Classification Of Cnidarians

Scientists recognize four classes of cnidarians—Hydrozoa, Cubozoa, Scyphozoa, and Anthozoa. The members of these classes are known as hydrozoans, cubozoans, scyphozoans, and antho-zoans, respectively. Some species of hydrozoans live only as polyps, some live only as medusae, some alternate between these two forms, and some live as mixed colonies of polyps and medusae. Cubozoans and scyphozoans spend most of their lives as medusae, while anthozoans live only as polyps.

Class Hydrozoa

The class Hydrozoa includes about 3,700 species, most of which live as colonial organisms in the oceans. Examples of colonial hydrozoans are species of the genus Obelia. As Figure 33-6a illustrates, one such species has many polyps attached to branched stalks. Some of the polyps function in gathering food, while others are responsible for reproduction.

The hydrozoan shown in Figure 33-6b, the Portuguese man-of-war (genus Physalia), exists as a colony of medusae and polyps. Its gas-filled float, which can measure as much as 30 cm (1 ft) across, keeps the colony at the surface of the ocean. The polyps in the colony are specialized for feeding, digestion, or sexual reproduction. Tentacles up to 20 m (65 ft) long dangle from the feeding polyps and carry large numbers of cnidocytes. The Portuguese man-of-war preys mostly on small fish, but its cnidocytes contain a neurotoxin (nerve poison) that can be painful and even fatal to humans.

figure 33-6

These organisms are two examples of colonial hydrozoans. (a) This colony of the genus Obelia is made up of polyps, although organisms of the genus Obelia can produce medusae that reproduce sexually. (b) The tentacles of this Portugese man-of war, genus Physalia, have both polyps and medusae that have specialized functions.

figure 33-6

These organisms are two examples of colonial hydrozoans. (a) This colony of the genus Obelia is made up of polyps, although organisms of the genus Obelia can produce medusae that reproduce sexually. (b) The tentacles of this Portugese man-of war, genus Physalia, have both polyps and medusae that have specialized functions.

(a) (b)

One hydrozoan that has been extensively studied is the hydra. Hydras are not typical hydrozoans because they exist only as polyps, they are not colonial, and they live in fresh water. Hydras range from 1 to 4 cm (0.4 to 1.6 in.) in length. Most hydras are white or brown, but some, like the one shown in Figure 33-7, appear green because of the algae that live symbiotically inside cells of their gastrodermis. Hydras can be found in quiet ponds, lakes, and streams. They attach themselves to rocks or water plants by means of a sticky secretion produced by cells at the hydra's base.

A hydra can leave one place of attachment and move to another. This can happen when the base secretes bubbles of gas, which cause the hydra to float upside down on the surface of the water. Hydras can also move by tumbling. This movement occurs when the tentacles and the mouth end bend over and touch the surface to which the hydra was attached while the base pulls free.

During warm weather, hydras generally reproduce asexually. Small buds, such as the one you can see in Figure 33-8, develop on the outside of the hydra's body. These buds grow their own tentacles and then separate from the body and begin living independently.

Sexual reproduction usually occurs in the fall, when low temperatures trigger the development of eggs and sperm. The eggs are produced by meiosis along the body wall in swellings called ovaries. Motile sperm are formed by meiosis in similar swellings called testes. In some species, eggs and sperm are produced in the same hermaphroditic individual, as indicated in Figure 33-8. In other species, the individuals are either male or female. In either case, sperm are released into the water, and those that reach ovaries can fertilize egg cells. Each fertilized egg then divides and grows into an embryo. A hard covering protects the embryo through the winter, and in the spring the embryo hatches and develops into a new hydra.

Chlorohydra

figure 33-8

Hydras can reproduce either asexually, by forming buds, or sexually, by producing sperm that fertilize eggs.

figure 33-7

The green color of this hydra, Chlorohydra viridissima, comes from the algae that live inside the hydra's cells. (LM 30x)

figure 33-7

The green color of this hydra, Chlorohydra viridissima, comes from the algae that live inside the hydra's cells. (LM 30x)

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Hydras can reproduce either asexually, by forming buds, or sexually, by producing sperm that fertilize eggs.

figure 33-9

The common jellyfish, genus Aurelia, reproduces when sperm from an adult male medusa fertilize eggs from an adult female medusa. Each fertilized egg produces a blastula, which develops into larva known as a planula. The planula forms a polyp, which produces more medusae.

