Marine Invertebrates

Generally, marine organisms are classified by where they live, how they get their food, and their body structure. In order to understand how marine organisms live with one another, one must first understand their general classifications.


Plankton are any floating or feebly swimming organisms that live close to the surface of the ocean. In fact, the term plankton comes from the Greek word planktos which means “wanderer.” These organisms include animals, plants, bacteria, or any other organisms which can not resist the ocean current. Phytoplankton are plankton that are autotrophic and can produce their own food; whereas zooplankton are plankton that rely on other organisms for food. Additionally, there are bacterioplankton that resemble bacteria and archea and help recycle minerals and decaying organisms in the water column. Some plankton live their entire lives as plankton. These organisms are referred to as holoplankton. In contrast, there are organisms that live only part of their lives as plankton. These organisms are referred to as meroplankton.


Zooplankton are animal plankton. There are basically two types of animal plankton; those that spend part of their lives as plankton, and those that spend their entire lives as plankton. These two groups are referred to as meroplankton and haloplankton respectively. Meroplankton typically consist of eggs and larvae of larger invertebrates or fish. These organisms continue to develop and later become part of the nekton or benthos. Haloplankton consist of small invertebrates such as protozoans, copepods, amphipods, some coelenterates even some large fish!


Many people think that plankton are always small, but zooplankton can come in many different sizes. Nannoplankton range from .005mm to .06mm. This group includes mostly unicellular animals. Microplankton range from .06mm to 1mm and include mostly eggs and larvae of invertebrates. Macroplankton are larger than 1mm and contain organisms such as copepods and some worms. Megaplankton are large plankton such as Portuguese man-o-war and By-the-wind sailor jellyfishes.

Animal Phyla

Consumers are organisms that are incapable of synthesizing their own food; therefore, they must rely on producers as a source of energy. In marine environments, the most abundant consumers are animals.

Bryozoans are colonial animals that build calcium skeletons. An individual bryozoan animal is called a zooid, but because they are colonial, they are never found alone. Each zooid has tentacles to trap diatoms and algae and a U shaped gut to process the food. Bryozoans grow and colonialize on all types of hard substances such as sand, rocks, docks, and boats. They can produce skeletons in all shapes and sizes. Some bryozoan colonies tend to look like fans and twigs while others look like a hard covering on a piece of algae.

The name Porifera means “pore bearer”. Sponges are the most simplistic multicellular animals. Sponges are sessile which means they do not move, but remain attached to objects. There are over 5,000 species of marine sponges and only about 150 species of freshwater sponges. Sponges clean the water by filtering gallons of water per day! Sponges also have symbiotic relationships with other organisms. The dark canals within the sponge provide great homes for amphipods, shrimp, worms, and brittle stars.


Sponges eat detritus and plankton. The also clean the water by filtering gallons of water per day! Sponges also have symbiotic relationships with other organisms. The dark canals within the sponge provide great homes for amphipods, shrimp, worms, and brittle stars.

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The bodies of sponges are made a fibrous matrix of spikes made from calcium and silica. Spicules Sponges do not have a nervous system or distinct tissues. Their cells work independently almost as if they were in a colony. Sponges can reproduce sexually or asexually.

When you buy a sea sponge from the store, what you are buying is the remaining silica and calcium matrix. These are not to be confused with the brightly colored sponges that are used for cleaning. Those sponges are synthetic and are made by chemical means.

These animals are the first animals to develop with differentiated tissues and nerve cells. These organisms have mouths and an internal digestive cavity. They use their tentacles to help in capturing and ingesting prey. They prey on fish, crustaceans, and plankton. Coelenterates may be either free swimming or sessile. The term coelenterate actually refers to two animal phyla: the Cnidarians and the Ctenophora.

The term Cnidaria means “stinging nettle” which is the identifying characteristic of this phylum. Cnidarians have tentacles which are lined with pressurized cells which eject a barb into their prey. This barb contains toxins which immobilize or kill the prey.

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Corals also fall within this category. Corals are cnidarians which actually look like little sea anemones. Instead of catching fish like their cousins, the catch plankton. The coral is made up of a colony of polyps which function as a single organism.

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Ctenophora means “comb bearer” and refers to a group known as comb jellies. Although these organisms appear similar to jellyfish, they are quite different. Comb jellies do not sting and vary in their means of reproduction.

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