LANCELETS: Cephalochordata

PHYSICAL CHARACTERISTICS

Lancelets look like slender fish without eyes. They are 0.4 to 3 inches (1 to 8 centimeters) long and whitish to creamy yellow, sometimes with a tint of pink. Mucus secreted by cells in their body covering gives lancelets a pearly sheen. V-shaped lines on the outside of lancelets' bodies outline muscle blocks inside the body. Small bristles surround the mouth. A fin runs along the entire back of the animal and extends forward into a short fin over the snout and backward into a tail fin. There is a fin on the belly toward the rear of the animal. Paired fin-like folds in front of the belly fin extend to the front of the lancelet.

Lancelets have a notochord (NOH-tuh-koord), which is a flexible rod of cells supporting the body. The rest of the skeleton is made up of small, flexible rods between the gill slits and supporting the mouth bristles. A nerve cord runs along the top of the notochord, and various types of sensing cells are distributed in the body covering, especially near the snout. Lancelets have blood vessels but no heart. The chest has about two hundred gill slits that do not open to the outside but empty into a chamber inside the body wall. The chamber empties to the outside through a hole on the belly of the lancelet.

GEOGRAPHIC RANGE

Lancelets live in all the oceans of the world in warm and cool waters.

HABITAT

Lancelets live in sandy bottoms near the shore. The larvae may drift over long distances before settling. Larvae (LAR-vee) are animals in an early stage that change form before becoming adults.

DIET

Lancelets eat plant plankton and diatoms. Plankton is microscopic plants and animals drifting in water. Diatoms (DYE-uh-tahms) are a type of algae that have a shell. Algae (AL-jee) are plantlike growths that live in water and have no true roots, stems, or leaves.

BEHAVIOR AND REPRODUCTION

Lancelets can swim vigorously forward and backward, but they spend most of their time buried halfway in the sand. They live in masses of more than nine thousand animals per square yard (meter). Depending on the coarseness of the sand, lancelets assume different feeding positions. In coarse sand they bury their entire body with only the head exposed to the water. In fine sand they lie on the bottom. Lancelets continuously produce a mucus net that they move over their gill slits to capture food particles in the water. The food particles and mucus net are rolled into a mass that passes into the digestive tract.

Lancelets have separate sexes. There are equal numbers of males and females in a population. Eggs and sperm are released into the water, where the fertilized (FUR-teh-lyzed) eggs, those that have united with sperm, develop into larvae. When twelve to fifteen pairs of gill slits have formed, the larvae sink to the bottom and transform into young lancelets. From that point on the animals grow, and additional gill slits and muscle segments develop while the reproductive organs grow to maturity.

LANCELETS AND PEOPLE

In southern China local fishermen using traditional techniques fish for and eat lancelets. The greatest importance of lancelets to people, however, is in the study of evolution.

LANCELETS AND VERTEBRATES

Scientists consider lancelets the closest living relatives to vertebrates (VER-teh-brehts), or animals with a backbone. The two groups share traits such as the organization of the main body muscles into separate segments and the organization of the blood vessels. Lancelets also have a structure that is a simple form of the liver of vertebrates. The nerve cord of lancelets has a central canal that is enlarged at the head end, making it similar to parts of the vertebrate brain.

CONSERVATION STATUS

Lancelets are not considered threatened or endangered.

FLORIDA LANCELET (Branchiostoma floridae): SPECIES ACCOUNT

Physical characteristics: Florida lancelets are whitish to creamy yellow, sometimes with a tint of pink. They have fifty-six to sixty-four muscle blocks and reach a length of 2.5 inches (6 centimeters).

Geographic range: Florida lancelets live in the Gulf of Mexico.

Habitat: Florida lancelets live on sandy bottoms in shallow seawater.

Diet: Florida lancelets eat plant plankton and diatoms.

Behavior and reproduction: Florida lancelets can swim but spend most of their time buried halfway in the sand. They capture food particles with a mucus net. They breed in Tampa Bay, Florida, from late spring to late summer. They can release eggs and produce and release more eggs in the same breeding season.

Florida lancelets and people: Florida lancelets are used for studies of embryonic development.

Conservation status: Florida lancelets are not considered threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Gee, Henry. Before the Backbone. London, Weinheim, New York: Chapman & Hall, 1996.

Simmer, Carl. Evolution: The Triumph of an Idea. New York: Harper-Collins, 2001.

Periodicals:

Stokes, M. D., and N. D. Holland. "Ciliary Hovering in Larval Lancelets (Amphioxus)." Biological Bulletin (June 1995): 231–233.

Web sites:

"Branchiostoma belcheri." Community-Based Conservation Management, China and Vietnam. http://ois.xmu.edu.cn/cbcm/english/resource/save/save04.htm (accessed on March 3, 2005).

"Introduction to the Cephalochordata." University of California, Berkeley, Museum of Paleontology. http://www.ucmp.berkeley.edu/chordata/cephalo.html (accessed on March 3, 2005).