As the most predictable occupant of anemones in tropical waters, clownfish make for the perfect case study of anemone symbiosis.

As a divemaster, I know there are some days when the ocean is reluctant to share its many wonders; when no matter how hard you try, you struggle to find anything of interest to show your guests. On days like these, when even the reef’s resident species seem to have disappeared from their customary haunts, there is one foolproof way to save the dive, and that’s to look for anemones. Even on the unluckiest days, these tiny ecosystems are fertile ground for exciting creatures, often including vividly colored clownfish that are as obstreperous as they are photogenic. Even if the anemone isn’t home to these territorial beauties, closer inspection will usually reveal something of interest, whether it’s a delicate porcelain crab or a transparent anemone shrimp. Anemones are a safe bet when it comes to spotting marine life because they offer great benefits to those species that have adapted to living within them, and the anemone usually benefits from the arrangement as well. These mutually symbiotic relationships mean that each party depends equally upon the other.

As the most predictable occupant of anemones in tropical waters, clownfish make for the perfect case study of anemone symbiosis. All over the world, clownfish and anemonefish can be found living in conjunction with anemones, with approximately 26 different species of fish relying on this particular kind of symbiosis. Which species lives in which anemone is far from random; only certain species of clownfish are compatible with certain species of anemone. Often, the former depends upon finding a host for survival, although both the fish and the anemone rely heavily on their relationship for success. Perhaps the biggest advantage for the clownfish is protection, offered by the anemone’s stinging cells, which prevent predators from penetrating its fronds. Clownfish have developed immunity to these stings, thanks to a thick layer of protective mucus coating their bodies. In return, clownfish ward off other fish that try to prey on the anemone’s tentacles and perform housekeeping duties by cleaning the anemone of potentially harmful parasites. Their waste also provides their host with valuable nutrients, making for one of the most successful mutually symbiotic relationships in the animal kingdom.

Other creatures also enjoy similarly reciprocal relationships with anemones; two of the most innovative are the boxer or pom-pom crab and the anemone hermit crab. Instead of residing inside the anemone, both crabs carry them around, using the anemones as forms of portable defense. For the boxer crab, this involves carrying a small pair of anemones in its claws, which it uses to threaten potential predators, brandishing the stinging tentacles like a medieval knight with a sword. The anemone’s tentacles are not only used for defense in this case; the crabs also use them to trap food particles, upon which both parties then feed. The anemone hermit crab similarly uses anemones to protect itself against aggressors, gathering them from the seafloor and planting them upon their shells. There, the anemones act as a stinging shield against the crab’s most common predator, the octopus. Anemones benefit from the locomotion afforded to them by their mobile homes, which exposes them to more plentiful and consistent food sources.

Other crabs and shrimp have developed a commensalistic form of symbiosis with certain anemones, which means that although the relationship doesn’t necessarily benefit the host, it doesn’t adversely affect it either. Porcelain crabs and anemone shrimp are both examples of this phenomenon, reaping the benefits of the anemone’s protection. So even on dives when there are plenty of sights to see, take the time to inspect anemones as you come across them — you never know what you might find inside.

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