Class Cubozoa

The cubozoans, or box jellies, were once classified in the class Schyphozoa. As their name implies, cubozoans have cube-shaped medusae. Their polyp stage is inconspicuous and has never been observed in some species. Most box jellies are only a few centimeters in height, although some reach 25 cm (10 in.) tall. A tentacle or group of tentacles is found at each corner of the "box." The cnido-cytes of some species, such as the sea wasp, can inflict severe pain and even death among humans. The sea wasp lives in the ocean along the tropical northern coast of Australia.

Class Scyphozoa

The name Scyphozoa (sie-foh-ZOH-uh) means "cup animals," which describes the medusa, the dominant form of the life cycle of this class. There are more than 200 species of scyphozoans, known commonly as jellyfish. The cups of the medusae range from 2 cm (0.8 in.) to 4 m (13 ft) across, and some species have tentacles that are several meters long. Pulsating motions of the cup propel the jellyfish through the water. Like the Portuguese man-of-war, some jellyfish carry poisonous nematocysts that can cause severe pain and even death in humans.

The common jellyfish, genus Aurelia, is a scyphozoan whose life cycle includes both medusa and polyp forms. As you can see in Figure 33-9, step Q, adult medusae release sperm and eggs into the water, where fertilization occurs. The resulting zygote divides many times to form a blastula, as shown in step ©. In step ©, the blastula then develops into a ciliated larva called a planula (PLAN-yuh-luh). In step ©, the planula attaches to the ocean bottom. The planula becomes a polyp by developing a mouth and tentacles at the unattached end as shown in step ©. As the polyp grows, shown in step ©, it forms a stack of medusae. Finally, as shown in step O, the medusae detach and develop into free-swimming jellyfish.

Jellyfish Critical Thinking

Class Anthozoa

The name Anthozoa means "flower animals," which is a fitting description for the approximately 6,100 marine species in this class. Two examples of anthozoans are sea anemones and corals, which are shown in Figure 33-10.

Sea anemones are polyps commonly found in coastal areas, where they attach themselves to rocks and other submerged objects. Anemones feed on fishes and other animals that swim within reach of their tentacles. However, some anemones in the Pacific Ocean have a symbiotic relationship with the clownfish, as Figure 33-11 demonstrates. The two animals share food and protect each other from predators. The movements of the clown-fish also help prevent sediments from burying the anemone. The clownfish produces a slimy mucus that prevents the anemone from firing its nematocysts when the clownfish touches the anemone's tentacles.

Corals are small polyps that usually live in colonies. Each polyp cements its calcium carbonate skeleton to the skeletons of adjoining polyps in the colony. When the polyps die, their hardened skeletons remain, serving as the foundation for new polyps. Over thousands of years, these polyps build up large, rocklike formations known as coral reefs. Only the top layer of the reef contains the living polyps. Coral reefs provide food and shelter for an enormous and colorful variety of fishes and invertebrates.

Nearly all coral reefs are restricted to a band of ocean within 30 degrees north or south of the equator. Most form at shallow depths in warm, clear waters. These conditions are necessary in order for photosynthesis to be carried out by the algae that live symbiotically inside coral cells. These corals depend on the algae to provide oxygen and to speed up the accumulation of calcium from the sea water. The algae in turn depend on the corals to supply vital nutrients.

figure 33-10

figure 33-10

Anthozoans, including this crimson anemone, Cribrinopsis fernaldi(a), and this golden cup coral, genus Tubastraea (b), live as polyps along ocean coasts.

figure 33-11

The clownfish, Amphiprion oceiiaris, lives symbiotically among the tentacles of sea anemones. The anemone's stinging tentacles protect the clownfish from predators. The clownfish, in turn, drives away other fish that try to feed on the anemone.

figure 33-11

The clownfish, Amphiprion oceiiaris, lives symbiotically among the tentacles of sea anemones. The anemone's stinging tentacles protect the clownfish from predators. The clownfish, in turn, drives away other fish that try to feed on the anemone.

Word Roots and Origins colloblast from the Greek kolla, meaning "glue," and blastos, meaning "bud"

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