Knock down, drag 'em out
Humans have found many ways around the vicious game of biological competition. You didn't crawl out of the hospital nursery past hordes of hungry predators, and you probably didn't fight anyone for your breakfast this morning. But other species compete all their lives for limited resources of food, space, and mates.
When resources are plentiful, competition weakens. But when resources are scarce, competition can become an all-out war for survival and reproduction. While competition seldom gets truly violent, the outcome often determines which of the competitors will get its genes into the next generation.
The winners will pass on their genes -- and, thus, their winning traits -- to their offspring, while the losers' genes will disappear. Thus, each successive generation faces stiffer competition than the last, as individual competitors become better and better adapted ("fit") to their environments.
Read about different competitive relationships on the reef.
Bottlenose dolphin <-> Bigeye trevally (fish)
The bottlenose dolphin (Tursiops truncatus) and other toothed whales are solitary predators who remotely detect prey by echolocation. Emitting a series of clicks and whistles, the dolphin then waits to analyze returning echoes. Echolocation is so precise that a dolphin can determine the size, direction, and distance of a school of prey fish, and even sizes of individual fish in the school. To feed, dolphins use a fast, surprise attack, and, unlike the hunting packs of trevallies, dolphins can successfully hunt alone.
Bigeye trevallies (Caranx sexfasciatus) prey on schools of small fish using a very different approach than do bottlenose dolphins. These trevallies have evolved to cooperate with each other, much like wolves in a pack, to surround, corner, confuse, and finally kill their prey. A single trevally would have a very difficult time attacking a school of prey fish, with its hundreds of pairs of watchful eyes and ability to move as a unit to avoid predators, so each trevally benefits from participating in the hunting party.
Spanish dancer (slug) <-> Sea sponge
Male Spanish dancers (Hexabranchus sanguineus) essentially enter dance competitions to win their mates. The judge is the desired female, who decides which writhing, scarlet red male wins the right to father her offspring. This process is an example of "sexual selection," in which a male "wows" a female into mating with him. Often, as in the case of Spanish dancers, the male risks his life to put on a winning show, since his mating behavior makes him more conspicuous to predators. Fortunately, Spanish dancers possess a potent toxin, which deters predators.
Sea sponges and other sessile (anchored) organisms compete fiercely with each other for space using physical and chemical warfare. Over millions of years of turf wars, sponges that evolved anti-sponge toxins, like the Microcionidae, were often victorious over non-toxic varieties. Thus, most sponges living today produce potent toxins, which provide a secondary benefit of discouraging all but the most highly adapted predators, such as the sea slugs.
Parrotfish <-> Long-nosed butterflyfish
Competition for a common resource can exert pressure on a species to alter not only their physical form, but also their behavior. For instance, neither parrotfish (Scarus spp.) nor their butterflyfish competitors have evolved effective defensive structures. Instead, they avoid predation by different adaptive behaviors. Parrotfish graze among schools of venomous, spined rabbitfish, which are seldom attacked by predators.
Long-nosed butterflyfish (Forcipiger flavissimus), like their parrotfish rivals, are not physically well protected against predators. Instead, both species have evolved behaviors that help them avoid being eaten while they graze for benthic algae. Butterflyfish typically swim in pairs near a particular clump of coral. If threatened, they expertly wedge themselves between coral branches and erect fin spines so they are almost impossible to dislodge.
Saddled butterflyfish <-> Clown anemonefish
Saddled butterflyfish (Chaetodon ephippium) mainly subsist on a diet of benthic algae; but they have also developed a taste for meat, and will rip tentacles from sea anemones if given the chance. Having evolved resistance to the anemones' toxins, the butterflyfish must still get past the smaller, but fiercely territorial, clownfish to steal a meal. An anemone robbed of its clownfish guards will quickly fall prey to the voracious butterflyfish.
Like most of their damselfish kin, clown anemonefish (more commonly known as "clownfish" -- Amphiprion percula) are fiercely territorial. While many damsels cultivate and guard tiny algae gardens, clownfish take up residence among the tentacles of one of two species of sea anemones. They aggressively defend their protective host even against large and tenacious invaders, such as saddled butterflyfish.
Bluestriped fangblenny (fish) <-> Bluestreak cleaner wrasse
The bluestriped fangblenny (Plagiotremus rhinorhyncos) has evolved to look and act just like a bluestreak cleaner wrasse. Why? So it can take advantage of the bluestreak's symbiotic relationship with larger fish. The bluestreak gently nibbles away parasites from its customers, but the mimic blenny unveils fang-like teeth that quickly strip healthy flesh from unsuspecting victims.
The bluestreak cleaner wrasse (Labroides dimidiatus) advertises its station by darting back and forth. Many fish regularly visit the station to have parasites removed by the bluestreak. Unfortunately, the bluestriped fangblenny has evolved to look and act just like a bluestreak. Not only do fangblennies compete with bluestreaks for customers, they also attack them and may make fish less trusting of true bluestreaks.
Cuttlefish <-> Cuttlefish
Cuttlefish (Sepia spp.) are masters of disguise, changing appearance by opening and closing pores in their skin (called chromatophores) to reveal or hide underlying layers of different colors. Males competing for females put on their flashiest zebra-striped patterns, but the show isn't meant for the females at all. Instead, larger, dominant males seem to win breeding rights to females by intimidating their opponents with both size and more intense zebra patterns.
Ghost crab <-> Hermit crab
As animal carcasses wash up on the beach and bake in the sun, they become storehouses of bacteria that easily spread diseases to living animals. Fortunately, stalk-eyed ghost crabs (Ocypode spp.), which live in burrows beneath the sand, relish dead animals and have evolved excellent chemical senses that enable them to quickly locate a newly arrived food source. They have much competition, though, including hermit crabs, which lay claim to carcasses still in the water.
Hermit crabs (Dardanus megistos) are aquatic, near-shore scavengers that can grow to almost a foot across. Lugging a "borrowed" shell on their backs, they crawl across the sandy bottom searching for dead animals. Since their heavy shells prevent them from moving efficiently on land, stalk-eyed ghost crabs usually win the feast there. But underwater, carcasses swarm with hermit crabs, which quickly tear animals apart using their dexterous mouthparts.