Reef activity changes to predation as illumination fades. The nighttime creatures come out of their hiding places and look for food. Some of these animals are the brittlestars, sea urchins, coral shrimps, crabs, and nudibranchs.
Choreographed and orchestrated with the precision of an elaborate Broadway musical, myriad players on coral reefs around the world flawlessly perform their daily roles in the ocean’s carefully scripted play. There are two main acts in this performance and twilight is the time of transition.
The colorful tropicals that flit about the reef during the day are the first to disappear from sight; fidgeting nervously in the shadowy crevices they anxiously await the fall of night. Opportunistic feeders such as sharks, eels, seabass and jacks go on the prowl, capitalizing on the temporary disorientation of their prey. Meanwhile night dwellers, such as Bigeyes, Cardinalfish and Soldierfish, begin to stir in their shadowy beds.
When the sun finally melts into the arms of the horizon, taking with it the bright light of day, an eerie silence pervades – announcing the second act – the moment of change. Then the curtain slowly opens and a new cast of characters crawls, slithers and slinks onto the nighttime stage.
It is believed the nighttime activity of many animals has evolved to aid their survival. With voracious daytime feeders such as triggerfish and wrasse asleep for the night, the reef is a much safer place for many invertebrates.
Urchins are usually the first to leave their sheltered daytime retreats. Attuned to some primal cue, when daylight disappears they wiggle out of their protective custom-carved crevices and appear magically, en mass, on the reef. Many urchins return to the same hole at sunrise each day. Urchins appear stationary but if you watch closely you’ll see them methodically inching their way across the reef, grazing on algae and other edible matter. Urchins have a powerful five part jaw on the undersides of their armored bodies and rows of tiny tube feet that propel them across the reef.
Brittlestars fill the nighttime stage underwater. There are more than 1,800 species found around the world. Brittlestars shun light and typically slither into the deepest crevice they can find during the day; you can often see the spiny tips of their arms sticking out of cracks, crevices and especially tube sponges, one of their favorite hideouts. Guided by tiny light receptors in their spines and tube feet (podia), they emerge at night to feed on bacteria, algae and larval fish. Although brittlestars have tube feet similar to those of urchins, they maneuver their sinewy arms to move along the reef. Like other echinoderms, most brittlestars have five arms, some have six or seven. Brittlestars are so well adapted for survival that, if threatened, they can easily shed their arms and make a quick getaway. Their disc-like body contains all digestive and reproductive organs and new arms are quickly regenerated.
Shortly after the twilight stage is set, crabs and other nighttime scavengers take their place on the reef. Coral crabs are usually abundant. As the light fades you’ll see them darting from place to place using quick, distracting movements. Searching for something to eat, they are always ready to scamper away. Crabs can also generate new appendages so they are quick to give up a claw if necessary. As night settles over the reef you’ll often see crabs sitting in the open, shoveling food into their mouths, using their claws as a spoon.
Many of the shells you see roving the reef at twilight are inhabited by Hermit Crabs. Hermit crabs carry their homes – empty shells – with them wherever they go and are most active after twilight.
As twilight deepens and your eyes adjust to the light, the reef will start to flicker with tiny iridescent eyes as shrimp crawl out of their caves. Just like their terrestrial relatives – insects – shrimp and other crustaceans are plentiful underwater, often numbering in the thousands. The Banded Coral Shrimp (Stenopus hispidus), one of the most striking, is found in both the Caribbean and the Pacific. Its glitzy barber pole-like design makes it fairly easy to spot when snorkeling, even during the day when the shrimp are tucked snugly into the reef’s dark recesses. Banded Coral Shrimp are infinitely more flamboyant after twilight; often seen in the open performing a ritualistic cleaning display. The Banded Coral Shrimp gets free meals in relative safety by servicing fish and removing parasites.
If you stay long enough after the light of day fades you may catch a glimpse of the reef’s graceful inhabitants – the nudibranchs. Some nudibranchs are active during the day, typically found grazing on sponges, soft corals and other sedentary organisms. Others, like the showy Spanish Dancer, prefer the shelter and relative safety of the night. The Spanish Dancer (Hexabranchus sanguinues) is usually found cruising along the reef but can perform a stunning U/W ballet, fluttering effortlessly through the water column. Nudibranchs are snails without shells, many have exposed gills and others breathe through their skin. They appear defenseless but all rely on some subtle method of defense; some use camouflage, others are toxic.
Like intermission between the acts of a play, dusk is a time of change – fleeting and fascinating – the link between night and day. It is without a doubt a great time to snorkel.