Brachiopod

Abrachiopod is a type of bottom-dwelling marine invertebrate with a shell similar to that of a clam or mussel. Also known as lampshells, brachiopods belong to the phylum Brachiopoda, which has existed for around 550 million years. Although remarkably abundant during the Paleozoic era as their vast fossil record indicates, brachiopods largely died out following the Permian-Triassic extinction that occurred around 250 million years ago. Only about 300 to 400 species found mostly in the cold waters of Antarctica, Alaska, New Zealand, and the Pacific Northwest survive today. Despite the visual similarities they share with clams and other bivalve shellfish, brachiopods are actually quite distinct from their modern counterparts. Most notably, brachiopods have a pair of asymmetrical shells and an appendage called a pedicle that they use to anchor themselves to the ocean floor or other surfaces. While they are few in number today, brachiopods are an important part of the animal kingdom that plays a key role in scientists' understanding of the evolution of life on Earth.

Background

The term brachiopod is derived from the Greek words brachio, which means "arm," and poda, which means "foot." This name reflects the fact that brachiopods have both an armlike internal structure used for feeding and respiration called the lophophore, and a footlike stalk known as the pedicle.

Fossil records show that the first brachiopods began to appear during the Early Cambrian period. Several different orders and classes of brachiopods developed over time. The pentamerida class, for example, first appeared during the Upper Cambrian epoch. Other types of brachiopods that eventually emerged included the articulates, inarticulates, and lingulates. While the diversity of all types of brachiopods increased during the Ordovician period, the articulates quickly became dominant. While the inarticulates and lingulates eventually fell into decline and were reduced to a handful of stable species, it is these species that ultimately survived into the modern world. The articulates, on the other hand, reached their greatest evolutionary extent during the Ordovician, when new orders like the strophomenids, rhynchonellids, and spiriferids were added. During the subsequent Silurian period, another new articulate order called the terebratulids appeared as some of the earlier orders began to decline. The Devonian period, which immediately followed the Silurian, was a time of both expansion and extinction for brachiopods. Most notably, the pentamerids were lost as a result of the mass invertebrate extinction that took place at that time. While brachiopods continued to expand even in the later stages of the Paleozoic era, that expansion abruptly stopped as a result of the major extinctions that occurred between the Permian and Triassic periods. This extinction wiped out almost all the articulate brachiopods, leaving only the terebratulids and the rhynchonellids. These two orders account for the only articulate brachiopods that still survive today.

Scientists believe that, in addition to the Permian-Triassic extinction, the gradual decline of brachiopods in the Mesozoic era was the result of increasing competition from bivalve organisms like clams. The extinction of the strophomenids allowed bivalve species to establish habitats in the sediment. While these habitats offered bivalves protection from predators, their presence effectively denied the same to brachiopods. As a result, the decline of brachiopods was worsened by predation.

Overview

Brachiopods are small marine organisms that bear a distinct resemblance to mollusks such as clams or mussels. That being said, however, brachiopods have a number of anatomical features that set them apart from other similar species. Most notably, they have a footlike stalk called a pedicle and a special feeding apparatus called a lophophore. Brachiopods are also distinguished by the symmetry of their valves. In brachiopods, the line of symmetry runs straight through each valve so that the left and right sides of each shell appear identical even though the two shell halves are actually different sizes and shapes. In bivalves like clams, the line of symmetry runs between the shells so that the two halves appear identical to each other even though the left and right sides of each shell are different.

The lophophore is a feeding apparatus that functions as a filter that separates food particles from water. Essentially, the lophophore is a circle of small tentacles that surrounds the brachiopod's mouth. Each tentacle holds numerous tiny hairlike organelles called cilia. The brachiopod beats the cilia to create a current that pulls water toward its mouth. At the same time, the cilia also filter plankton out of the moving water. The filtered plankton subsequently travels down special grooves leading to the mouth so that it can be taken into the brachiopod's digestive system. The current created by the lophophore also helps to ensure that the brachiopod has a continuous oxygen supply.

The pedicle is an equally important anatomical feature that brachiopods rely on for their survival. Further, the two major categories of brachiopods—articulate and inarticulate—are differentiated based on how each type uses the pedicle. The pedicle serves as a sort of anchor for articulate brachiopods, firmly fixing them to a hard surface. Articulate brachiopods remain attached to that surface with the help of the pedicle for their entire lives. Inarticulate brachiopods, meanwhile, use the pedicle as a muscular tool for burrowing through sand or other soft sediments. Given this ability, inarticulate brachiopods are somewhat more mobile than their articulate counterparts.

Articulate and inarticulate brachiopods are also distinguished by certain aesthetic and structural differences in their shells. The generally clam-shaped shells of articulate brachiopods are made of a substance called calcium carbonate and feature interlocking pegs, or teeth, that form a hinge between the valves. While one of the valves holds the teeth themselves, the other contains the sockets into which the teeth fit. Inarticulate brachiopods have oval-shaped shells made of calcium phosphate. Unlike those of articulate brachiopods, the shells of inarticulate brachiopods do not feature hinge teeth or sockets.

Regardless of type, most brachiopods feed on plankton and any particles of dead organic matter that they can filter out of water. As opposed to clams, mussels, and other mollusks, brachiopods are not usually preyed upon by other animals. In terms of reproduction, most brachiopods are dioecious, which means that there are both male and female members of the species. To produce offspring, male and female brachiopods each release germ cells that meet and fertilize in the open sea to form zygotes.

Bibliography

"Brachiopod." Sam Noble Museum, samnoblemuseum.ou.edu/common-fossils-of-oklahoma/invertebrate-fossils/brachiopods. Accessed 8 Nov. 2017.

"Brachiopoda." The Digital Atlas of Ordovician Life, www.ordovicianatlas.org/atlas/brachiopoda. Accessed 8 Nov. 2017.

"Brachiopoda Lamp-Shells." Animal Diversity Web, animaldiversity.org/accounts/Brachiopoda. Accessed 8 Nov. 2017.

"Brachiopods." British Geological Survey, www.bgs.ac.uk/discoveringGeology/time/Fossilfocus/brachiopod.html. Accessed 8 Nov. 2017.

"Brachiopods." Jurassic Coast, jurassiccoast.org/fossilfinder/focus-on-fossils/brachiopods. Accessed 8 Nov. 2017.

"Brachiopods." Queensland Museum, www.qm.qld.gov.au/~/media/Documents/Learning+resources/QM/Resources/Fact+Sheets/fact-sheet-brachiopods.pdf. Accessed 8 Nov. 2017.

Carlson, Sandra J. "The Evolution of Brachiopoda." Annual Review of Earth and Planetary Sciences, vol. 44, 2016, pp. 409–38.

Nannan, Zhang. "Scientists Construct Organs

"What Are Brachiopods?" WiseGeek, www.wisegeek.com/what-are-brachiopods.htm. Accessed 8 Nov. 2017.