Invertebrates are animals that do not have a bony internal skeleton, although many do have hard outer coverings that provide structure and protection. More than 90% of all animals are invertebrates and they are classified into at least 33 major groups, or phyla. Nearly every phylum of invertebrates has members that live in the oceans.
Six phyla of invertebrates that are commonly found in the oceans are: Porifera (sponges); Cnidaria (corals, jellyfish, and sea anemones); Annelida (segmented worms); Molluska (snails, clams, mussels, scallops, squid, and octopuses); Arthropoda (crabs, shrimp, barnacles, copepods, and euphausids); and Echinodermata (sea stars, sea urchins, and sea cucumbers). Each phyla is divided into smaller groups called classes, which is then split into families and then species.
The sponges are the least complex multicellular animals. They generally live attached to a surface and have three basic shapes, encrusting, vase-like, and branching. Sponges live in intertidal (between the tides) zones as well as in the deep ocean. They can be a few inches (centimeters) to 10 feet (3 meters) in diameter. There are nearly 10,000 species of sponges and all but two families are only found in ocean environments.
The general body of a sponge is a central cavity surrounded by a fleshy body riddled with holes. The cells lining these holes pump water into the central cavity. As the water moves through their bodies, the sponge absorbs nutrients, and filters out particles from the water as their food. The name Porifera means “hole bearer,” reflecting the many holes in the animals’ bodies.
The phylum Cnidaria includes jellyfish, sea anemones, and corals. The word Cnidaria comes from the root word knide, which means “nettle.” It refers to the special stinging cells that the animals in this group have for protection and predation (hunting an animal for food). These cells contain coiled threads that are fired at predators and prey. The threads may contain substances that paralyze or sticky substances that entangle their target.
Cnidarians have two body plans: polyp and medusa. Corals and sea anemones are typical polyps. They are jar-shaped animals with a mouth at the opening of the jar. Tentacles rim the mouth and are used to pull food into the stomach, which is located on the inside of the jar. Sea anemones have a basal disc (tooth-like structure), which is located where the bottom of the jar is, and it is used to burrow (dig a tunnel or hole) into the sand or rocks.
Corals have a skeleton made of the mineral calcium carbonate, which cements the individual coral polyps together into a colony (group). Jellyfish have a medusa shape, and this can be visualized as a polyp turned upside-down so that it looks like a bell. Jellyfish swim by contracting their bodies and forcing water out of the bell. They have long tentacles (appendages) surrounding their mouths that are used to capture prey.
The phylum Annelida includes worms that are segmented, meaning that the body is made up of sections. Each body section may have a specialized purpose, such as reproduction, locomotion, or sensing the environment. These segments are apparent on the outside of the worm’s body and make it look ringed. The word Annelida comes from the word annelus, meaning “ringed.” The most familiar member of Annelida is the earthworm, which is not a marine species.
The class Polychaeta (meaning “manyfooted”) is the largest class of Annelida, with about 10,000 species, most of which are marine (of the ocean). Almost all of them have paired appendages on their segments that can be used for swimming, burrowing, or walking.
Polychaetes often have very well developed heads with a variety of sensory organs that detect prey by touch, vision, and smell. They range in appearance from very colorful to very plain-looking. Some swim through the water, others crawl on the sand or rocks, and others live cemented to the seafloor, building and living in tube-like structures.
The mollusks are an extremely large phylum with over one hundred thousand species, most of which are marine. Most mollusks have a head, a foot, and a body that is covered by a shell-like covering called a mantle. The three most common classes of mollusks are the snails; the clams, oysters, scallops, and mussels; and the squid and octopuses.
The gastropods are the largest class of mollusks with over eighty thousand species. They include snails, slugs, abalone, and limpets. Most gastropods crawl along the seafloor among rocks, grazing on algae (tiny rootless plants that grow in sunlit waters).
Some, however, hunt for their food among plankton, which are organisms that drift through the ocean. Other gastropods filter water for food particles. The majority of gastropods live in coiled shells, which provides protection from predators and protection from the force of waves. The shells are also used to protect animals that live in the areas between low and high tides from becoming too dry.
The bivalves, meaning “two doors,” are mollusks that have two shells like clams, scallops, mussels, and oysters. These animals generally live in sediments (sand, gravel, and silt) on the bottom of the ocean and gather food by filtering particles out of the water.
