Phoronids: Phoronida

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PHORONIDS: Phoronida



Phoronids (for-OH-nihds) are long, thin, wormlike animals that live inside slender tubes that they make with their own bodies. When relaxed, adults normally measure up to 18 inches (450 millimeters), but some phoronids can extend their bodies nearly five times that length. Their body thickness measures 0.006 to 0.2 inches (0.15 to 5 millimeters). There is no distinct head. The slitlike mouth is covered by a flap of skin and is found between the horseshoe-shaped, tentacle-bearing ridges that make up the lophophore (LO-fo-for). The tentacles of the lophophore help phoronids to breathe, eat, and protect themselves. The ends of the horseshoe-shaped ridges are coiled like springs. The slender body trunk is swollen, or bulb-shaped, at the end. This region of the body contains most of the internal organs.

Inside they have a body cavity and a u-shaped digestive system. The nervous system includes a nerve center between the mouth and the anus. The anus is located at the end of the digestive system and is where solid waste leaves the body. At the base of the lophophore is a ringlike nerve structure. Phoronids have blood that circulates inside a system of tubes, or vessels. They have a pair of kidneylike organs that not only remove wastes from the blood, they also work as part of the reproductive system.


Phoronids are found in all oceans and seas, except the Antarctic Ocean.


Adult phoronids live along coastlines, from the zones that are affected by the tides, rarely going deeper than 180 feet (54 meters). A few species have been found at depths of approximately 1,310 feet (400 meters). They live inside flexible, layered, parchmentlike tubes. Some species live alone, buried straight up and down in sand, mud, or fine gravel. Others live in large groups, their tangled tubes often covering rocks and shells. They are free to leave their tubes and make another one somewhere else. One species intertwines its tube with that of a tube-living sea anenome. Some species do not make a tube. They bore into rock and live inside the tunnel.


Phoronids eat algae (AL-jee), small animal larvae (LAR-vee), and other plankton floating in the water.


Phoronids have extremely flexible bodies. They will quickly direct their lophophores to take full advantage of food-carrying ocean currents. Tiny bristles, or cilia (SIH-lee-uh), on their lophophores help carry bits of food trapped by sticky mucous on the tentacles down special grooves all the way to the mouth. They are also quick to respond to danger and will withdraw inside their tubes when they are threatened. If injured by a predator, phoronids can regenerate lost or damaged body parts in just a few days.

Phoronids reproduce in different ways. Sometimes both males and females are required to reproduce. In other species, individuals may have both male and female reproductive organs at the same time. This way, every phoronid is a potential mate. Phoronids can also reproduce without eggs and sperm at all. Instead, individuals simply divide their bodies, breaking off at the middle into halves. Each half quickly develops into a complete animal.


The name Phoronida comes from the Latin Phoronis, the last name of the mythological Io. Zeus, the king of gods, turned Io into a cow because he wanted to hide her from his wife. Io Phoronis wandered the Earth for many years, until she was returned to her original body. Phoronids were first discovered and known for many years only as larvae. It was only much later that they were associated with their tube-building adults.

Reproduction usually takes place from spring through fall. In most species, eggs and sperm are released, and fertilization usually takes place in the water. In a few species, the males produce sperm packets and transfer them directly to the female. Fertilization takes place inside the female's body cavity. The eggs develop in special tissues in the lophophore or inside the body cavity. The newly hatched larvae of all phoronids swim freely and live with other plankton. After about twenty days they settle to the bottom. In less than thirty minutes, they can transform into slender, young phoronids ready to burrow into their surroundings.


Phoronids do not impact people or their activities.


No species is considered threatened or endangered.


Physical characteristics: Fully stretched out, these flesh-colored or clear-bodied phoronids may reach a length of 5 inches (120 millimeters) and are 0.02 to 0.08 inches (0.5 to 2 millimeters) thick. The lophophore sometimes is clear and has white spots. The lophophore is horseshoe-shaped, or coiled with a single spiral, and has as many as 230 tentacles. Each tentacle is 0.08 to 0.2 inches (2 to 5 millimeters) long.

Geographic range: This species is found in the eastern and western Pacific Ocean and the northwestern Atlantic Ocean.

Habitat: This species lives along coastlines, in zones affected by tides down to depths of approximately 33 feet (10 meters). They are found in groups of intertwined tubes covering rocks, wood, and other hard surfaces. They sometimes burrow in rock, algae, coral, and the shells of mollusks (mussels, clams, snails, and their relatives).

Diet: They capture plankton with their sticky tentacles.

Behavior and reproduction: They protect themselves by quickly withdrawing inside their tubes. Injured animals can replace damaged or lost body parts in just a few days.

This species has both male and female reproductive organs. The fertilized eggs are brooded in two groups on the tentacles. Fully grown larvae are approximately 0.03 inches (0.8 millimeters) long. Adults also reproduce by dividing their bodies in two.

Phoronis ijimai and people: This species is not known to impact people or their activities.

Conservation status: Phoronis ijimai is not considered threatened or endangered. ∎



Emig, C. C. "The Biology of Phoronida." Advances in Marine Biology (1982): 1-89.

Garey, J. R., and A. Schmidt-Rhaesa. "The essential role of "minor" phyla in molecular studies of animal evolution." American Zoologist 38, no. 6 (December): 907-917.

Web sites:

Introduction to the Phoronida. (accessed on March 28, 2005).

[email protected] (accessed on March 28, 2005).

"Phoronida. Horseshoe Worms." (accessed on March 28, 2005).

"The Phoronida." (accessed on March 28, 2005).