Sea anemone

Sea anemones are colorful animals that live in the ocean. They belong to a group called Cnidaria, which also includes corals, jellyfish, and Hydra. Unlike jellyfish, sea anemones stay in one place and do not move around like a floating medusa.

Most sea anemones live attached to hard surfaces, but some can live in soft mud or even float near the water's surface. They have a trunk with a mouth and tentacles around it. These tentacles can catch food and have special stinging cells called cnidocytes that help them catch prey.

Many sea anemones have a special friendship with tiny plants called zooxanthellae or zoochlorellae that live inside their cells, giving them extra food. Some sea anemones also live together with clownfish or hermit crabs, helping each other. They reproduce by releasing eggs and sperm into the water or by splitting into pieces that grow into new anemones.

People sometimes keep sea anemones in special tanks called reef aquariums, but taking them from the wild to sell can harm their populations in some places.

Anatomy

A typical sea anemone is a fixed animal attached to a surface by a sticky foot called a basal or pedal disc. It has a column-shaped body with a flat top called an oral disc. Most sea anemones are small, ranging from 1 to 5 cm wide and 1.5 to 10 cm long, but some can be much larger.

The oral disc has a central mouth surrounded by tentacles. These tentacles have special cells that help catch food and defend against threats. Many sea anemones can also release thin filaments for extra protection. While most are safe for humans, a few types can cause serious harm with their stinging cells. Some small fish, like clownfish, live among the tentacles without getting stung.

Life cycle

Sea anemones are different from other sea creatures because they do not have a swimming stage in their life. Instead, they start as small, swimming babies called planula larvae, which grow directly into new sea anemones. They can reproduce in two ways: by combining eggs and sperm or by splitting themselves into two parts.

Some sea anemones can change their gender during their life, while others are only one gender. They can also reproduce without needing another anemone, by growing a new copy of themselves from a piece of their body. This helps them survive and spread to new places.

Behaviour and ecology

Sea anemones can change their shape a lot. They have muscles that let them stretch, shrink, bend, and twist. They can even turn some parts inside out or pull their tentacles inside.

Most sea anemones stay in one place for weeks or months, sticking to rocks or shells. But they can move slowly, creeping along the ground. Some can even swim a little by moving their tentacles quickly. One type can both walk and swim, moving like a caterpillar or using its tentacles to push through the water.

Sea anemones catch food by grabbing it with their tentacles. They can eat small fish, crabs, and other sea creatures. Some sea anemones work together with tiny plants called algae. The algae give the anemone food and oxygen, and the anemone protects the algae.

Many sea creatures, like clownfish, live safely among the anemone's tentacles. The anemone protects them, and they give it food in return. Some crabs and shrimp also live with anemones in similar ways.

Sea anemones live in oceans all around the world, from deep water to shallow coasts. They are most common in warm tropical waters but can also live in colder places. Many attach to rocks or shells, while some bury themselves in sand or mud.

Relationship with humans

Sea anemones and the special fish that live with them can be beautiful to watch in aquariums. But taking them from the wild to put in tanks can hurt their populations because there are fewer of them left in their natural homes.

People in some places cook and eat sea anemones. For example, in southwestern Spain and Sardinia, a type called the snakelocks anemone is marinated, coated in a batter like for calamari, and fried. In other areas, such as the east coast of Borneo, the Thousand Islands in Southeast Asia, and Taizhou in Zhejiang, people also eat sea anemones. In Thailand, a special soup called kaeng khua hed lub uses a type of sea anemone, but this species is protected by law there.

Fossil record

Most sea anemones do not leave behind hard parts that can be recognized as fossils. However, a few fossils of sea anemones have been found. Mackenzia, from the Middle Cambrian Burgess Shale in Canada, is the oldest known fossil of a sea anemone.

In 2024, more than 100 fossils of sea anemones were discovered in Brazil, dating back to the Silurian period. The species, Arenactinia ipuensis, is the oldest anemone found in Latin America and the only fossils of soft-bodied anemones preserved in three-dimensional form. These fossils give us valuable information about how anemones evolved and lived during the Paleozoic.

Taxonomy

Sea anemones belong to a group called Actiniaria. They are part of the phylum Cnidaria, class Anthozoa, and subclass Hexacorallia. Scientists have suggested a new way to group Actiniaria using DNA information.

The Actiniaria group includes different suborders and superfamilies such as:

• Suborder Anenthemonae
  • Superfamily Edwardsioidea
  • Superfamily Actinernoidea
• Suborder Enthemonae
  • Superfamily Actinostoloidea
  • Superfamily Actinioidea
  • Superfamily Metridioidea

Phylogeny

Actiniaria is an order that belongs to the subphylum Anthozoa and the class Hexacorallia. You can learn more about this by looking at the taxonomy of Anthozoa.