In grade school biology, you may have heard of the term "symbiosis," referring to a give-and-take relationship between two organisms. But, in fact, symbiosis refers to any number of different kinds of partnerships – some are mutually beneficial and some are not. Partnerships in which the host is harmed are termed "parasitic" and include diseases as well as mere symbiont exploitation until the host dies. Nevertheless, in the tropical marine world, almost every creature lives in symbiosis with another in some way. It's simply an easier way to live.

A symbiotic relationship can take on many forms, which are classified depending on how the creatures share their living space or use each other. Understanding what sorts of relationships may exist in the natural world between species will help you to better understand your own pets as you watch them maneuver in quarters you've created for them.

The specialized defenses of certain species are often used by those that have little or none of their own, and is called inquilinism. A defenseless little reef fish would want to take advantage of the natural protection provided by a sea-urchin neighbor's razor-sharp, and sometimes venomous, spines. Clownfish dance among the battery of the anemone's stinging cells; other fish have adapted to a drifting life in the jellyfish's armory of stinging cells or swim close to the nearly invincible barracuda or shark.

Some creatures use others as camouflage. Majid crabs snip pieces off of sponges and other nearby organisms and embed them into their shells, sometimes even carving the sponge into a cap that neatly fits on their carapace. Other crabs plant sea anemones onto their shells devising a built-in self-maintaining shield of stinging cells – or hold one in each claw, and like a boxer, attempt to punch the offender with its borrowed battery. Scientists believe inquilinistic relationships merely evolved from creatures living in the vicinity of one another.

Endoecism refers to animals that live in the shelters created by their host, most typically burrows. Proximity is also a likely reason for such a relationship to grow over the millennia, combined with a frequent hunt for shelter by what eventually becomes the symbiont. The arrow goby of North America is commonly found in the burrows of various invertebrates, favoring crab holes. The symbiont usually benefits the most in this relationship; however having a little visitor has its advantages. Should the goby find a chunk of food too large for it to handle, it will give it to the crab. The crab chops it up as it devours its free snack, allowing the goby to take a few shredded pieces back. Sometimes the live-in symbionts feed on the hosts' waste products, earning their space as housekeepers.
Many reef species are relegated to settling on a surface before adulthood, where they remain and live out their life. But these "sessile" organisms can sometimes become mobile should they settle down on the right spot. When one animal uses another for transportation, the symbiotic relationship is called phoresis. Barnacles, for example, may settle on a crab carapace, snail shell, or on the back of a whale.

Sometimes, creatures will grow on top of one another merely because they would otherwise have no place to settle. This is especially true for creatures like oysters that need a hard surface on which to establish, but may live or have drifted into an area that has a shifting, sandy or muddy bottom. Such relationships are called epizoism.

The two most obvious symbiotic relationships involve food associations (commensalism) and associations in which both host and symbiont benefit (mutualism). These two are very close, but in commensalism, the issue is only food and it's usually only the symbiont that benefits directly. Many shrimp, crabs and copepods, for example, live on the surface of corals and other cnidarians, eating their mucus coating, dead skin or any adhering organic particles. The crustaceans usually cause no harm to their coral hosts – and may even pluck off the occasional parasite.

Mutualism is the most well-known type of symbiosis, and is marked by an interdependence of host and symbiont. In most mutualistic relationships, one could not survive without the other, which makes these sorts of relationships among the most fascinating. Hermatypic (reef-building) corals and their algal symbionts, zooxanthellae, are a common example of such crucial interdependence. The zooxanthellae live inside the coral's tissue, taking advantage of its waste products and transforming them into substances the coral can use to grow and maintain its calcium carbonate home. During times of high stress, such as prolonged high temperatures, the coral will eject zooxanthellae into the water column. Although the coral can live with a small amount of algal symbionts over short periods of time, a long absence of their usual population of symbionts spells certain death.

Cleaning symbiosis is similar, but the symbionts don't live within their hosts' tissues. A large fish will literally pull into a "cleaning station," which is nothing more than an area where cleaner shrimp and fish live. Like Indy 500 pit mechanics, the cleaners scramble from their crevasses and overhangs, picking off parasites, algae, and detritus for the fish, while scoring an easy meal.

Understanding these interactions can provide valuable insight into the health of your pets and also let you know that some behavior – regardless of how odd or dangerous it may appear – is actually normal.