Brachiopods

Brachiopods are a phylum of marine animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection. Articulate brachiopods have toothed hinges and simple opening and closing muscles, while inarticulate brachiopods have untoothed hinges and more complex muscles. In a typical brachiopod a stalk-like pedicle projects from an opening in the hinge or from a hole in the larger valve, attaching the animal to the seabed but clear of silt that would obstruct the opening.

Brachiopods have a mantle that secrete and lines the shell, and encloses the internal organs. The body occupies typically about one-third of the internal space inside the shell, nearest the hinge. The rest of the mantle encloses a water-filled space containing the lophophore, a crown of tentacles that filters food particles out of the water. In all species the lophophore is supported by cartilage and by a hydrostatic skeleton. The lophophore filters food, mostly phytoplankton, out of the water. From there the food is transported in succession to: the grooves along the bases of the lophophore's tentacles; the mouth; pharynx; oesophagus; and finally the stomach, where the food is digested. Nutrients are transported from the stomach throughout the coelom (main body cavity), include the mantle lobes, by cilia. The wastes produced by metabolism are broken into ammonia, which is eliminated by diffusion through the mantle and lophophore. The lophophore and mantle are the only surfaces that absorb oxygen and eliminate carbon dioxide. Oxygen seems to be distributed by the fluid of the coelom. The heart is above the stomach, and the blood vessels connect it to the major organs. However, the main function of the blood may be to deliver nutrients. The maximum oxygen consumption of brachiopods is low, and their minimum requirement is not measurable.

The "brain" of adult articulates consists of two ganglia, one above and the other below the oesophagus. Adult inarticulates have only the lower ganglion. Nerves run to the lophophore, the mantle lobes and the muscles that operate the valves. Many brachiopods close their valves if shadows appear above them, but the cells responsible for this are unknown. Some brachiopods have statocysts which detect changes in the animals' balance.

Lifespans range from 3 to over 30 years. Ripe gametes (ova or sperm) float from the gonads into the main coelom and then exit into the mantle cavity. The larvae of inarticulate brachiopods are miniature adults, with lophophores that enable the larvae to feed and swimming for months, until the animals become heavy enough to settle to the seabed. Larvae of articulate species are different from the adult forms, live only on yolk, remain only among the plankton for only a few days, and then metamorphosing.

In addition to the traditional classification into inarticulate and articulate brachiopods, two approaches appeared in the 1990s: grouping the inarticulate Craniida with articulate brachiopods, as both used the same material in the mineral layers of the shell; and making the Craniida a third group, as their outer organic layer is different from that of either the others. However, some taxonomists believe it is premature to suggest higher levels of classification such as order, and recommend a bottom-up approach that identifies genera and then groups these into intermediate groups. Traditionally brachiopods have been regarded as members of or as a sister group to the deuterostomes, a super-phylum which includes chordates and echinoderms. One type of analysis of brachiopods' evolutionary relationships has always placed brachiopods as protostomes, while another type has split between placing brachiopods among the protostomes or the deuterostomes.

In 2003 it was suggested that brachiopods evolved from an ancestor similar to Halkieria, a slug-like animal with "chain mail" on its back and a shell at the front and rear end, and that the ancestral brachiopod converted its shells into a pair of valves by folding the rear part of its body under its front. However, new fossils found in 2007 to 2008 showed that tommotiids' "chain mail" formed the tube of a sessile animal, and that one resembled phoronids, which are close relatives or a sub-group of brachiopod, while the other tommotiid bearing two symmetrical plates that might be an early form of brachiopod valves. Lineages that have both fossil and extant brachiopods appeared in the early Cambrian, Ordovician and Carboniferous periods respectively. Other lineages have arisen and then become extinct, sometimes during severe mass extinctions. At their peak in the Paleozoic era the brachiopods were among the most abundant filter-feeders and reef-builders, and occupied other ecological niches, including swimming in the jet-propulsion style of scallops. Brachiopod fossils have been useful indicators of climate changes during the Paleozoic era. However, after the Permian-Triassic extinction event, brachiopods recovered only a third of their former diversity. A study in 2007 concluded that brachiopods were especially vulnerable to the Permian-Triassic extinction, as they built calcareous hard parts (made of calcium carbonate) and had low metabolic rates and weak respiratory systems. It was often thought that brachiopods were in decline after the Permian-Triassic extinction, and were out-competed by bivalves. However, a study in 1980 concluded that: both brachiopods and bivalves increased all the way from the Paleozoic to modern times, but bivalves increased faster; after the Permian-Triassic extinction, brachiopods for the first time were less diverse than bivalves.

Brachiopods live only in the sea, and most species avoid locations with strong currents or waves. Articulate species have larvae that settle in quickly and form dense populations in well-defined areas, while inarticulate larvae swimming for up to a month and have wide ranges. Brachiopods now live mainly in cold and low-light conditions. Fish and crustaceans seem to find brachiopod flesh distasteful and seldom attack them. Among brachiopods only the lingulids have been fished commercially, on a very small scale. One brachiopod species may be a measure of environmental conditions around an oil terminal being built in Russia on the shore of the Sea of Japan.