Brachiopods ( / ˈ b r æ k i oʊ ˌ p ɒ d / ), phylum Brachiopoda, are a group of animals with hard "valves" (shells) on the top and bottom surfaces. This is different from bivalve molluscs, which have shells on the left and right sides. Brachiopod shells are hinged at the back and can open at the front for feeding or close for protection.
There are two main types of brachiopods: articulate and inarticulate. Articulate brachiopods have tooth-and-groove structures in their shell hinges, while inarticulate brachiopods do not. This feature helps scientists identify these groups in fossils. Articulate brachiopods have simple, vertical muscles to open and close their shells. Inarticulate brachiopods have weaker, toothless hinges and a more complex system of vertical and diagonal muscles to keep their shells aligned. Many brachiopods have a stalk-like structure called a pedicle that extends from one shell. This pedicle helps the animal stay attached to the seabed and avoid sediment that might block the opening.
Brachiopods can live for three to over thirty years. When they are ready to reproduce, their eggs or sperm move from the gonads into the main body cavity and then into the mantle cavity. The larvae of inarticulate brachiopods look like tiny adults with lophophores (feeding organs made of tentacles) that help them swim and eat for months before settling on the seabed. The larvae of articulate brachiopods look like small, round blobs with yolk sacs and stay in the water for only a few days before changing into adults and leaving the water.
Brachiopods only live in the ocean and avoid areas with strong currents or waves. Articulate brachiopod larvae settle quickly and form large groups in specific areas, while inarticulate larvae swim for up to a month and live in a wide range. Fish and crustaceans usually avoid eating brachiopods.
The word "brachiopod" comes from the Ancient Greek words brachion ("arm") and podos ("foot"). They are sometimes called "lamp shells" because the curved shells of the Terebratulida class resemble oil lamps.
Brachiopods look similar to bivalves but are not closely related. They developed their two-shell structure independently, an example of convergent evolution. Brachiopods belong to the group Lophophorata, which also includes Bryozoa and Phoronida. These groups share the feature of lophophores.
Brachiopods are believed to have evolved from "tommotiid" ancestors during the Early Cambrian period. They were very diverse during the Paleozoic era, more so than bivalves. However, their diversity was greatly reduced by the end-Capitanian and end-Permian mass extinctions. Their numbers never recovered to Paleozoic levels, and bivalves became more dominant in marine ecosystems. Today, there are about 400 living brachiopod species, compared to around 9,200 bivalve species. Most brachiopods now live in cold, low-light waters.
Only lingulid brachiopods (such as Lingula species) have been fished commercially, but on a very small scale.
Anatomy
Modern brachiopods range in size from 1 to 100 millimeters (0.039 to 3.937 inches) long. Most species are about 10 to 30 millimeters (0.39 to 1.18 inches) long. The largest living brachiopod is Magellania venosa. The largest brachiopods ever found, Gigantoproductus and Titanaria, were up to 30 to 38 centimeters (12 to 15 inches) wide and lived during the Upper Carboniferous period.
Brachiopods have two shell parts, called valves, that cover the top (dorsal) and bottom (ventral) surfaces of the animal. This is different from bivalve mollusks, whose shells cover the sides. The valves are not mirror images of each other and have their own symmetrical shapes. The shell forms during growth using genes that are also used by mollusks, including homeobox genes.
The brachial valve is usually smaller and has brachia (arms) on its inner surface. These arms support the lophophore, which helps the brachiopod feed and breathe. The pedicle valve is usually larger and has an opening near the hinge for the pedicle, a stalk-like structure that attaches the brachiopod to the seafloor. Some scientists argue that the terms "dorsal" and "ventral" are not always accurate because the "ventral" valve may have formed from a fold of the upper surface. In most living brachiopods, the ventral valve lies above the dorsal valve when the animal is in its natural position.
In many living articulate brachiopods, both valves are convex and may have growth lines or other patterns. Inarticulate lingulids, which burrow into the seabed, have flatter, smoother valves that are similar in size and shape.
Articulate brachiopods have a tooth-and-socket system that allows the valves to hinge and lock together. Inarticulate brachiopods do not have this system; instead, their valves are held together by muscles.
All brachiopods have adductor muscles inside the pedicle valve that close the valves by pulling on the brachial valve. These muscles include "quick" fibers for emergency closing and "catch" fibers that keep the valves closed for long periods. Articulate brachiopods open their valves using abductor muscles (diductors) near the rear of the shell. Inarticulate brachiopods open their valves by reducing the size of the coelom (body cavity), which pushes the valves apart. Both types open their valves to about a 10-degree angle. Inarticulate brachiopods can also use their muscles to operate the valves like scissors, which helps lingulids burrow.
