Sea anemone are some of the most unique animals of the ocean. They are related to both coral and jellyfish and are not plants, despite looking like one and being named after one.
Sea anemone are single polyps attached to a rock using a basal or pedal disc. The top of the sea anemone is where its tentacles are. The tentacles are used to capture prey such as small fish and crustaceans, and are very dangerous. When something brushes past its tentacles, nematocysts, organelles that contain venom, send a mini-harpoon into the prey, which gets paralyzed and eaten. Sea anemone can also use their tentacles to fight each other for space. When there are 2 sea anemone that are extremely close, they will lean down towards each other and both engage their tentacles in for a fight.
In the middle of the tentacles is an opening that can serve as the mouth, the anus, and the opening where egg or sperm are released. Through the opening is the pharynx, and below the pharynx is the gastrovascular cavity. The gastrovascular cavity is where the sea anemone can digest food. It is also surrounded by a stomach lining and produces digestive enzymes.
Sea anemone are able to move, though they look stationary. It is impossible to see their movement with a naked eye because of how slow they can be. They move by contracting their foot in a wave-like formation, or casting themselves free of the rock they are bound to.
Sea anemone have life cycles that are similar to polyps and jellyfish. A sea anemone’s life begins when neighboring sea anemone cast out their sperm or eggs into the water at the same time, called broadcast spawning. The egg and sperm join together to form planula (a larvae), which sinks down to the ocean bottom to start growing into a sea anemone. This is similar to coral and jellyfish life cycles because coral also use broadcast spawning and jellyfish also first form a planula by releasing eggs and sperm into the water at the same time.
below: a sea anemone larvae
Sea anemone are not endangered. Their predators are the loggerhead sea turtle and the sea star.
A symbiotic relationship is an interaction between two species of animals where both are able to benefit. Because sea anemone are extremely good at protecting themselves, they can also protect other species as long as the other animal is immune to their stings.
Sea anemone are very well known for their symbiotic relationship with clownfish. They benefit from clownfish because the clownfish are able to poop into the sea anemone, giving them nutrients. The clownfish, in return, receives protection from predators because of the sea anemone’s sting. They are not affected by the sea anemone because they have a thick layer of mucus on their skin that the tentacles are unable to penetrate.
Besides clownfish, sea anemone also form relationships with photosynthetic algae. These algae live in the stomach of anemone and help by sharing some products of photosynthesis, such as oxygen and glucose. The algae are given protection from micro feeders.
Sometimes, you might see a sea anemone sitting on top of a hermit crab. This is also a symbiotic relationship. The sea anemone is able to get more nutrients because they are no longer stationary, and the crab gains protection. In fact, crabs will intentionally seek out sea anemone because they know how powerful its protection can be.