Free rides and win-win situations: Commensals and Mutualists
Between predator-prey and competitive relationships, it may seem that organisms are alone in the world, fighting to survive and reproduce. But many organisms have evolved cooperative strategies for survival and reproduction. In these species "partnerships," at least one partner benefits, and neither is harmed.
If only one partner benefits, and the other is not much affected, the relationship is called commensalism. Consider barnacles that attach themselves to whales. The whales aren't harmed, but the filter-feeding hitchhikers get ferried around the ocean and may find more food than if they were stuck in one place.
If both partners feel the effects of the other, their relationship is called mutualism. In this case, each species tends to evolve adaptations to the other (that is, they coevolve) in order to maximize benefits and minimize losses caused by their close association.
Read about commensalistic and mutualistic relationships on the reef.
Manta ray <-> Remora
Among the largest living fishes, manta rays can reach 20 feet in width and weigh more than two tons. Like most marine behemoths, they are filter feeders. Using an unusual pair of "head flaps," they funnel tiny prey, such as small fish, crustaceans, and comb jellies, into their gaping mouths. Whatever morsels escape might be wasted, but are frequently caught and eaten by hitchhiking remoras.
Fast-swimming predators, like the manta ray, are messy eaters who leave behind a trail of food scraps. Remoras, or "suckerfish," have evolved a highly specialized body that allows them to exploit that resource. Fast swimmers, they easily catch up with a host and attach from below, using a powerful suction disc -- which evolved from an ordinary dorsal fin -- on top of their heads. The remoras' streamlined shape allows them to hitchhike without slowing down their hosts.
Hermatypic coral <-> Zooxanthellae
It takes a lot of energy to secrete the calcium carbonate exoskeletons (hard outer structures) that make up coral reefs. Reef waters are typically very low in nutrients, so most coral animals can't filter out enough food to provide the extra energy they need. To make up for this deficiency, hermatypic corals shelter microscopic algae (zooxanthellae) within their tissues; in exchange, the algae supply the corals with carbohydrates so the corals have enough energy to build reefs.
Zooxanthellae (pronounced "zoe-zan-thelly") are microscopic algae that live within the tissues of host animals, including hermatypic coral animals. Like all plants, zooxanthellae make their own food by a process called photosynthesis. Using solar energy absorbed by special pigments, they transform carbon dioxide into carbohydrates and oxygen. What they don't need themselves passes directly into the coral's gut cavity, providing the extra energy the coral needs to produce a calcium carbonate exoskeleton.
Calcareous algae <-> Branching coral
Some multicellular algae on the reef produce calcium carbonate (limestone) skeletons very similar to those made by hard corals. These calcareous algae play a major role in barrier reef construction, acting as a sort of living mortar that holds together individual coral colonies. Growing between corals and wrapping around the bases of branching corals, calcareous algae protect the corals from erosion, especially in high-energy areas.
Individual coral colonies, especially branching corals, can easily be toppled in high-energy reef zones, such as the reef front and rock rim. Waves can easily scour away sediments from a colony's base, uprooting it and pushing it along like tumbleweed. So how do branching corals ever get a solid foothold in such zones? Calcareous algae grow between corals and around their bases, preventing erosion and stabilizing the reef structure.
Parrotfish <-> Goldlined rabbitfish
On the reef, carnivores have diversified into many more species than have herbivores. Competition among carnivores has produced a treacherous environment for prey, in which hungry jaws lurk around every corner, during all hours of the day. To escape predation, some relatively defenseless herbivores, such as parrotfish (Scarus spp.), have evolved to graze with schools of their better-protected rabbitfish relatives.
Goldlined rabbitfish (Siganus lineatus), locally called spine-feet fish, are so named for the defensive venomous spines at the ends of each of their pelvic fins. But spines are a last-ditch defense. To avoid being thrust into a risky spine-to-fang battle, rabbitfish employ their expert color-changing talents to avoid predator detection in the first place. Schools of rabbitfish thus provide an excellent refuge for their poorly defended relatives, the parrotfish.
Clown anemonefish <-> Sea anemone
Nestling among the venomous stinging tentacles of a sea anemone seems like a very bad survival strategy -- unless you and the anemone have some kind of an arrangement. Clown anemonefish (Amphiprion akindynos) and sea anemones have evolved just such a relationship. As juveniles, clownfish perform a ritual of "anemone rubbing." Initially protected from stings by a thick mucus coat, the clownfish incorporates anemone mucus into its own coat until the anemone no longer stings it, apparently recognizing the fish as part of itself. From then on, they defend each other, and clownfish have even been seen dragging food to their host anemone.
Reef animals are masters of disguise, and sea anemones are no exception. Attached to the reef by a suction disc, tentacles swaying with the current, they are the animals perhaps most often mistaken as plants. The illusion is further reinforced by the presence of two or more commensal clownfish among the tentacles. But the clownfish and anemone are a predatory team, working side by side and sharing food. In addition, the clownfish fight off intruders, such as anemone-eating butterflyfish, and the stinging cells (nematocysts) of the anemone deter potential clownfish predators.
