HYDROIDS: Hydrozoa

PHYSICAL CHARACTERISTICS

Hydroids (HIGH-droyds) have two body forms. One is the medusa (mi-DOO-suh), a jelly-like, umbrella-shaped, freely swimming form with a mouth and tentacles that face down. The other is a colony of polyps (PAH-luhps), or tube-shaped sacs that have a mouth and tentacles that face up. The polyp form is fixed to the material on which it lives. Most hydroids have both a medusa and a polyp stage.

Medusae (mi-DOO-see, the plural of medusa) have a typical and easily recognizable body shape. The shape of polyps ranges from giant coral-like colonies through feathers and flowers to microscopic balls of tissue. The umbrella of medusae can be one–sixty-fourth of an inch to more than 6 feet (500 micrometers to 2 meters) in diameter. The medusae and polyps of most hydroids are clear and filmy. Among colored species, the color often comes from the food the hydroids eat. The most common color is red from crustaceans (krus-TAY-shuns), which are water-dwelling animals, such as shrimp, that have jointed legs and a hard shell but no backbone. Other colors are green, white, orange, yellow, blue, and purple.

Hydroid colonies are made up of different types of polyps that work together to serve the entire colony. One type catches prey and defends the colony's territory. Another type takes the prey from the catcher-defenders and swallows and digests it. A third type of polyp is in charge of reproduction.

GEOGRAPHIC RANGE

Hydroids live all over the world.

HABITAT

Hydroids live in all water habitats, from sea caves to deep-sea trenches, from lakes and ponds to rocky coasts and between grains of sand.

DIET

Hydroid polyps and medusae feed on almost all animals, from plankton, or microscopic plants and animals drifting in water, to fish.

BEHAVIOR AND REPRODUCTION

Some medusae remain immobile in the water, their tentacles outstretched, ambushing their prey. Others cruise across the water to catch their prey. Polyps simply extend their tentacles to catch passing prey. They also form currents by moving their tentacles to direct food particles toward their mouths.

Polyps compete for space and defend their territory. Medusae are sharply individual but can be gathered by winds and currents to form large swarms. When hungry, both polyps and medusae are always in search of food. When the body cavity is full of food, the tentacles usually are contracted, showing that these animals probably can control their stinger discharge.

Most polyps have separate sexes. Males release sperm in the water, and the sperm swim toward eggs that are on the female's body. Fertilization (FUR-teh-lih-ZAY-shun), the joining of egg and sperm to start development, usually takes place inside the female's body, and she releases the larvae (LAR-vee) into the water, where they fall to the bottom and settle near the parent colony. Larvae are animals in an early stage that change form before becoming adults.

WHAT'S IN A NAME?

The words hydroid and medusa come from Greek myth. The hydra was the many-headed monster that fought Hercules, who would cut off a head only to see two heads replace it. Medusa was one of the gorgons, three snake-haired sisters who would turn anyone who looked at them to stone.

Among medusae, males release sperm and females release eggs in the water, where fertilization occurs and larvae develop. The larvae reproduce asexually by budding, or producing bumps that develop to full size and then break off to live as new individuals. Asexual (ay-SEK-shuh-wuhl) means without, and sexual means with, the uniting of egg and sperm for the transfer of DNA from two parents. Therefore, a single larva that develops from a bud produces a polyp colony that in its turn produces many adult medusae.

HYDROIDS AND PEOPLE

The medusae of hydroids prey on the eggs and larvae of fish that people need for food. Some hydroids are used for scientific research. Others inhibit the functioning of power plants by clogging their pipes and reduce the speed of ships by attaching themselves to their hulls. Some medusae and polyp colonies inflict severe stings on humans.

CONSERVATION STATUS

Hydroids are not threatened or endangered.

FIRE CORAL (Millepora alcicornis): SPECIES ACCOUNTS

Physical characteristics: The polyps of fire coral colonies form massive, horn-shaped, upright branches. Large feeding polyps are surrounded by smaller catching-defending polyps. The feeding polyps are short and stout, with four to six short tentacles around the mouth. The catching-defending polyps are long, slender, and mouthless. The reproductive polyps are embedded in the surface of the coral. Fire coral is usually yellow or brown with white tips. The medusae of fire coral do not have tentacles and live for only a short time.

Geographic range: Fire coral lives in the Pacific and Indian oceans and in the Red and Caribbean seas. Specific distribution map not available.

Habitat: Fire coral lives on ledges and reefs in water less than 98 feet (30 meters) deep.

Diet: Fire coral feeds on animal plankton, mostly crustaceans.

Behavior and reproduction: Scientists do not know how fire coral behaves. It reproduces by the budding of tiny medusae from the polyp colony. These medusae live only a few hours, but before dying, they release eggs and sperm. Fertilization occurs in the water, and larvae develop, settle, and form new colonies.

Fire coral and people: The stings of fire coral cause severe burns.

Conservation status: Fire coral is not threatened or endangered. ∎

NO COMMON NAME (Distichopora violacea): SPECIES ACCOUNTS

Physical characteristics: Distichopora violacea (abbreviated as D. violacea) lives as a polyp colony that looks like coral. It is purple with white tips. There are an equal number of catching-defending polyps on both sides of a row of irregularly shaped holes on the surface of the colony.

Geographic range: D. violacea lives in the Indian and Pacific oceans and in the Red Sea. Specific distribution map not available.

