Ophiuroidea (Brittle and Basket Stars)

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Ophiuroidea

(Brittle and basket stars)

Phylum Echinodermata

Class Ophiuroidea

Number of families 16

Thumbnail description
Small- to medium-size echinoderms with a flattened disk often covered with a series of scales, granules and small spines; usually five long thin (in comparison with the disk) articulated arms that break off easily; a row of papillae (small nipple-shaped structures) known as the arm comb near the base of each arm; lower surface of the disk containing a central mouth leading to the stomach pouch; mouth divided by five jaws


Evolution and systematics

The fossil record of the ophiuroids extends back some 500 million years to the early Ordovician period. They are the most diverse class of echinoderms, with some 250 described genera and 2,000 species. The most recent phylogeny and upper-level taxonomy places extant ophiuroids into two subclasses: the Oegophiuridea with only one family, the Ophiocanopidae; and the Ophiuridea, which is divided into two distinct orders—the Euryalida, including the basket stars with branched arms, and the more familiar Ophiurida or brittle stars. The phylogenetic tree for ophiuroids suggest that they underwent a major diversification in the Triassic and early Jurassic periods; that is, about 200–250 million years ago. The classification of the ophiuroids, however, remains a subject of debate. The placement of the unusual Ophiocanops fugiens, thought to be a "living fossil," into the subclass Oegophiuridea, is especially interesting, since this subclass was thought to have become extinct after the late Carboniferous period. The families are as follows: Ophiocanopidae; Asteronychidae; Gorgonocephalidae; Asteroschematidae; Euryalidae; Ophiomyxidae; