Evolution: Survival: Coral Reef Connections

Predators and Prey

Eat or Be Eaten: Predators and Prey, Parasites and Hosts

Actually, it's more like eat and be eaten for most organisms. While plants and some bacteria can make their own food, other organisms must eat living things to survive. This makes them predators. You might not think of a grass-munching cow as much of a predator, but cows are indeed the predators of their grass prey.

Of course, the cows themselves are prey to other animals, like humans and coyotes. Now we have a simple food chain, with grass plants making food at the bottom, cows as middle predators, and then humans and coyotes as "top predators."

While predators usually kill their prey to eat them, parasites live on or in their prey (called a host), nibbling or sucking tiny bits so the host survives, nourishing them for many meals to come. Almost all organisms play host to parasites throughout their entire lifetimes.

To succeed at the evolutionary game, organisms must eat but not be eaten. As a result, in predator-prey (and parasite-host) relationships, something called coevolution can often occur: when one of them develops a new offense or defense, the other must develop a counter-weapon to survive.

Read about different predator-prey relationships on the reef.

Enormous schools of prey fish (many species, including cardinalfish (Cheilodipterus parazonatus)) glide across the reef in search of tiny zooplankton. The prey fish, in turn, are eaten by many species of larger fish, like bigeye trevallies. Schooling offers some protection from predators, since each fish can be on the lookout, but it requires precise choreography to work well. Schooling fish have evolved special vibration sensors along their lateral sides that allow them to literally feel each other's movements and stay in synch.

Covered with long, venomous spikes, the crown-of-thorns starfish (Acanthaster planci) is a voracious feeder that can eat living corals because of a unique adaptation: a wax-digesting enzyme system. Populations of the starfish were once kept low by a few key predators, namely the giant triton. Since humans decimated giant triton populations, crown-of-thorns starfish outbreaks periodically kill vast expanses of hard coral.

Hard corals have evolved to have large amounts of a wax (cetyl palmitate) in their tissues. Very few predators can digest the wax, which has allowed corals to flourish and produce massive reefs. Recently, epidemic populations of one predator -- the crown-of-thorns starfish -- have done extensive damage to many reef regions. Armies of grazing starfish leave a wake of destruction in their path, killing up to 95 percent of the hard corals in an area.

One common defense against predators is a protective covering, such as a shell. Another is to flee the predator. During its evolution, the green sea turtle (Chelonia mydas) sacrificed speed in favor of a thick, heavy shell (carapace). The carapace acts as armor, protecting the turtle's body from the sharp teeth of predators. But some, like the tiger shark, are powerful enough to bite right through the carapace and kill the turtle.

Sea sponges, such as those of the Microcionidae family, have escaped predation by all but a few species because they produce foul-tasting and sometimes toxic compounds. These compounds evolved as chemical weapons for use against other sponges, as well as against fouling organisms (creatures that grow on top of other creatures, thus decreasing their fitness) -- their defensive function was just a lucky side effect. But some predators, such as sea slugs, have evolved resistance to the toxins and even use those toxins against their own predators.

Named for their bright colors and beak-like mouths, parrotfish (Scaridae family.) are large herbivores that graze on the algae growing atop hard corals. Using their beaks and two pairs of crushing jaws, parrotfish are marvelously adapted for crunching and pulverizing chunks of algae-coated coral. They digest the algae and excrete the coral as fine sand. Unfortunately, they are poorly equipped to defend themselves against predators, such as barracuda, but some find protection by schooling with better-armored fish.

Algae occur in a kaleidoscope of forms and colors on the reef, but they have one main function: turning solar energy into food. Thus they are called "primary producers." Without them, the reef would quickly become a lifeless moonscape. One important algal group, benthic (bottom-dwelling) algae, rapidly grows over dead coral and other inert objects, providing a grazing yard for herbivores, such as parrotfish.

Packed with miniature toxin-loaded harpoons (nematocysts), the tentacles of sea anemones provide an excellent deterrent against almost all would-be predators. Saddled butterflyfish, though, have evolved resistance to the toxins and apparently relish the tentacles. Still, to grab a meal, the butterflyfish must get past the anemones' second line of defense: zealous clownfish guards.

Safely tucked in coral crevices or half-buried in sand and rubble, gobies (Gobiidae family) maintain a low profile on the reef to avoid predation. In addition, they have evolved independently swiveling eyes that constantly search the water for potential attackers. But their efforts can be foiled by ambush predators, like the smallscale scorpionfish, whose camouflage prevents gobies and other prey from seeing them until it's too late.

In the world of predators and prey, the normal rule is that big creatures eat smaller creatures. But sometimes the tables are turned, as in the case of the bluestriped fangblenny (Plagiotremus rhinorhyncos), a small but sinister predatory fish. Evolved to perfectly mimic the bluestreak cleaner wrasse, the fangblenny falsely advertises cleaning services to larger fish, such as the reef lizardfish. Once the bigger fish moves in close, the fangblenny attacks and darts away with a mouthful of sushi.

Gobies are the most diverse fish family on the reef, with more than 200 species described. With their generally small sizes and ability to adapt to a wide variety of specialized habitats, gobies have become the most diverse marine fish family in the world. This does not mean they are always successful at avoiding predators, though. For instance, the blueband goby (Valenciennia strigata) is eaten by a variety of predators, including the venomous coneshell.

The common silver gull (Larus novaehollandiae), like most gulls, will eat just about anything it can get its heavy, hooked bill into. Often scavenging recently dead animals and even trash, silver gulls help keep shore areas clear of debris. But once the debris is gone, they turn to other easy pickings: eggs and chicks of other seabirds, such as the black noddy.

Silvery schools of many species of juvenile fish (called fish fry) find some refuge from the intense predation of outer reef zones by living near the shoreline. But they can't let down their guard completely. While feeding on benthic algae and floating microscopic communities of plants and animals, they are easily visible from above the water's surface and often fall prey to hunting reef herons.

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