Brachiopods, also known as lamp shells, are marine invertebrates characterized by two hard valves or shells located on their dorsal and ventral sides, in contrast to the lateral shells of molluscan bivalves. This ancient group of organisms has existed for at least 600 million years and constitutes the phylum Brachiopoda. They are traditionally divided into two main groups: articulate and inarticulate brachiopods. Articulate brachiopods possess tooth-and-groove structures on their valve hinges, while inarticulate brachiopods lack these structures and rely on muscles to join their valves.
During the Paleozoic era, brachiopods were among the most prevalent and diverse organisms in marine environments, forming extensive reefs and occupying various ecological niches. There are around 30,000 known fossil species of brachiopods, though only about 330 species are alive today, primarily found in cold, deep-sea environments and polar regions.
Modern brachiopods typically range in size from 1 to 100 mm (0.039 to 3.937 inches), with most species measuring between 10 and 30 mm (0.39 to 1.18 inches) in length. The largest living species, Magellania venosa, can reach up to 100 mm. In contrast, the largest known fossil brachiopods, such as Gigantoproductus and Titanaria, were significantly larger, reaching widths of 30 to 38 cm (12 to 15 inches). In contrast, the tiniest extant species, Gwynia, measures just about 1 mm (0.039 inches).
All brachiopods have a hard shell with two valves covering the dorsal and ventral surfaces of their bodies (unlike bivalves, which have the shell covering the lateral surfaces). The dorsal valve, also called the brachial valve, is smaller and bears tiny arms called brachia on its inner surface that help support the feeding organ, the lophophore. The ventral or pedicle valve is larger and has an opening for the stalk-like pedicle for attachment to a substrate.
The articulate and inarticulate groups of brachiopods are distinguished based on the hinge pattern of the two valves. While articulates possess a tooth and socket arrangement for joining the pedicle and brachial valves, inarticulate brachiopods are held together only by muscles. In many extant articulates, both valves are convex and bear growth lines, whereas in inarticulate lingulids, the valves are of similar size and shape and are much smoother.
Layers
Each shell valve of a brachiopod consists of three distinct layers: an outer periostracum and two inner biomineralized layers. In articulate brachiopods, the periostracum is protein-based, followed by a primary layer of calcite and an innermost layer that combines proteins and calcite. Although inarticulate brachiopods share this three-layered structure, their chemical composition varies across different classes.
Members of the class Terebratulida have punctate shells characterized by mineralized layers perforated by tiny canals that are extensions of the mantle tissue. In contrast, impunctate shells are solid, without any such perforations.
In the classes Lingulida and Discinida, which have pedicles, the periostracum is composed of chitin. The primary layer is made of apatite, and the innermost layer consists of a mix of collagen, chitinophosphate, and apatite. This intricate layering and varied composition highlight the structural diversity and adaptability of brachiopods.
Like mollusks, brachiopods possess a soft and fleshy mantle that encloses the visceral organs and secretes the shell above it. Most of a brachiopod’s body is covered by the mantle lobes, flabby extensions surrounding a water-filled cavity that houses the lophophore. The body cavity or coelom extends into each mantle lobe as a network of canals carrying nutrients to the mantle edge.
In most brachiopods, the mantle edge is dotted with movable bristles called chaetae or setae that help funnel water in and out of the mantle cavity. They may also act as sensors and aid in defense.
The mantle also has hollow extensions called diverticula that penetrate the biomineralized valve layers and reach the periostraca. These extensions sometimes store chemicals, like glycogen, or secrete repellents to ward off enemies.
Brachiopods possess a U-shaped feeding organ called a lophophore, which comprises a bunch of ciliated tentacles that help sift food particles out of the water current. This non-retractable organ occupies about two-thirds of a brachiopod’s internal body space.
A hydrostatic skeleton and cartilage generally support the lophophore, but in some articulate brachiopods, a calcareous structure called the brachidium adds extra support.
Most modern brachiopods attach themselves to a substrate using a cylindrical extension of the body wall called a stalk or pedicel. However, some genera, like the inarticulate Crania and the articulate Lacazella, lack a pedicel and instead cement themselves by attaching their ventral valve to the substrate.
Some early brachiopods, like strophomenates, kutorginates, and obolellates, had a specialized structure called the pedicel sheath for attachment. This structure was an extension of the umbo of the pedicle valve.
In articulates, the pedicel is an extension of the coelom and is lined by a layer of longitudinal muscles. While members of the order Lingulida possess long pedicles, those of the order Discinida have short ones.
In inarticulates, the pedicel is not an extension of the coelom and has a compact core of connective tissue. The distal end of the pedicel usually has short root-like extensions or papillae that help attach to the substrate. In members of the genus Chlidonophora, the pedicel is branched.
Oxygen is transported throughout the body via blood pumped by a muscular heart located dorsally above the stomach. The blood circulates through a network of vessels that branch out to essential organs, including the lophophore, gut, gonads, nephridia, and muscles.
Brachiopods excrete nitrogenous metabolic wastes in the form of ammonia, which diffuses out of their bodies through the mantle and lophophore. Although these invertebrates possess metanephridia, an excretory organ in many phyla, it is only used for ejecting gametes.
Since most brachiopods’ diet is digestible, they barely produce solid waste.
In adult articulates, the brain comprises two ganglia, one above and the other below the esophagus, whereas in inarticulates, there is only the lower ganglion. A network of nerves spreads out from the ganglia (and, in articulates, from the commissures joining the two ganglia) to the lophophore, the mantle lobes, and the muscles regulating the two valves.