Reef lizardfish <-> Bluestreak cleaner wrasse
Many reef animals that can't groom themselves, like the reef lizardfish (Synodus variegatus), have evolved to secrete a mucus coating. The mucus offers some protection against parasites and also reduces drag as they swim. Unfortunately, mucus itself is an attractive food to some parasites and bacteria. What's a lizardfish to do? It visits a small cleaner fish, like the bluestreak wrasse, that gently eats away surface parasites from skin, mouth, and gills.
The resident fish doctor and dentist on the reef is the bluestreak cleaner wrasse (Labroides dimidiatus). With an easily identifiable bright blue stripe and stereotypical behavior, the bluestreak attracts larger fish, like the reef lizardfish, to its cleaning station. As it makes a meal out of the larger fish's parasites, the bluestreak gently tickles its customers, a behavior that seems to bring them back again and again.
Sponge crab <-> Sea sponge
Sponge crabs (Dromiidae family) avoid predators by carrying a disguise with them at all times. Their posterior legs are modified for grasping, and the crabs use them to carry live Halichondria sponges on their backs. Since the sponges are toxic to most potential predators, the undercover crab doesn't have to worry about being attacked and can concentrate on more important things, like finding food.
Many sea sponges have evolved chemical weaponry for use against other sessile organisms in the never-ending battle for space on the reef. Since the compounds tend to be distasteful and often toxic to predators, the sponges avoid most predation. Sponge crabs exploit this defense by carrying live sea sponges on their backs. And the sponges may benefit, too: By living atop a crab, they no longer have to battle for space.
Giant clam <-> Zooxanthellae
Myths about divers being caught and eaten by giant clams (Tridacna gigas) still abound. The clams, though immense (up to three feet across and weighing more than 200 pounds), are not man-eaters. In fact, they are filter feeders that strain tiny food particles from the water. They get whatever additional nutrition they need from symbiotic algae, such as zooxanthellae, similar to those found in reef-building corals.
Zooxanthellae are microscopic algae that live within the tissues of a variety of host animals, including giant clams. Like all plants, zooxanthellae use energy from sunlight to make their own food by a process called photosynthesis. Excess food leaks out of the algae and into the filter-feeding clam, which relies on the algae's extra energy to survive. Because zooxanthellae make food most efficiently in fairly shallow, well-lit waters, giant clams are most abundant there too.
Upside-down jellyfish <-> Zooxanthellae
Jellyfish are soft-bodied, free-swimming animals closely related to the corals. Most jellyfish are predators, using the tentacles that drape from their floating bell to ensnare and paralyze prey. A few species of so-called upside-down jellyfish (Casseiopea medusae), however, have literally "flipped their lids." They float upside-down, and their tentacles are blanketed with symbiotic zooxanthellae that use solar energy to make food for the jellyfish.
Zooxanthellae are microscopic algae that live within the tissues of host animals, including hermatypic corals, giant clams, and upside-down jellyfish. The jellies may be the best hosts of all because they can swim to a depth where the zooxanthellae have optimal sunlight levels. The tiny plants cover the tentacles of the jellies, making food by photosynthesis and releasing whatever they don't need right into the jellyfish's tissues.
Tern <-> Ghost crab
Every living organism eventually dies, whether killed by a predator, a disease, or just "old age." Leftover pieces of prey and whole carcasses comprise a valuable source of food. Not surprisingly, a large group of organisms, called detritovores, have evolved in a way that lets them take advantage of this resource in every environment. For example, stalk-eyed ghost crabs eat carcasses, such as dead terns, that wash up on cay beaches. In turn, living terns (Sterna spp.) benefit by being spared carrion-associated diseases.
Ghost crabs (Ocypode spp.) perform a great cleanup service as they get a meal. Quickly pinpointing the location of newly arrived carcasses, masses of crabs share in the feast while the carcass is still fresh. Living terns may even benefit from the crabs' work by being spared exposure to disease-causing bacteria that would otherwise build up on their rotting kin.
Giant triton <-> Hermit crab
When a giant triton (Charonia tritonis) dies, its tissues will likely be consumed by a group of detritovores, organisms evolved to eat dead and decaying organic matter. But the marvelous, and often enormous (up to 20 inches across), calcareous shell made by the living triton cannot be eaten. Instead, it will quickly be claimed by a hermit crab, which cannot make a shell of its own for protection.
Unlike other crabs, hermit crabs (such as Dardanus megistos) are unable to make a thick, protective shell (carapace) for its hind-end. While the head and legs are well protected from predators, the vulnerable back-end must be tucked inside a shell scavenged from the reef floor after its original owner dies. Each time the hermit crab outgrows its shell, it must find a larger one and then move in quickly to avoid being eaten.