Brachiopods

Brachiopods, or lampshells, are a phylum of small marine animals with a two-valved shell that, at first glance, resemble bivalved mollusks such as clams. The resemblance, however, is quite superficial. The orientation of the shells of brachiopods is very different from that of bivalved mollusks, and brachiopods have two additional structures virtually unique to them, the lophophore (a ciliated feeding apparatus) and pedicle (a muscular stalk). The valves of the brachiopod shell are symmetrical—being dorsal and ventral and covering the upper and lower parts of the body, with the hinge at the posterior end. In contrast, shells of a clam are on the right and left sides of the body, with the hinge dorsal.

The lophophore, which occupies much of the space in the anterior portion of the shell, resembles a circle of small tentacles surrounding the mouth. Each tentacle bears large numbers of tiny cilia that, when beating, create a water current that draws in water and suspended food particles down toward the mouth. Once trapped in the lophophore, tiny plankton are passed along special grooves that lead to the mouth, and from there, to a stomach and intestine. The water current also maintains a steady supply of oxygen to the animal. The lophophore is a complex feeding apparatus found in only a few other groups of marine and freshwater animals, chiefly the Bryozoa and Phoronida. Collectively, these three groups of marine invertebrates are sometimes referred to as lophophorates.

Brachiopods can be divided into two major groups, articulate and inarticulate, based on their use of the pedicle. The anatomy of an articulate brachiopod. Illustration by Hans & Cassidy. Courtesy of Gale Group. Articulate brachiopods are fixed directly to a hard substrate by the pedicle, a short piece of connective tissue at the posterior end of the shell. The brachiopod has a very limited range of motion and remains, for the most part, sessile. The inarticulate brachiopods are not fixed to one location. Instead, they use their specialized muscular pedicles to burrow through sand and other soft sediments. At the distal end of the pedicle a sticky substance is secreted that forms a sand anchor, enabling them to withdraw deeper into the sediment by contracting the muscular pedicle when threatened. The pedicle ranges from about 0.1 in (2.5 mm) in some species to more than 7.9 in (20 cm) in others. Some bivalved mollusks, such as oysters, also attach themselves to the substratum, but they lack a pedicle.

Brachiopods first appeared about 500 million years ago during the Paleozoic era, as shown by their common occurrence as fossils in many parts of the world. This accounts for their great interest to geologists. Over 30,000 species are believed to have evolved over the years. Today, roughly 300 living species are know to exist. Most lampshells are found in deepwater, so their shells are not commonly found on the beach. In the Pacific, Lingula unguis is commonly found living in vertical burrows in sand and mud. In Maine, Terebratulina septentrionalis is sometimes exposed to the air at low tide. Five species of brachiopods are readily collected from rocks off the shores of the South Island of New Zealand.

Most lampshells are dioecious, producing either male or female gametes. In the majority of species, when the gonads have ripened, the gametes are released into the coelom and pass through the nephridia to the sea. Fertilization takes place outside the body. Some species, however, brood their young by retaining their eggs and awaiting the arrival of male gametes with the incoming flow of water. The fertilized eggs develop into free-swimming larvae that are capable of feeding. Further development of the larvae depends on the species: in most articulate brachiopods, larvae undergo a transformation of the body shape and structure before settling, while the larvae of inarticulate brachiopods already resembles the final adult stage apart from its diminutive size. As the shell develops in the latter species, the larvae are encouraged to settle on the seabed.

Since brachiopods are fixed in position after the larval phase, there is little observable behavior beyond the opening and closing of valves. The valves may remain closed for periods of 5-22 hours, during which there is no feeding and little or no oxygen uptake. Lampshells obtain oxygen from the water that passes over the lophophore, which functions as a respiratory surface although brachiopods tolerate lack of oxygen (anoxia) well. Oxygen consumption rates of brachiopods are generally lower than those of bivalves of similar size.

The food of brachiopods is mainly algal cells of the phytoplankton, which are strained out of water currents passing over the lophophore in a process called filter-feeding. Clearance rates of several species are in the same range or a little lower than rates shown by bivalve mollusks. Research on the physiology of brachiopods is heavily slanted toward comparisons with mussels and clams because of the obvious similarity in protective shell and mode of feeding. The fossil record indicates the rise of one group followed by the decline of the other, as if they might have been competitors. Unlike mollusks, brachiopods are not significantly preyed upon or harvested. Some observations suggest that fish find lampshells distasteful.