal and nocturnal populations cross paths. During this time behaviors alter radically. One set of creatures is seeking safe haven for rest while others are waking up ... hungry. Predation is at it’s peak. As snorkelers, this is one of the very most exciting times to observe the reef community.

Visual ability is a fundamental survival skill among creatures of the reef, especially fish. Primarily through vision, most fish accomplish the rigors of their daily routine that allow them to stay alive--finding food, shelter, mates, and avoiding predators. This is no easy task considering the highly competitive nature of reef life.

By necessity, the eyes of fish are remarkably sophisticated. In contrast to archaic beliefs of a distorted "fish-eye" view, more recent studies show that the lenses of fish eyes yield acute, distortion-free sight. Many fish are also capable of binocular vision, allowing a keen sense of depth perception necessary for navigating the complex, three-dimensional coral world. Vision requires light, and we find that different species of fish have adapted successfully by evolving eyes that function best under specific lighting conditions.

In general, fish that thrive in the abundant light of day, have eyes with small pupils facilitating excellent focus. Additionally, the backs of their eyes are literally packed with light-sensitive cone cells that give them color sensitivity and visual acuity in some cases more highly developed than our own. With this capability, daytime fish are able to able to discriminate between miniscule particles of food, various types of coral and other elements of habitat, as well as species of fish that appear similar, or threatening. But what happens when light levels drop?

Specialization is not without its cost. While these eyes are extremely well adapted for the bright light of day, when illumination diminishes, so does visual ability. At the end of the day colors are muted, and small pupils are unable to conduct enough light to see clearly. In this compromised state of impaired vision, diurnal fish become vulnerable.

Just as the daytime fish see clearly under full sun, other fish see much better in the dim light of dusk. Their eyes are large, with huge black pupils designed to gather and utilize low levels of light. Instead of cone cells used for seeing color, these eyes are backed with large rod cells, yielding sight primarily in black and white. What they lose in visual acuity, they gain in sensitivity. These are the eyes of the twilight hunters.

During the day these fish shun the light of the bright sun, hiding in the dark holes, and crevices of the reef. But as dusk approaches, they begin to emerge, and they are ready to eat. To hunt successfully, these fish have little use for color vision. Rather, they must discern form (often by outline), movement, size, and location of potential prey. And for this their eyes are perfectly adapted.

Predation is only one of the reefs behavioral changes that occur at dusk. With fading light, and the exit of voracious daytime feeders, a host of invertebrates begin to emerge from the secluded safety of the reef. Sea urchins, crinoids, and other echinoderms, rise from the protective nooks and crannies of the coral and begin their nocturnal habits. Crabs, shrimp, lobster, and other crustaceans, cloistered in coral crevices by day, also come into the open, where they are easily visible to twilight snorkelers. And without the beaks of marauding parrotfish, the coral animals, retracted by day, extend their tentacles to capture small planktonic prey.

While I first discovered the thrill of twilight snorkeling simply by staying in the water "too long," conscientious dusk snorkelers will enjoy the experience much more by employing some simple preparations. The predominant factor, obviously, is that snorkelers enter the water late in the day with ample light to see, but exit the water in what can be nearly complete darkness.

To avoid unwanted complications, I always bring along a submersible light. As the sun goes down, and a shadowy gloom replaces brilliant color, our own vision becomes seriously compromised, just like the daytime fish. Although unnecessary at the start, in the ensuing darkness, using a waterproof light reveals many of the newly "arrived" nocturnal animals. Just as important, using a light will help us locate shallow rocks, corals, and other potential hazards.

In planning a dusk snorkel I also attach (usually with electrical tape) a small flashlight or chemical light stick to my snorkel. This makes it very easy to locate your snorkel buddy if necessary.

We all recognize that skin abrasions, especially coral cuts are best avoided, both for our own comfort as well as the health of the coral. In the dark, this is always more challenging. As a precaution, I usually "dress up," at dusk, being sure to cover my body with some type of protective layer. Lycra suits work perfectly, but just about any type of clothing will suffice.

In addition to unwanted contact with coral or other objects, in the dark, it’s also easier to become disoriented. Some shore entries are done through narrow channels in the reef, or from small beaches that while obvious in the light, can become remarkably obscure in the dark. It’s important to pay special attention to your location when you get into the water so that you may safely find your way back out later on. Some dusk snorkelers leave a small flashlight, flashing strobe, or other illumination on the beach to mark their point of entry. Several months ago in Fiji, we simply tied a Coleman lantern to a palm tree and this proved highly effective.

Whenever possible, I prefer to snorkel the area first in the day. This give me a good familiarity with the physical layout under well-lit conditions. It also lets me better appreciate the daytime ecology, making the changes that occur at dusk all the more dramatic.

One final consideration: in the day, with the warmth of the sun, I rarely worry about bringing a towel. In the dark, however, after a long immersion in the water, a warm, dry towel, is a welcome comfort.

With these few tips, I’m sure you will safely and comfortably enjoy the excitement of snorkeling at dusk. And, at least in terms of reef ecology, you will have the chance to see for yourselves how the "other half" lives.