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Coral Reef Connections
  Reef Relationships | Predators and Prey | Competitors | Partners | Conclusion

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.

Trevally (fish) -> Prey fish
Triton (snail) -> Starfish -> Hard coral
Tiger shark -> Sea turtle
Sea slug -> Sea sponge
Barracuda -> Parrotfish -> Algae
Butterflyfish -> Sea anemone
Scorpionfish -> Goby (fish)
Fangblenny (fish) -> Lizardfish
Coneshell (snail)-> Goby (fish)
Seagull -> Noddy (bird)
Heron (bird) -> Fish fry


Bigeye trevally (fish) -> Cardinalfish (prey fish)
Like lions on the prowl, bigeye trevallies (Caranx sexfasciatus) form hunting schools to patrol the reef for dense schools of prey fish, like cardinalfish. Voracious predators, bigeye trevallies have evolved lightning swiftness and exceptional cooperation that allows them to surround prey schools, herding them in close to the reef. The prey panic, and their tightly organized schools turn to chaos, giving the trevallies a chance to attack.

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.


Giant triton (snail) -> Crown-of-thorns starfish -> Hard coral
One of the few predators of the crown-of-thorns starfish, the giant triton (Charonia tritonis) has evolved a tolerance to the starfish's toxins. Unfortunately, tritons can no longer keep starfish populations in check since they've been overharvested for their beautiful shells. With too few tritons on the reef, crown-of-thorns starfish populations can explode, jeopardizing the living coral that makes up reefs.

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.


Tiger shark -> Green sea turtle
Lurking around the edges of reefs during the day, tiger sharks (Galeocerdo cuvier) have evolved as highly aggressive "top" predators that grow at least 25 feet long. They detect prey using an array of finely tuned senses, including electrical current detection. Rows of razor-sharp teeth and powerful jaws allow them to crack though even the thick carapace (shell) of full-grown sea turtles.

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 slug -> Sea sponge
Many sea sponges, like anemones, use toxins to repel would-be predators. Some species of sea slugs, however, such as Platydoris scabra, have evolved immunity against the toxins of specific sponge families (in this case, Microcionidae). This adaptation benefits the slugs in two ways. First, they don't have to compete with many other organisms for the sponges. The sea slugs can also concentrate the sponge toxins to foil their own predators -- at least until the slugs' predators also evolve immunity to the toxins.

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.


Barracuda -> Parrotfish -> Benthic algae
Barracuda (Sphyraena spp.) are fierce predatory fish that patrol outer reef areas in large schools. Like many predators, they have evolved as extremely fast swimmers, with streamlined, torpedo-like bodies. And they are efficient killers, using conical, razor-sharp teeth to quickly rip prey to shreds. In addition, they are resistant to the toxin found in the bodies of many of their prey, such as parrotfish.

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.


Saddled butterflyfish -> Sea anemone
Saddled butterflyfish (Chaetodon ephippium) seem to flutter, rather than swim, above the algae-covered coral they commonly graze on. Their gentle disposition disappears, however, in the presence of another favorite food: sea anemones. When feeding, the butterflyfish turn into vicious predators, darting in to rip off the anemones' fleshy tentacles. Having evolved resistance to the anemones' toxins, they need only get past clownfish guards to pick off a delicious meal.

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.


Smallscale scorpionfish -> Goby (fish)
Just as some prey species have evolved cryptic coloration and patterns that help them avoid predation, some predators have evolved camouflage that lets them hide themselves and ambush their prey. One such predator, the smallscale scorpionfish (Scorpaenopsis oxycephala), closely resembles the reef's rocky, algae- and coral-encrusted bottom, where it lies in wait for crustaceans and small fish, such as gobies.

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.


Bluestriped fangblenny (fish) -> Reef lizardfish
Many large fish, including the reef lizardfish (Synodus variegatus), regularly visit the bluestreak cleaner wrasse to have skin, mouth, and gill parasites removed. The two fish benefit by the association; a third fish, however, has evolved to take advantage of them both. Using a devious disguise and copycat behaviors to attract larger fish, the fanged ambush predator, called a fangblenny, rips living tissue from surprised prey.

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.


Coneshell (snail) -> Blueband goby (fish)
Coneshells (Conus spp.) are gastropod mollusks, closely related to the more familiar and harmless land snails. With beautiful, ornately designed shells, coneshells are highly sought-after by shell collectors. These gastropods have evolved as deadly predators, however; a single puncture from their venomous radula (modified tooth) can rapidly paralyze and even kill a human. Of course, coneshells evolved not to defend themselves against collectors, but to efficiently kill prey, such as the blueband goby.

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.


Silver gull -> Black noddy
Coming ashore only to breed (as do most true seabirds), black noddies (Anous minutus) form immense colonies of up to 100,000 nesting pairs on larger reef islands. Noisily chattering as they work, mating pairs build nests of guano-cemented leaves and grasses in fig trees. But even in the shelter of the trees, chicks and eggs are often stolen and eaten by marauding silver gulls.

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.


Reef heron -> Fish fry
Reef herons (Egretta sacra), unlike true seabirds, live onshore year-round. While seabirds hunt far out beyond the reef, reef herons fish along the cay and reef flat. They have evolved to hunt during low tide, allowing them to wade the shallows. With excellent eyesight and marksman-like aim, they expertly spear fish fry, adult fish, and crustacean prey from beneath the water's surface.

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.

  -> Knock down, drag 'em out
  Reef Relationships | Predators and Prey | Competitors | Partners | Conclusion
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