Each valve has three layers: an outer periostracum made of organic compounds and two biomineralized layers. Articulate brachiopods have a protein-based periostracum, a calcite layer beneath it, and a mix of proteins and calcite inside. Inarticulate brachiopods have similar layers but with different compositions that vary among species. Some brachiopods, like Terebratulida, have punctate shells with tiny canals that connect to the mantle. These canals allow tissue to extend into the shell. Impunctate shells are solid, while pseudopunctate shells have bumps formed from calcite rods and are only found in fossils.
Lingulids and discinids, which have pedicles, have a matrix of glycosaminoglycans (long, unbranched sugars) that hold other materials like chitin, apatite, and collagen. Craniids, which attach directly to hard surfaces, have a chitin-based periostracum and calcite layers. Shell growth occurs in three ways: holoperipheral (equal growth around the edge), mixoperipheral (growth toward the other shell), and hemiperipheral (flat, forward growth).
Brachiopods have an epithelial mantle that produces the shell and encloses their internal organs. The body occupies about one-third of the shell’s interior, near the hinge. The rest of the space is filled with mantle lobes that surround a water-filled cavity where the lophophore is located. The coelom (body cavity) extends into the mantle lobes as a network of canals that deliver nutrients.
New cells on the mantle’s edge secrete the periostracum first, then later produce the mineralized layers of the shell. Most brachiopods have movable bristles (chaetae or setae) on their mantle edges, which may help with defense or sensing water flow.
In most brachiopods, hollow extensions of the mantle called diverticula pass through the shell layers into the periostracum. These structures may store chemicals, release repellents, or aid in respiration. Experiments show that covering the shell with petroleum jelly reduces oxygen consumption, suggesting the diverticula are involved in respiration.
Like bryozoans and phoronids, brachiopods have a lophophore, a U-shaped structure of tentacles that filters food from water. The lophophore is supported by cartilage and a hydrostatic skeleton (internal pressure). In larger species, the lophophore is folded into complex shapes like loops or coils to fit within the limited space inside the shell.
Biology
Water enters the lophophore from the sides of the open valves and leaves at the front of the animal. In lingulids, the entrance and exit channels are formed by groups of chaetae that act like funnels. In other brachiopods, the entry and exit channels are shaped by the structure of the lophophore. The lophophore captures food particles, especially phytoplankton (tiny photosynthetic organisms), and sends them to the mouth through brachial grooves along the bases of the tentacles. The mouth is a small slit at the base of the lophophore. Food passes through the mouth, muscular pharynx ("throat"), and oesophagus ("gullet"), all of which are lined with cilia and cells that secrete mucus and digestive enzymes. The stomach wall has branched ceca ("pouches") where food is digested, mainly within the cells.
Nutrients are transported throughout the coelom, including the mantle lobes, by cilia. Wastes produced by metabolism are turned into ammonia, which is removed by diffusion through the mantle and lophophore. Brachiopods have metanephridia, used by many phyla to excrete ammonia and other dissolved wastes. However, brachiopods show no signs of podocytes, which perform the first phase of excretion in this process, and their metanephridia seem to be used only to release sperm and ova.
Most of the food brachiopods eat can be digested, so they produce very little solid waste. The cilia of the lophophore can change direction to remove isolated particles of indigestible matter. If the animal encounters larger lumps of unwanted matter, the cilia lining the entry channels pause, and the tentacles in contact with the lumps move apart to form large gaps. Then, the tentacles slowly use their cilia to push the lumps onto the mantle lining. The mantle has its own cilia, which wash the lumps out through the opening between the valves. If the lophophore becomes clogged, the adductors snap the valves sharply, creating a "sneeze" that clears the blockage. In some inarticulate brachiopods, the digestive tract is U-shaped and ends with an anus that removes solids from the front of the body wall. Other inarticulate brachiopods and all articulate brachiopods have a curved gut that ends blindly, with no anus. These animals bundle solid waste with mucus and periodically "sneeze" it out using sharp contractions of the gut muscles.
The lophophore and mantle are the only surfaces that absorb oxygen and release carbon dioxide. Oxygen is distributed by the fluid of the coelom, which is circulated through the mantle and moved by contractions of the coelom lining or by beating of its cilia. In some species, oxygen is partly carried by the respiratory pigment hemerythrin, which is transported in coelomocyte cells. The maximum oxygen consumption of brachiopods is low, and their minimum requirement is not measurable.
Brachiopods also have colorless blood, circulated by a muscular heart located in the dorsal part of the body above the stomach. The blood flows through vessels that extend to the front and back of the body and branch to organs, including the lophophore at the front and the gut, muscles, gonads, and nephridia at the rear. The blood circulation does not appear to be completely closed, and the coelomic fluid and blood must mix to some extent. The main function of the blood may be to deliver nutrients.