Many clams are able to burrow into sand or clay. Scallops can swim by forcing water through their shells. Oysters tend to cement themselves to hard surfaces. Mussels produce tough strings called byssal threads that attach their shells to surfaces in wave-swept areas.
The most complex of the Mollusca phylum are the squid, octopuses, nautiluses, and cuttle fish. Each of these animals has a head that is surrounded by tentacles. They swim through the water using propulsion from their tentacles, much like swimmers kick their legs underwater, and creep along the bottom of the ocean using their tentacles as legs.
Cuttle fish and squid have a shell like other mollusks, but it has been internalized. In octopi, the shell is completely absent. The members of this class have excellent eyesight and are considered intelligent.
Arthropods are the most numerous invertebrate phylum with over one million species identified. Some scientists expect that there may be as many as ten million arthropods on Earth. All arthropods have a strong external skeleton that protects them from predation and supports their body structures. They have a type of muscle called a striated muscle that allows them to move quickly. They also have legs, antennae, and other appendages that are jointed.
The phylum Arthropoda has three major divisions or subphyla. The subphylum Crustacea includes about thirty thousand different species, most of which live in the ocean. Crustaceans include many different types of marine animals that are divided into several classes. Brine shrimp, which are important fish food and live in very salty water, belong to the class Branchiopoda.
The class Maxillopoda includes barnacles and copepods. Barnacles are specialized crustaceans that spend the adult part of their life cemented head-down on hard surfaces like rocks, piers, the bottoms of ships, and even the undersides of whales.
Their legs have developed into featherlike appendages that they use to generate water currents to bring food particles into their mouth. Copepods are small shrimp-like animals that are extremely important to the planktonic food web, the network of plankton that form the base of the food chain in the oceans. They are the most numerous animals in the ocean, sometimes reaching densities of more than a million per cubic yard (meter).
The class Malacostraca includes shrimp, lobsters, crabs, and euphausids. There is an enormous amount of diversity among the members of this class, which includes about twenty-five thousand different species. Some malacostracans spend their lives swimming among plankton, others walk along the ocean floor scavenging for food, while others live in burrows and attack prey that come nearby. Many members of this group live and feed off of fish or even other crustaceans. This class is very important to the economy, both as food for humans and as pets in the aquarium industry.
All of the six thousand members of the phylum Echinodermata are marine. The root word echino means “spiny” and the root word derma means “skin.” The name Echinodermata refers to the bony structures called ossicles found in the skin of these animals. Echinoderms do not have well developed sensory organs or brains.
They all have a water vascular system that is used to circulate nutrients and gasses through their bodies. All echinoderms share the same general appearance, which is based on five similar sections that radiate out from a central point.
There are five classes of echinoderms: feather stars, sea stars, brittle stars (sea stars), sea urchins, and sea cucumbers. Brittle stars are fairly uncommon. There are about six hundred species of brittle stars, living mostly in shallow waters.
Sea stars (commonly known as starfish) use their water vascular system to operate suction cups located on the bottoms of their legs. These suction cups are called tube feet and they are used both for predation and for gas exchange. Sea stars eat by gripping both shells of a clam or mussel with its tube and pulling the prey open. Then it inverts its stomach inside the shells and digests the victim.
Ophiuroids usually look like a small disc surrounded by five long worm-like arms. They are called brittle stars because when they are attacked, they will simply detach the arm that has been the target. Later, the ophiuroid will regenerate its arm.
Sea urchins are usually pin cushion-shaped and covered with sharp spines. These spines are used for locomotion as well as for defense. Between the spines, sea urchins have special appendages called pedicellariae that look like tiny claws. These are used for capturing prey and for cleaning. The mouth of the sea urchin is on its underside and is composed of five tooth-like plates.
Sea cucumbers live on the sea floor and look like cucumbers with five soft ridges. Many live on coral reefs and are extremely colorful. Sea cucumbers feed by extruding feathery appendages that can capture prey that swim too close. When attacked, many sea cucumbers will suck in water and then use the water pressure to eject their internal organs, including their digestive and respiratory systems.
The predator becomes confused among all the tissues in the water and may even become entrapped in some of the sticky material. After such an attack, a sea cucumber will regenerate its internal organs over a period of several weeks.