The chitinous chaetae on the mantle possess sensory units and probably send tactile signals to the receptors on the mantle epidermis.
Some brachiopods, like Lingula anatina, also possess statocysts as balancing organs.
The term Brachiopod is derived from the Ancient Greek words’ brachion,’ meaning arm, and ‘podos,’ meaning foot. This naming refers to the brachia or arms of the lophophore found in these animals. With time and research, scientists have devised different taxonomic classifications for brachiopods.
The traditional classification, proposed in 1869, divided all brachiopods into two broad groups: Inarticulata and Articulata. While articulates possess toothed hinges between the valves, the inarticulate valves are joined by only muscles.
A revised classification based on the chemical composition of brachiopod shells was proposed. Members with calcite shells were placed under the class Calciata, while those with shells made of chitin and calcium phosphate were classified under the class Lingulata.
Around this time, another classification, the three-part scheme, was also proposed. In this scheme, the family Craniida was placed under the subphylum Craniiformea, while the orders Lingulida and Discinida were grouped within the subphylum Linguliformea. The two families, Rhynchonellida and Terebratulida, were placed under the subphylum Rhynchonelliformea.
Molecular and genetic research since the 1990s has provided us with new insights into the classification of modern brachiopods. These invertebrates were initially considered members of the superphylum Deuterostomia (along with echinoderms and chordates); however, scientists now propose their placement within the superphylum Lophotrochozoa (under the broad clade Protostomia). 18S rRNA sequencing suggests that horseshoe worms or phoronids (phylum Phoronida) are the closest relatives of the inarticulate brachiopods.
Currently, around 330 living species fall under three subphyla, three extant classes, and five extant orders.
Brachiopods have a long and rich palaeontological history. More than 12,000 fossil species have been recognized, with the earliest undisputed brachiopod dating back to the Early Cambrian Period.
According to Claus Nielsen’s (1991) ‘brachiopod fold’ hypothesis (later adapted by Cohen and colleagues in 2003), brachiopods are descendants of an ancestor similar to Halkieria, a small, slug-like animal. The hypothesis proposes that the first brachiopod folded the rear part of its body under its front, giving rise to the paired valves.
A revised hypothesis based on new fossils found in 2007 and 2008 suggested that brachiopods evolved from tommotiids, an extinct group of invertebrates from the Cambrian Period.
Brachiopods are exclusively marine animals that inhabit environments with minimal strong currents or waves. They are generally found in cold, low-light conditions, such as crevices, caves, under rocky overhangs, continental shelves, and deep ocean floors. While articulate brachiopod larvae quickly settle and form dense populations, inarticulate larvae can swim for extended periods, resulting in a broader distribution.
Some species, like those in the genus Pelagodiscus, have a cosmopolitan distribution, while others, such as certain rhynchonelliforms, are often endemic to specific regions.
They live between three to as high as thirty years.
Brachiopods collect their food using an ‘upstream collecting’ mechanism. In this feeding mechanism, water enters the lophophore from the sides of the valves, and the food particles are trapped in the ciliated tentacles of the organ. The particles are then funneled into the brachial groove (which runs around the bases of the tentacles), from where they move to the slit-like mouth at the base of the lophophore.
The food then moves into the pharynx and the esophagus, both lined by cilia and specialized secretory cells producing digestive enzymes. It finally gets digested in specialized pouches or ceca in the stomach. The assimilated nutrients are transported throughout the body by the lashing of cilia lining the coelom.
These invertebrates often alter the direction of the beating of cilia to eliminate any indigestible matter. For example, when an individual encounters large lumps of unwanted food, the cilia lining the entry channels stop beating, and the tentacles move apart to form large gaps. The lumps are then made to pass through the gaps and are finally deposited onto the mantle lining.
Most adult brachiopods are gonochoric (separate male and female sexes) and possess four gonads, two in each valve. The gametes (sperm and ovum) develop within the gonads and then move to the main coelom, from where they are released into the surroundings (for external fertilization) through the metanephridia. The females of some species, however, retain their eggs within (internal fertilization), and embryos develop in specialized brood chambers till they are ready to hatch into larvae.
During development, the embryo’s cells undergo radial (cells form stacks of rings above each other), holoblastic (complete cleavage of the cells), and regulative cleavage (the fate of a cell is determined by interactions with the adjacent cells) to form a coeloblastula, which further develops into a lobate larva in most species.
The larvae of inarticulate brachiopods are planktotrophic, meaning they feed on plankton, and resemble miniature adults with valves, mantle lobes, a coiled pedicle, and a small lophophore. These larvae exist as plankton and can remain in this floating state for several months. As they develop, they form valves and shells and eventually settle on the ocean floor to become sessile adults.
In contrast, the larvae of articulate brachiopods are lecithotrophic, deriving their nutrition from the egg yolk. These larvae have a short planktonic phase of about two days. After this stage, they sink, attach to a suitable substrate, and metamorphose into juveniles. As they mature into adults, the front lobe of the mantle develops into the lophophore and other organs, while the mantle folds over the front lobe and begins secreting the shell.
Since brachiopods have very little body tissue and are guarded by hard shells, they are rarely fed upon. However, recent studies on modern brachiopod populations suggest that these invertebrates face moderate levels of predation by echinoderms, crustaceans, fish, and migratory birds.
Welcome to AnimalFact - your go-to source for fascinating insights into the amazing world of animals!
