Life Forms in the Ocean

Fifty percent of life on earth lives at sea. Life in the ocean ranges in size from microscopic invertebrates to vertebrates as large as humpback whales.

When we think of sea life, we probably think of fish, seals and whales. Do you ever think about echinoderms, coelenterates or sponges? How about arthropods, mollusks and annelids - the invaluable invertebrates of the sea?


              Microscopic view of phytoplankton
Phytoplankton are tiny cells that have enormous impact on our planet.

Through the process of photosynthesis, these abundant cells produces something like half the oxygen in our air!

Phytoplankton form the base of the prolific marine food chain, which ultimately also helps feed people and other terrestrial life.

Throughout Earth's history, phytoplankton have also played an important role in regulating Earth's climate. They remove huge amounts of the greenhouse gas carbon dioxide from the atmosphere, turning it into organic matter via photosynthesis. Much of this organic carbon is consumed by animals in upper ocean waters. Some falls to the seafloor, as dead organisms or fecal pellets, where it is consumed or converted over time into oil and gas deposits.


All echinoderms live in the sea. They have no head or tail, and are said to have a 'five-fold' symmetry. One of the best known is the starfish, but the group also includes sea-urchins and sea-cucumbers. Curiously their larvae vaguely resemble very small fish and they are thought to be quite closely related to the vertebrates.

Various Echinoderms


  Aurelia, or Moon Jellies, by the reefs off
Lizard Island, Australia

There are over 90,000 species of cnidarians (also called coelenterates, in the world.

These cnidarians are invertebrates, meaning they have no back bone.

Cnidarians are animals that have tentacles surrounding their mouth. The tentacles contain stinging cells that make it easy to capture their next meal, and contain poison to kill their prey.

Some of the animals that are cnidarians are jellyfish, octopus, corals, hydra, sea fans, and sea anemones.

Cnidarians bodies have two layers of cells. The outer layer of cells covers the body while the inner layer lines the digestive system, which is where the food is broken up to be made into both energy and nutrients.

The body can either have the shape of an umbrella, a bell, or a cylinder and it comes in any of the three colors: red, white, or green.

Their bodies contain a nervous system, and contain sensory organs. It also has a muscular skeleton.

A sea fan
Cnidarians use their environment to get oxygen to stay alive.

They get rid of waste through their mouth since it has no anus.

It has radical symmetry which means  their body parts are symmetrically arranged around a central point.

They eat both animals and plants. 

These types of species can produce both sexually and asexually. It reproduces sexually by releasing either sperm or egg into the water. Cnidarians produce asexually by budding.


Arthropods are animals with segmented bodies and six or more jointed legs.

They are the largest animal group on Earth. In fact, more than three out of four of all animals are arthropods. They are found everywhere - on land, in trees, in freshwater and saltwater, and even underground.

Arthropods are generally small. Most are less than 1 cm long. Some arthropods, however, are quite large. The giant king crab, for example, measures over 3.2 cm from the tip of one outstretched leg to another.

Some of the most familiar arthropods of the sea are shrimp, lobsters, octopus, crabs and anenome's

Some anthropods of the sea: (for a larger view, click on each image)
Spotted Anemone Crab   Spiny Lobster   Hinge Beak Sprimp


Mollusks were among the first inhabitants of the Earth.  Fossils of mollusks have been found in rocks and date back over 500 million years. Mollusk fossils are usually well preserved because of their hard shell.

Most mollusks have a soft, skin-like organ covered with a hard outside shell. Some mollusks live on land, such as the snail and slug. Other mollusks live in water, such as the oyster, mussel, clam, squid and octopus.

Ocean living mollusks move or swim by jet propulsion. They propel themselves by ejecting water from their body. For example, the squid ejects water from a cavity within its body, and the scallop ejects water to move by clamping its shell closed.

Other ocean living mollusks, like the oyster, attach themselves to rocks or other surfaces, and can't move. They feed by filtering small food particles from water that flows through them.

Mollusk Movies:

Clams   Cuttlefish   Nautilus   Octopus   Oyster   Snail   Squid
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Movies courtesy of:




Annelids is the proper name for worms.  There are about 9,000 species of Annelids known today, including worms and leeches. They can be found almost anywhere in the world. Annelids have existed on Earth for over 120 million years.

Annelids, commonly known as worms

Annelids have bodies that are divided into segments. They have well-developed internal organs. One common characteristic of annelids is that they don't have any limbs.

Some annelids may have long bristles. Others have shorter bristles and seem smooth, like the earthworm.

Marine worms are a smaller group that live mainly in the sea.  Either they swim or shimmy around rocks and feed on small animals (ragworm) or else they live in tubes and extend their tentacles to catch plankton, or filter small animals from mud or sand. The worm casts found low down on sandy Atlantic beaches are formed by the lugworm.

Leeches make up the third group of segmented worms. There are only 300 known species.  All feed on blood.


Vertebras are marine mammals but also marine birds, marine reptiles, and - of course - marine fish.

Mammals represent a wide range of shapes, sizes and colors.  One characteristic of all mammals is that they have hair. This is more obvious in some animals, such as a seal who often have visible fur, than in others, like whales whose hair has sometimes disappeared by the time they are born!

Mammals are also endotherms, commonly known as "warm-blooded."

Most mammals have teeth, although some, like the baleen whales, do not. Since mammals range widely in habitat and food preferences, they have a wide range in feeding styles and preferences.

In marine mammals, whales feed using teeth or baleen, and on a variety of prey, including small fish, crustaceans and sometimes other marine mammals. Pinnipeds feed using teeth, usually eating fish and crustaceans. Sirenians also have teeth, although they also use the power of their strong lips when grasping and ripping aquatic vegetation.

Mammals reproduce sexually and have internal fertilization. All marine mammals are placental mammals, meaning they give birth to live young, and the unborn young are nourished in the mother's uterus by an organ called the placenta.

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Marine life:

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