Biscayne National Park - Prowling the Reef

From the mariner's point of view, reefs have always been dangerous places, capable of taking many a life. But from the naturalist's, their role is in the fact giving life: for coral reefs are the houses, the forests, the hiding places of the sea. The orange and maroon tentacles of feather duster worms reach into the water to trap small living things; a passing shadow will cause them to bolt back into their leathery tubes. Hoary old loggerhead sea turtles sleep with the fronts of their barnacle-encrusted shells wedged tightly under coral rock. Hungry green moray eels, some 10 feet long, wait for unsuspecting octopuses to pass by their lairs. Arrow crabs balance on top of the coral, delicately bracing their tiny triangular bodies on pin-striped, spidery legs. Damselfish and surgeonfish, among others, leave their hideaways every day (they feed on nearby algae and sea grass), but they never travel very far, and they soon return to the shelter of the coral. Their effect lingers far longer than they do: the intense foraging destroys the grass close to the reefs and creates a bare zone of sand and shell fragments, a "fish halo", that surrounds the reef like a moat.

Shelter is not all a coral reef offers. During the day, schools of triggerfish, butterflyfish, angelfish, and other rainbow-hued swimmers dart about the coral, constantly on the prowl for food. Just about anything they can pluck out of a crevice is fair game, anything, that is, except inch-long cleaner shrimps. Standing brazenly out in the open, these peppermint-striped creatures sway back and forth as a signal that their cleaning business is open and that they are not frightened prey. Fish infected with parasites eagerly line up to await the shellfish's services. When the shrimp is ready, it crawls about on the fish's scales and onto its gills and mouth, picking off the parasites with sharp little nippers. As soon as one fish is cleaned, it leaves, and the next swims into place.

When the last rays of sunlight filter into the water and darkness comes, nocturnal brittle stars, a type of starfish with five long, hairy arms winding out from a tiny round body, snake their way over the bottom. One brilliantly luminescent variety transmits waves of blue-green light when disturbed. Spiny lobsters, of Florida crayfish, test the water for smells and vibrations, waving long thorny antennae as they tiptoe over sand and grass, eagerly picking away at worms, sand dollars, and snails. (Though they lack the big, protective, crushing claws of their New England relatives, they make up for it in agility.) At daybreak, as many as 17 spiny lobsters crowd together under a rock ledge, their bristling antennae facing outward in defense.

Not even the deepest coral caves can offer total protection from an octopus. Changing into almost any color of the rainbow faster than a chameleon, this master of camouflage can blend into any background, even matching the texture of slippery brown seaweed. Blushing a dark red, then fading to gray or white as it moves agilely across the bottom, the eight-armed mollusk scans one coral cave after another, looking for prey. Using powerful suction discs on its arms, it drags a spiny lobster out of hiding. With one bite, it injects an immobilizing poison into the lobster; moments later, it spews out the remains.

Shoreward from this stony realm are gentler do mains, where sea whips, sea fans, and other soft corals replace their limestone cousins. Growing in colonies several feet high, they too are composed of tiny polyps, but their skeletons are far more flexible, gently swaying with the currents.

At night huge hammerhead sharks, some 10 or 12 feet long, cruise these shallows. On the alert for prey, the hammerhead uses sensory cells on the underside of its mallet-shaped head to detect electrical impulses from stingrays buried in the sand. Stingrays are famous, some would say infamous, for the venomous barbs on their lashing tails, but their weapons are useless against the sharks.

As the reef holds back the ocean's power, so another protective barrier, a chain of islands, creates shelter where underwater meadows can flourish. Horse-shaped seahorses, pipe-shaped pipefish, and star-shaped starfish all thrive in island-lined Biscayne Bay. Sea hares, mottled with purple and bearing rabbit-ear tentacles on their heads, nestle among the grass blades by day and gracefully undulate to the surface at night. If bothered, these snaillike creatures let out puffs of vivid purple ink, something that most other animals find it worth their while to avoid.

Looking for all the world like festive pink, white, and green pincushions, thousands of sea urchins graze on the algae that encrust the grasses. (Aside from the occasional manatee or green sea turtle, few creatures eat the leaves themselves.) Many of the sea urchins hold flat pieces of shell against their round bodies like shields, protecting themselves from ultraviolet light that penetrates the shallow water.