What is Sea Anemone?

The two swimmers put on their masks and snorkels and swam out through the clear, shallow tropical waters. Small, bright-colored fish darted around them. Most amazing of all was a beautiful flower waving back and forth in the gentle currents. But flowers don’t grow underwater! They looked again, and the flower curled up. This was not a plant after all. It was a sea anemone.

Type of Sea Anemone

• A sea anemone (Actinauge richardi)
• Beadlet anemone (Actinia equina)
• Strawberry anemone (Actinia fragacea)
• Sandalled anemone (Actinothoe sphyrodeta)
• Cloak anemone (Adamsia palliata)
• Trumpet anemone (Aiptasia couchii)
• Dead man's fingers (Alcyonium digitatum)

Body structure / Anatomy

A sea anemone is an animal. It is shaped like a cylinder. It has a short, thick stump with a crown of tentacles around the top. Most sea anemones are brightly colored. Their shape and colors make them look like large flowers. They belong to a class of animals known as anthozoans. In fact, the word Anthozoa means “flower animals.”

The sea anemone lives in all oceans around the world. Most sea anemones live in shallow coastal water up to 164 feet (50 m) deep. Not all sea anemones are found in shallow water, though. They can live at all water depths. Some even live deeper than 32,800 feet (9,997 m). Scientists estimate that there are about 1,200 different species of sea anemones. Sea anemones are closely related to reef-building hard corals.

They are also related to soft corals like sea fans. Unlike corals, which are colonial animals, the sea anemone is a solitary creature. Although they may occur in groups, each sea anemone is separate from the others. Most sea anemones stay attached to the substrate, but they are able to move. One way sea anemones can move is by gliding along the bottom of the ocean. Some sea anemones can flex their bodies and swim short distances. There are even a few pelagic species that float.

Spectacular Tentacles

The sea anemone’s tentacles surround an opening that leads into the gut of the creature. This single opening serves as both the mouth and the anus of the animal. Yes, food and waste go in and out of the same opening. YUK! Most sea anemones are not very big. The base usually ranges from less than 0.25 inches (0.5 cm) to about 4 inches (10 cm) in diameter. But there are bigger ones. Some sea anemones are more than 5 feet (1.5 m) in diameter. Most are only a few inches tall, but a few can stretch to nearly 3 feet (1 m) tall.

The sea anemone does not have a skeleton. Its thick base is very muscular and filled with fluid. When threatened, many sea anemones can contract their muscles to pull the tentacles and body inward to form a tight ball. The tentacles are the most spectacular feature of the sea anemone. There can be one or more rings of tentacles that surround the mouth. Some sea anemones have long, slender, threadlike tentacles. Others have stout tentacles like strands of thick spaghetti. In some sea anemones, the tentacles are short and stubby. The tentacles often have bright colors and patterns.

The Venomous Sting

What makes the tentacles of the sea anemone so special is that they contain unique stinging cells. These special cells contain a nematocyst. One end of it has venomous spines, and the other end is attached to the tentacle by a slender thread. The outside of the nematocyst has sensory hair. When something touches the sea anemone, the sensory hair is triggered, and the nematocyst will shoot out. The venom of most anemones is not harmful to humans. A few sea anemones are very toxic. Their sting can cause great pain, even death. It is best to look, but don’t touch when you see a sea anemone.

Anemone Feeding

The sea anemone is a carnivore. Sea anemones eat a variety of foods, including fish, crabs, shrimp, mussels, and small plankton. They use their stinging tentacles to capture their food. Like a bullet, the nematocyst travels more than 6.5 feet (2 m) per second. It penetrates the prey and paralyzes it with the toxin. Then the prey is wrapped by the anemone’s tentacles, which bend inward and bring the prey to the mouth. Food is drawn into the central cavity inside the base of the anemone. This central cavity is where digestion takes place.

All of the digested food is absorbed into the cells of the body. Waste materials will be mixed with the water inside this cavity and then pushed back out of the mouth. Sea anemones can feed in three different ways. They can actively capture their prey with their tentacles. Another method is to filter floating food particles as they land on the tentacles. Sea anemones can also capture food that has been released by wave action or other predators. The amount and type of food captured depend on the number and size of an anemone’s tentacles.

Sea anemones have another way to obtain nutrition. They house algae inside their tissues. Like all plants, algae use sunlight to produce food through photosynthesis. This relationship benefits both the algae and the anemone. The algae are protected by the anemone and the anemone can obtain nutrients from photosynthesis by the algae.

Relationship with Anemone fish and Snail

Sea anemones have an interesting relationship with clownfish. The clownfish has protective mucus that prevents it from being stung by the nematocysts. Protected from predators, the clownfish live in the anemones. The clownfish chases away other fish that may try to eat the anemone.

Sea anemones also have an interesting partnership with snails. Some sea anemones will hitch a ride on a snail. The anemone can offer the snail protection from predators. The anemone benefits by having free transportation to areas where food is available. The snail also provides a way for the anemone to escape its predators. This doesn’t only happen with snails. Sometimes the sea anemone will attach to a hermit crab shell.

Anemone Reproduction

Where do baby sea anemones come from? And how can you tell a male sea anemone from a female? First, there is no way to tell the males and females apart. Sometimes the same anemone produces both eggs and sperm!

Sea anemones can reproduce in several ways.

One way is by splitting apart. A single anemone can split in half and form two anemones. Instead of splitting completely, some sea anemones grow smaller baby anemones on the side of their base. New anemones are also formed when small pieces break off the bottom of the anemone’s base.

Yet another way that sea anemones reproduce is by shedding eggs and sperm into the water. The egg and sperm will meet and form a planula larva. The planula larva is a very tiny organism covered in fine hairs called cilia. The cilia are like tiny oars that beat back and forth. The planula larva can move using its cilia.

This larva will eventually develop into an adult anemone. What is amazing is that most sea anemones don’t stick to just one type of reproduction. Sea anemones will reproduce by splitting, budding, or by producing eggs and sperm.

Anemone Enemies   

Animals

The nematocysts of a sea anemone are a good defense. But they are not enough to ward off all predators. Many animals will attack and eat anemones, such as sea stars, sea spiders, snails, fish, and nudibranchs. These predators are often very specific.

Human

One predator species will attack only one particular species of the sea anemone. Sea anemones are not endangered. They are not good to eat, so there is no concern about humans overfishing them. Only one species is eaten as a delicacy, by people in southern Italy and Spain.

However, the main threat to sea anemones is humans. Large numbers of sea anemones are collected in the wild and sold in pet stores around the world. When anemones are removed from the wild, the animals that depend on them are affected.

Environmental Factors

Sea anemones are also threatened by changes in the ocean environment. Most anemones live in shallow water near the coastline. These habitats are impacted by increased human development and population growth. Chemicals from pollution can harm delicate anemones. Pollution also makes the water cloudy. If sunlight cannot penetrate the water, the algae living within the anemone will die. Without their beneficial algae, many anemones would be unable to thrive. Sea anemones are also at risk when the animals they prey on the decline due to pollution and overfishing.

Sea anemones are among the most beautiful creatures in the ocean. Their unusual bodies and bright colors are fascinating. Scientists continue to study sea anemones to unlock their many mysteries. For example, studying the venom in nematocysts may lead to new medical treatments for humans. We must protect our oceans so these strange and beautiful creatures can continue to survive.