Habitat: D. violacea lives in shallow seawater.

Diet: Scientists do not know what D. violacea eats.

Behavior and reproduction: Scientists do not know how D. violacea behaves or reproduces.

Distichopora violacea and people: D. violacea has no known importance to people.

Conservation status: D. violacea is not threatened or endangered. ∎

NO COMMON NAME (Aglantha digitale): SPECIES ACCOUNTS

Physical characteristics: Aglantha digitale (abbrviated as A. digitale) is a medusa with a cylindrical umbrella that is about three-eighths of an inch to 1.5 inches (10 to 40 millimeters) high. The umbrella is about twice as tall as it is wide and has a small cone at the top. The mouth has four simple lips. Eight long, sausage-shaped sex glands hang freely inside the hydroid just under the top of the umbrella. A. digitale has eighty or more very short tentacles at the base of its umbrella.

Geographic range: A. digitale lives in the Arctic Ocean and in the northern parts of the Atlantic and Pacific oceans. Specific distribution map not available.

Habitat: A. digitale lives in the open ocean from the surface down to about 2000 feet (600 meters).

Diet: A. digitale eats animal plankton.

Behavior and reproduction: A. digitale uses its entire umbrella for jet propulsion. Males release sperm and females release eggs into the water. When fertilization occurs, a free-floating larva develops.

Aglantha digitale and people: A. digitale has no known importance to people.

Conservation status: A. digitale is not threatened or endangered. ∎

NO COMMON NAME (Aequorea victoria): SPECIES ACCOUNTS

Physical characteristics: The polyp colonies of Aequorea victoria (abbreviated as A. victoria) are small and unbranched and have creeping stems. The outer covering is thin and has a cap made of many flaps. The colony of A. victoria has twenty tentacles connected by a membrane at the base. The medusae of A. victoria are small when they leave the polyp colony, having only two tentacles and four canals. They can grow up to almost 5 inches (13 centimeters) wide and 1.5 inches (4 centimeters) high with as many as 150 tentacles. The umbrella is saucer to half-dome shaped, and the mouth is fringed.

Geographic range: A. victoria lives in the northeastern part of the Pacific Ocean. Specific distribution map not available.

Habitat: The polyp colonies of A. victoria grow on mussel shells. The medusae are plankton in the coastal and open parts of the sea.

Diet: Scientists do not know what the polyp colonies of A. victoria eat. The medusae feed on fish larvae and on jellylike plankton.

Behavior and reproduction: A. victoria secretes, or gives off, a protein that produces a blue light when it reacts with the calcium in seawater. The polyp form and newly released medusae are tiny and rarely seen. Medusa production is very intense, because the species is found in swarms. The eggs and sperm released by these swarms of medusae produce great quantities of larvae.

Aequorea victoria and people: The light-producing protein secreted by A. victoria is used to measure cellular levels of calcium.

Conservation status: A. victoria is not threatened or endangered. ∎

PORTUGUESE MAN OF WAR (Physalia physalis): SPECIES ACCOUNTS

Physical characteristics: A Portuguese man of war colony consists of a large, purplish blue gas-filled balloon that floats on the sea surface carrying the polyps. The balloon can reach a length of almost 12 inches (30 centimeters). A sail running lengthwise along the top of the balloon allows the man of war to move with the wind. The polyps form clusters at the mouth end of the balloon. Each cluster has a reproductive polyp and a feeding-defending polyp. Each feeding-defending polyp has a tentacle covered with extremely poisonous stingers. The tentacles can be several yards (meters) long.

Geographic range: Portuguese men of war live in the warm waters of the Pacific, Indian, and Atlantic oceans and in the Mediterranean Sea. Specific distribution map not available.

Habitat: Portuguese men of war live at the surface of the ocean. They frequently are blown ashore by strong winds.

Diet: Portuguese men of war eat plankton and small fish that they kill with their tentacles.

Behavior and reproduction: Portuguese men of war gather in large groups called navies. The sail on a man of war is either right-oriented or left-oriented, much as people are right-handed or left-handed. The wind moves "left-handed" men of war to the right and right-handed men of war to the left. Portuguese men of war have separate sexes and release their eggs and sperm into the sea, where fertilization occurs and larvae develop. The larvae bud and grow into adults that reproduce sexually.

Portuguese men of war and people: Portuguese men of war inflict very painful stings on swimmers who become tangled in their tentacles and on waders who step on them.

Conservation status: Portuguese men of war are not threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Carson, Rachel. The Edge of the Sea. 1955. Reprint, Boston: Mariner, 1998.

Niesen, Thomas M. The Marine Biology Coloring Book. 2nd ed. New York: HarperResource, 2000.

Web sites:

Borneman, Eric. "Venomous Corals: The Fire Corals." Reefkeeping.http://www.reefkeeping.com/issues/2002-11/eb/ (accessed on January 26, 2005).

"Introduction to Hydrozoa." University of California, Berkeley, Museum of Paleontology. http://www.ucentimetersp.berkeley.edu/cnidaria/hydrozoa.html (accessed on December 17, 2004).

"Portuguese Man-of-War (Bluebottle—Physalia spp—Hydroid)." Hawaiian Lifeguard Association. http://www.aloha.com/lifeguards/portugue.html (accessed on January 27, 2005).