Wrapped in undulating fronds of kelp anchored to the ocean floor, a sea otter floats on its back, deftly cracking open sea urchins on a rock lying on its chest. Unbeknownst to the otter, it is a keystone species in a food web, its relationship with kelp essential to an entire ecosystem. Kelp forests grow in shallow coastal waters off coasts near Korea, Japan, South Africa, New Zealand, Australia, islands in the Southern Ocean and western coasts of North and South America. These prolific ecosystems host a number of organisms that share symbiotic relationships.
Sea Otters and Kelp
A large alga or seaweed, kelp exists both rooted to the ocean floor at holdfasts and free floating. High in biomass, kelp forests host many organisms, including sea otters, which tangle themselves in the fronds to keep from drifting. A mutually beneficial relationship between sea otters and kelp was first discovered after sea otters were hunted almost to extinction for their fur. The kelp began to disappear. Scientists discovered that one of the otter’s main foods, sea urchins, exploded in number when otters dwindled, destroying not only kelp but also the homes of other kelp forest creatures, leaving desolate urchin barrens. As hunting was halted and sea otter populations grew again, kelp forests slowly returned.
Sheephead and Señorita Wrasses and Kelp
Kelp and the sheephead wrasse, a fish, and kelp and the señorita wrasse enjoy mutualistic partnerships as well. Sheephead wrasses, like sea otters, feed on sea urchins and other mollusks, keeping the urchins from overgrazing the kelp. Señorita wrasses, as cleaner fish, eat parasites found on other fish – especially predatory fish -- and also eat lacy bryozoans, invertebrates that feed on and damage kelp. The predatory fish, aside from providing tasty parasites, also do señorita wrasses the favor of not eating them -- usually. Meanwhile, the kelp provides wrasses shelter from predators and nurseries for their young.
Purple Sea Urchins and Lined Chitons
Purple sea urchins and lined chitons engage in a different kind of symbiotic relationship. Armored with spines for protection, sea urchins tend to gather near kelp holdfasts to feed -- and where predators look for prey. Their spines not ominous enough, urchins also burrow into rock for protection, scraping away depressions in stony surfaces with tiny teeth. Oval, pillbug-like mollusks called lined chitons, camouflaged on the ocean floor, possess no spines or other defenses. They surreptitiously scoot under the spines of the sea urchin and, uninvited, share its burrow or use its vacant burrows to evade predators. Their relationship, called commensalism, benefits the chiton, but it apparently neither benefits nor harms the sea urchin.
Gumboot Chitons and Scale Worms
Another chiton, the large gumboot, gets a kind of "cousin’s payback" in a commensal relationship it shares with scale worms and other creatures. Gumboot chitons have armor-like segments covering their gills. Within the gill space lurk red-banded, red, and pale yellow scale worms, dodging predators. Small pea crabs also tuck themselves into these crowded gill chamber "hostels," yet the chiton gains no benefits. In addition, gumboot chitons may act indirectly commensal to coralline algae -- by eating their cousins. By consuming fleshy red algae growing on the surface of coralline algae “skeleton,” chitons might reduce problems caused by algae overgrowth.