Articulata (Articulate Lampshells)

views updated

Articulata

(Articulate lampshells)

Phylum Brachiopoda

Class Articulata

Number of families 20

Thumbnail description
Brachiopods that live within a rounded, hinged, and mostly calcareous shell composed of two bilaterally symmetrical but dissimilar valves, and that generally attach themselves to hard substrates with a pedicle (foot-like structure) supported by connective tissue

Evolution and systematics

Lampshells were a dominant life form during the Cambrian period (570–500 million years ago [mya]). They flourished during the Paleozoic (570–240 mya) and Mesozoic eras (240–65 mya). The brachiopods reached the zenith of their evolutionary diversification during the Ordovician period (500–435 mya). At that time, the articulate lampshells appeared and underwent their most important phase of evolution. Since the close of the Paleozoic era, however, when their populations were decimated by mass extinctions from unknown causes, their numbers have been steadily decreasing.

Fossil remains have shown that articulates were once widely distributed and abundant throughout the marine world. Although over 30,000 species are known from fossil records between the Cambrian period and the present day, only 250 to 325 species are thought to be extant. The majority of present-day brachiopod species are assigned to the class Articulata rather than the class Inarticulata. The two classes are distinguished primarily by the way in which the two valves are attached along the rear line of the organism, and their method of contact. As of 2003, there are three living orders of articulates and four extinct ones.

Physical characteristics

Most articulate lampshells are less than 1 in (25 mm) across; in some instances, however, they can grow as much as 2 in (50 mm) in width. They contain two very conspicuous hinged valves (or shells) that are composed of scleroproteins and calcite (a form of calcium carbonate). The shells are segmented in a radial pattern; for the most part, they are usually brown in color, or sometimes greenish when surrounded by algae. Only a few lampshells are naturally white. The upper (dorsal) valve is smaller than the lower (ventral) valve. The valves are connected by a joint that can articulate or pivot when teeth located on the ventral valve are inserted into sockets on the dorsal valve along a hinge line. In this manner, the shell can close in front of the hinge and open from behind the hinge, thus locking itself at the rear. The hinge restricts the organism's movements to simple opening and closing. Only a slight anterior gap of about 10° is apparent when the valves are locked in place. This gap at the apex of the two shells resembles the shape of the spout of ancient Roman and Greek oil lamps—whence the English name "lampshell."

The space inside the shell is divided approximately into two halves, the mantle cavity and the body or coelomic cavity. The body of an articulate lies between the shell valves, sheathed in mantle tissue that secretes the shell valves. The coelom lies between the body wall and the gut, and surrounds the lophophore and its tentacles. The lophophore is a crown-shaped structure of hollow tentacles with a pair of arms or brachia (from which the name brachiopod is derived), one brachium on each side of the mouth. The lateral arms of the lophophore are partially supported by a cartilaginous axis on the dorsal valve and a fluid-filled canal within each brachium. The arms climb in a spiral pattern into each half of the mantle cavity.

Articulate lampshells have three sets of muscles that control the movements of their valves. The largest are the adductor muscles, used to close the shell. These muscles usually have two, occasionally four, closely spaced foundations on the ventral valve and four distinctly separated foundations on the dorsal valve. The posterior adductors close the shell rapidly, while the anterior adductors hold the shell tightly closed over longer periods of time. The diductor muscles are used to open the shell for feeding; they have two large foundations on the ventral valve and an attachment area on the rear tip of the dorsal valve. The adjustor muscles, positioned for moving the shell relative to the pedicle, have two attachments near the posterior end of each valve. The pedicle itself is a cylindrical stalk of horny material used to anchor the organism on such substrates as pebbles, stone particles, or coral. The pedicle in articulates ranges from absent in a few species to short and nonmuscular to very long, flexible, muscular, and branched. The pedicle does not expand where it leaves the shell but emerges through a slit or notch on the ventral valve. The pedicle lacks internal muscles and a coelomic lumen; is supported by connective tissue; and is composed of short papillae on its distal end that may branch, wrap around a substrate, or penetrate a soft substrate with acid secretions.

