Brachiopods

The Brachiopoda is a phylum of marine animals whose bodies are protected by bivalved external shells. Although brachiopods are not very diverse in modern seas, with about 300 living species, they occur in a diversity of habitats, from the intertidal zone to the deep sea. They were much more diverse historically, with at least 12,000 fossil species known. Because the brachiopod fossil record is continuous from the Early Cambrian (about 570 million years ago) through the modern era, bra-chiopods are important in stratigraphy and in studying rates and patterns of evolution. Studies of living species have in turn been useful in understanding the lives of species known only as fossils.

The body of an adult brachiopod (which includes the gut, gonads, and shell muscles) is covered by a dorsal and a ventral valve of the shell.

Brachiopods are sometimes mistaken for bivalve mollusks (for example, clams), but those have lateral shell valves, not dorsal and ventral. The shell also encloses a large space anterior to the body, the mantle cavity. Projecting from the body into the mantle cavity are the two brachia (arms) of the lophophore, a large organ involved in feeding and respiration. The brachia, which are typically coiled in complex folds and spirals, bear fine tentacles whose cilia propel seawater through the mantle cavity and trap small particles carried in on the currents. Brachiopods feed on these particles. Most brachiopods release eggs and sperm into seawater, where they are fertilized and develop into planktonic larvae; these eventually return to the bottom and metamorphose into juveniles.

Brachiopods are divided into two groups, articulates and inarticulates. Articulates include species in which the two shell valves— composed of calcium carbonate—are connected by a hinge. Modern articulates live permanently attached to rocks (though some extinct forms lived unattached on soft substrates), attached by the pedicle, a stalk that protrudes from a hole in the posterior of the ventral valve. The guts of articulates are incomplete (the stomach opens into a blind intestine, and the remains of indigestible food must be shed from the mouth). In contrast, inarticulate brachiopods have shell valves that are not connected by a hinge and are usually made of chitin and calcium phosphate. Their guts are complete. Inarticulates have more

Figure 1

Diagrammatic Cross Section of an Articulate Brachiopod, Viewed from the Side

Figure 1

Diagrammatic Cross Section of an Articulate Brachiopod, Viewed from the Side

Source: Pearse, Vicki, et al. 1987. Living Invertebrates. Palo Alto, CA: Blackwell Scientific Publications, p. 664. (Reprinted with permission)

Note: The valves of the shell enclose the body and the mantle cavity. Only one of the two brachia of the lophophore is shown. The adductor muscles pull the two valves closed; the diductor muscles open them. The nephridium is the structure through which eggs or sperm (produced in the gonad) are released to the outside.

diverse ways of living than articulates: some live in burrows in sand or mud, and others live attached to rocks, cemented by one of their valves or a pedicle.

The evolutionary relationships between articulates and inarticulates and between bra-chiopods and other animal phyla are not well understood. Brachiopods appeared in the fossil record about 570 million years ago, with inarticulates appearing slightly earlier than articulates. Since then they have undergone several phases of diversification and extinction, with major crises including an end-Ordovician decline for inarticulates about 430 million years ago and an end-Permian decline for articulates about 245 million years ago. Modern species are not directly at risk of extinction as a result of human activities, because most are not edible or otherwise economically important.

—Bruno Pernet

See also: Evolutionary Biodiversity; Extinction, Direct

Causes of; Paleontology

Bibliography

James, Mark A., et al. 1992. "Biology of Living Brachiopods." Advances in Marine Biology 28: 175-387;

Rudwick, Martin J. S. 1970. Living and Fossil Brachiopods. London: Hutchinson.

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