The "brain" of adult articulates consists of two ganglia, one above and one below the oesophagus. Adult inarticulates have only the lower ganglion. Nerves from the ganglia and the commissures where they join extend to the lophophore, mantle lobes, and the muscles that operate the valves. The edge of the mantle likely has the greatest concentration of sensors. Although not directly connected to sensory neurons, the mantle's chaetae probably send tactile signals to receptors in the mantle's epidermis. 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' position.
Lifespans range from 3 to over 30 years. Adults of most species are of one sex throughout their lives. The gonads are masses of developing gametes (ova or sperm), and most species have four gonads, two in each valve. In articulates, the gonads lie in the channels of the mantle lobes, while in inarticulates, they lie near the gut. Ripe gametes float into the main coelom and then exit into the mantle cavity through the metanephridia, which open on either side of the mouth. Most species release both ova and sperm into the water, but females of some species keep the embryos in brood chambers until the larvae hatch.
The cell division in the embryo is radial (cells form in stacks of rings directly above each other), holoblastic (cells are separate, although adjoining), and regulative (the type of tissue into which a cell develops is controlled by interactions between adjacent cells, rather than rigidly within each cell). While some animals develop the mouth and anus by deepening the blastopore (a "dent" in the surface of the early embryo), the blastopore of brachiopods closes, and their mouth and anus develop from new openings.
The larvae of lingulids (Lingulida and Discinida) are planktotrophic, feeding and swimming as plankton for months. They resemble miniature adults, with valves, mantle lobes, a pedicle that coils
Taxonomy
Brachiopod fossils display many different shapes of shells and lophophores, but modern brachiopods have fewer differences and often show soft body features. Both old and current brachiopods have traits that make it hard to create a complete classification system based only on their physical features. The group has also seen convergent evolution, where similar traits developed in unrelated groups, and reversals, where newer groups lost traits found in older groups. Because of this, some scientists think it is too early to define higher classification levels like orders and instead suggest starting with identifying genera and then grouping them.
Other scientists believe some traits are stable enough to support higher classification levels, even though there are disagreements about what those levels should be. The traditional classification system was created in 1869, and two more systems were developed in the 1990s.
About 330 living brachiopod species are known, grouped into more than 100 genera. Most modern brachiopods belong to the rhynchonelliform group (Articulata).
DNA studies since the 1990s have changed how scientists understand relationships between organisms. It is now clear that brachiopods are not part of the Deuterostomia group (like echinoderms and chordates), as once thought, but instead belong to the Protostomia group, specifically a subgroup called Lophotrochozoa. Although adult brachiopods look very different from phoronids (horseshoe worms), DNA analysis shows phoronids are the closest relatives to inarticulate brachiopods, not articulate ones. However, the exact relationships among inarticulate brachiopods are still unclear. Some scientists suggest adding phoronids to the Brachiopoda group as a class called Phoronata, along with Craniata and Lingulata, in the subphylum Linguliformea. The other subphylum, Rhynchonelliformea, includes only one living class, which is divided into three living orders: Rhynchonellida, Terebratulida, and Thecideida.
This shows the current classification of brachiopods down to the order level, including extinct groups, which make up most species. Extinct groups are marked with a (†) symbol.
Ecology
Brachiopods are a marine group with no known species living in freshwater. Most avoid areas with strong water movement and are found in places like rocky overhangs, crevices, caves, steep slopes of continental shelves, and deep ocean floors. Some articulate species attach to kelp or live in sheltered areas of intertidal zones. The smallest living brachiopod, Gwynia, is about 1 millimeter (0.039 inch) long and lives between gravel grains. Rhynchonelliforms have larvae that feed only on their yolk and settle quickly, often forming dense populations in specific areas. Young brachiopods sometimes attach to the shells of older ones. In contrast, inarticulate brachiopods have larvae that swim for up to a month before settling, allowing them to live in wide ranges. Members of the discinoid genus Pelagodiscus are found worldwide.
Brachiopods have a low metabolic rate, about one third to one tenth that of bivalves. They were common in warm, shallow seas during the Cretaceous period, but most of their former habitats are now occupied by bivalves. Today, most brachiopods live in cold, low-light environments.
Brachiopod shells sometimes show signs of predator damage and repair. Fish and crustaceans seem to avoid eating brachiopod flesh. Fossil records show that predators like gastropods attacked mollusks and echinoids 10 to 20 times more often than brachiopods, suggesting brachiopods were targeted by mistake or when other prey was scarce. In areas with little food, the snail Capulus ungaricus steals food from bivalves, snails, tube worms, and brachiopods.
Only lingulid brachiopods are fished commercially, and only on a small scale. The fleshy pedicle is the part eaten. Brachiopods rarely settle on artificial surfaces, likely due to sensitivity to pollution. This may make the population of Coptothyrus adamsi useful for monitoring environmental conditions near an oil terminal being built in Russia on the Sea of Japan’s shore.