The short U-shaped intestine of articulates begins at the lower end of the stomach and ends in a blind pouch with no anus. The excretory system consists of segmental organs known as metanephridia, which are short tubes ending in large nephrostomes. Articulate brachiopods have a simple circulatory system containing colorless blood with only a few cells. The system includes a dorsal vessel that functions as a primitive pumping heart. The simple nervous system consists of a small dorsal ganglion, or group of nerve cells, behind the mouth and a large ventral ganglion in front of it. A nerve ring around the esophagus connects the various parts of the nervous system, including a nerve running into each tentacle. Articulates do not possess special sense organs, but the mantle margin is most likely an important area of sensory reception. The setae (bristles) on the mantle are thought to transmit stimuli to receptors in the mantle epidermis.

Distribution

Articulate lampshells are scattered around the world in marine environments, mostly in colder waters. Where they are found, they usually occur in dense numbers.

Habitat

Lampshells are found at depths ranging from the intertidal zone (between high and low tide levels in coastal areas) to the deep sea as far as 17,410 ft (5,300 m). Most articulate lampshells live in moderately deep water as low as 1,500 ft (450 m) in polar waters and cryptic (hidden) environments. They are classified as epifaunal; that is, they live on the sea floor or attached to other animals or objects underwater. Most species live on rocks or other solid substrates, but some dig vertical burrows in sandy or muddy bottoms.

Behavior

Articulate lampshells are sedentary or sessile in their lifestyle, attaching themselves to a substrate by the use of a cord-like pedicle on their lower valves. The lower valve is usually positioned on top. Lampshells with pedicles tend to orient themselves with regard to the water so that the front-to-back axis of the shell is at right angles to the current, with the commissural plane parallel to the current. The commissure is the line of junction between the edges of two valves. The few species of articulates that lack pedicles attach themselves to the substrate by the lower valve.

Articulate lampshells protect themselves by closing their shells. When disturbed, they will also contract their pedicles and pull themselves downward toward the substrate. They are capable of only limited lateral motion. Most of their movement involves opening the shell for feeding and closing it for protection.

Feeding ecology and diet

Articulates are generally suspension feeders, taking in minute particles of nourishment from the water. The lophophore is primarily a feeding and respiratory organism. To feed, the lampshell valves open a small gap in the front to allow water to flow over the lophophore. The hollow tentacles, with cilia on the outside, can reach to the front of the valves, while being held near the upper valve. Suspended microorganisms and food particles are trapped in mucus-covered loops and swept by the beating cilia with the intake current down to the mouth through a brachial groove. The mouth leads into an esophagus and stomach, which digests the food. Coelomocyte cells collect nitrogenous material throughout the body and release these wastes into the nephridia, or simple kidneys. The two nephridia have a thin duct leading into the mantle cavity from the coelom so that the wastes can be passed into special outflow currents that carry them away through the animal's mouth. The continual flow of water over the lophophoral arms allows for respiratory exchange of oxygen and carbon dioxide.

Reproductive biology

Most articulates have separate sexes. In addition, most species have no special breeding season, although some breed only at certain times of the year. There are usually four gonads, which are masses of developing gametes located beneath the mesothelium of the coelom. The ova or sperm are found in patches on the coelomic epithelium. They pass into the coelom and shell cavity through tubes in the segmental organs. In most species, large numbers of gametes are produced and discharged into the water, where fertilization takes place; in a few species, however, embryonic development takes place within the parent's shell, sometimes in a brood pouch.

Free-swimming larvae develop from the fertilized ova, propelling themselves through the water by rows of cilia on their three-lobed bodies. After a few days they sink to the bottom and undergo metamorphosis. At this point, the larvae have three developing regions: body, mantle, and pedicle lobes. No shell appears until the post-larval stage. The posterior lobe becomes elongated into a stalk, the middle lobe secretes a shell, and the remaining lobe forms the lophophore.

Conservation status

Articulate lampshells have diminished dramatically from the height of their diversity over 450 million years ago. Extant species are not, however, considered threatened. No species are listed by the IUCN.

Significance to humans

Articulate brachiopods have little economic significance but are important in understanding the fossil record and the evolutionary history of invertebrates.

Species accounts

List of Species

Black lampshell
Argyrotheca cistellula
Lampshell
California lampshell

Black lampshell

Hemithyris psittacea

order

Rhynchonellida

family

Hemithyrididae

taxonomy

Hemithyris psittacea Gmelin, 1791.

other common names

None known.

physical characteristics

Black lampshells possess a shell that is small to medium in size, with a maximum adult width of 1.38 in (35 mm). The shell is moderately thick with a highly convex shape and a brownishpurple color. The ventral valve narrows to a long and curved beak. There are two spirolophous lophophore spirals, which are dorsally directed and partially supported by crura (hard processes extending forward from the socket region of the dorsal valve) and two pairs of metanephridia. The dorsal floor contains a low median ridge. The slender curved radulifer crura are attached to small outer hinge plates. The pedicle is somewhat short, almost as broad as the foramen, and not used for locomotion. The intestine is curved distally and is enlarged at its end. The adductor muscles possess two attachments on the ventral valve. The mantle canals contain two primary trunks in each mantle lobe. There are no spicules in the soft parts of this lampshell.

distribution

Circumpolar, primarily in the Pacific Ocean and Atlantic Ocean.

habitat

Black lampshells inhabit areas with tropical to cold surface waters. They are found at depths ranging from intertidal zones at about 33 ft (10 m) to deeper waters at about 4,265 ft (1,300 m).

behavior

Sessile lifestyle; generally attached to substrates by a pedicle.

feeding ecology and diet

Feed on dissolved organic matter.

reproductive biology

The posterior portion of each mantle lobe contains a specialized pair of pillared areas for the gonads.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Argyrotheca cistellula

order

Terebratulida

family

Argyrotheca

taxonomy

Argyrotheca cistellula Searles-Wood, 1841.

other common names

None known.

physical characteristics

Argyrotheca cistellula has a shell width of about 0.59 in (15 mm). The shell contains a yellowish brown, four-cornered ventral valve and a five-cornered dorsal valve. The lophophore loops are schizolophous, which means that they consist of lobed discs with only a small number of marginal filaments. The peduncle is very small. This species generally lacks setae.

distribution

Argyrotheca cistellula is found off the coast of Europe and is common in the Mediterranean Sea.

habitat

This species is believed to occur over a wide range of depths, but is most frequently found between 7–200 ft (2–60 m).

behavior

Sessile lifestyle; generally attached to substrates by the pedicle.

feeding ecology and diet

Feeds on dissolved organic matter.

reproductive biology

Little is known about the reproductive cycle of A. cistellula. It is thought to breed continuously, on the basis of the maturing eggs and larvae that are found in its brood pouches throughout the year. Each embryo goes through a gastrula stage as well as two-lobed and three-lobed stages. Before leaving the brood

pouch, the larvae have an apical lobe with a ring of long cilia, a mantle lobe with a mid-ventral strip of cilia, and a pedicle lobe that lacks cilia. The larvae lack setae. Unlike most lampshells, A. cistellula is hermaphroditic, which means that each individual possesses both male and female gonads.

conservation status

Not listed by IUCN.

significance to humans

None known.