Brachiopods are the state fossil of Kentucky.
Evolutionary history
More than 12,000 fossil species have been identified. These species are grouped into over 5,000 genera. The largest modern brachiopods are 100 millimeters (3.9 inches) long, but some fossils are as wide as 200 millimeters (7.9 inches). The earliest confirmed brachiopods are found in the early Cambrian period. Inarticulate brachiopods appeared first, followed by articulate brachiopods. Three unmineralized species from the Cambrian period seem to belong to two different groups that evolved from mineralized ancestors. The inarticulate Lingula is called a "living fossil" because similar species have been found dating back to the Ordovician period. Articulate brachiopods experienced major diversification but also suffered severe mass extinctions. The most diverse modern groups, Rhynchonellida and Terebratulida, first appeared at the start of the Ordovician and Carboniferous periods, respectively.
Since 1991, Claus Nielsen proposed a hypothesis about brachiopod evolution. In 2003, Cohen and others adapted this idea to explain the earliest evolution of brachiopods. The "brachiopod fold" hypothesis suggests that brachiopods evolved from an ancestor similar to Halkieria, a slug-like animal with "chain mail" on its back and shells at both ends. The hypothesis claims that the first brachiopod formed two valves by folding its body.
Fossils found after 2007 support a new idea about the evolution of brachiopods. These fossils suggest that brachiopods may have evolved from tommotiids, a group of Early-Cambrian animals. Tommotiids had armor made of organophosphatic compounds, unlike Halkieria, which had calcite armor. A new tommotiid, Eccentrotheca, had an assembled armor coat that formed a tube, suggesting it was a sessile animal rather than a slug-like one. Eccentrotheca’s tube resembled that of phoronids, which are sessile animals that feed using lophophores and are closely related to brachiopods. Another fossil, Paterimitra, had two symmetrical plates at the bottom, similar to brachiopod valves but not fully enclosing the body.
During the Paleozoic era, brachiopods were among the most common filter-feeders and reef-builders. They also occupied other ecological roles, such as swimming using jet propulsion like scallops. After the Permian–Triassic extinction event, often called the "Great Dying," brachiopods recovered only one-third of their previous diversity. Some thought brachiopods declined because bivalves outcompeted them. However, in 1980, Gould and Calloway found that both brachiopods and bivalves increased in diversity from the Paleozoic to modern times, but bivalves increased faster. The Permian–Triassic extinction was more severe for brachiopods than for bivalves, causing brachiopods to become less diverse. In 2007, Knoll and Bambach concluded that brachiopods were among the groups most vulnerable to the Permian–Triassic extinction because they had calcareous hard parts, low metabolic rates, and weak respiratory systems.
Brachiopod fossils have helped scientists study climate changes during the Paleozoic. When global temperatures were low, such as in much of the Ordovician, the large temperature difference between the equator and poles led to different fossil collections at different latitudes. In warmer periods, like much of the Silurian, smaller temperature differences allowed the same few brachiopod species to colonize seas at low to middle latitudes.
From the 1940s to the 1990s, family trees based on embryological and morphological features placed brachiopods among or as a sister group to deuterostomes, a super-phylum that includes chordates and echinoderms. Closer examination found problems with this classification. Nielsen believes brachiopods and phoronids are related to deuterostome pterobranchs because their lophophores are driven by one cilium per cell, unlike bryozoans, which are considered protostomes and have multiple cilia per cell. However, pterobranchs are hemichordates, likely closely related to echinoderms, and there is no evidence that their ancestors were sessile or used tentacles to feed.
From 1988 onwards, molecular phylogeny studies, which compare biochemical features like DNA, placed brachiopods among the Lophotrochozoa, a protostome super-phylum that includes molluscs, annelids, and flatworms but excludes Ecdysozoa, which includes arthropods. This conclusion is supported by all molecular studies that use a wide range of genes, including rDNA, Hox genes, mitochondrial protein genes, and nuclear protein genes.
Some studies from 2000 and 2001, using both molecular and morphological data, support brachiopods as Lophotrochozoa. Others from 1998 and 2004 concluded that brachiopods were deuterostomes. Phoronids feed using a lophophore, burrow, or attach to surfaces, and build three-layered tubes made of polysaccharide, possibly chitin, mixed with seabed material. Traditionally, phoronids were considered a separate phylum, but molecular studies from 1997 to 2000 showed they are a subgroup of brachiopods. However, a 2005 study suggested phoronids are a subgroup of bryozoans.
All molecular phylogeny studies and half of the combined studies until 2008 concluded that brachiopods are Lophotrochozoa. However, these studies could not identify which Lophotrochozoa phylum is most closely related to brachiopods, except for phoronids, which are a subgroup of brachiopods. In 2008,