Lampshell

Terebratulina retusa

order

Terebratulida

family

Cancellothyrididae

taxonomy

Terebratulina retusa Linnaeus, 1758.

other common names

None known.

physical characteristics

Adults of this species are small to moderately large brachiopods, with a maximum shell width of 1.18 in (30 mm). The shell is roughly oval in shape, and the anterior commissure is slightly grooved. Externally, the shells have numerous and somewhat coarse radial ribs. These ribs become knobby, forming smooth, rounded tubercles (nodules) at the rear and side margins of both valves, although they are most developed on the sides of the ventral beak or umbo. The beak represents the initial point of growth of a valve. A wide number of concentric growth lines are present. Shell color ranges from white to yellowish gray. The small deltidial plates are separated. The species is endopunctate, which means that there are tiny canals called endopunctae extending from the inner surface of the valve almost to the outside. In T. retusa, the endopunctae are usually arranged radially (corresponding to the external ribs) on the inside of the valves. Spicules, or hard calcareous plates, are found in the body tissue near the lophophore as well as in the tissue covering the mantle canals. The spicules help to support the soft body tissues. The tip of the pedicle divides into short rootlets that are able to penetrate the substrate.

distribution

Terebratulina retusa is found in the eastern part of the Atlantic Ocean around the British Isles, excluding the eastern coast of England and the southern Irish Sea. The species also occurs in the North Atlantic from Scandinavia to the eastern coast of Greenland, and as far south as the Mediterranean Sea.

habitat

Terebratulina retusa is found in tropical to cold surface waters, at depths ranging from the intertidal zone (about 50 ft [15 m]) to the deep sea (about 4,850 ft [1,480 m]).

behavior

This species is generally sedentary or sessile in its lifestyle, generally attaching itself to substrates by the use of the pedicle.

feeding ecology and diet

They primarily eat dissolved organic matter.

reproductive biology

The shell of T. retusa often acquires a brighter color during the breeding season, as the yellow or orange of the ripe gonads in females or the cream color of the male gonads often show through the external surface. Female gonads are almost always orange, red, or brown in color, however, even when the shell has no coloration.

conservation status

Not listed by IUCN.

significance to humans

None known.

California lampshell

Laqueus californianus

order

Terebratulida

family

Laqueidae

taxonomy

Laqueus californianus Koch, 1848.

other common names

English: Smooth lampshell.

physical characteristics

The shell of California lampshells is medium to moderately large in size, with an adult dimension of 0.79–1.97 in (20–50 mm). The tan-colored shell is biconvex and may be smooth or ribbed. The pedicle is short to moderately long. The loop that supports the lophophore passes through one or more bilacunar phases. The lophophore is plectolophous, which means that it has longer lateral lobes and a coiled median lobe. Spicules occasionally occur scattered over the mantle canals. Dental plates are present. The ventral median septum (partition) is weak. Hinge plates are variably developed.

distribution

California lampshells are found near the break between the continental shelf and slope along the western coast of the United States.

habitat

California lampshells live in the continental shelf zone at depths of 325–650 ft (100–200 m). They are also found in tropical to cold surface waters and in deeper waters.

feeding ecology and diet

California lampshells are believed to eat dissolved silt as well as organic matter, diatoms, dinoflagellates, and phytoplankton, but may not be limited to these items.

behavior

California lampshells are generally sedentary or sessile in their lifestyle, attaching themselves to substrates by their pedicles. They are usually found with the ventral shell uppermost and the valves slightly apart.

reproductive biology

The eggs of California lampshells are 0.00512–0.00551 in (130–140 μm) in diameter, and the sperm are unmodified. At 50°F (10°C) an embryo develops within 72 hours, and gastrulation occurs within 24–38 hours. A three-lobed larva with an attachment disk develops in about seven days. The larvae, however, die within one day at 77°F (25°C). At 68°F (20°C), development is normal but may result in abnormal settlement of larvae. At 59°F (15°C), 50°F (10°C), and 41°F (5°C), most larvae achieve competence in five, seven, and nine days, respectively. Settlement and metamorphosis can occur within one day after the larvae make contact with the substrate.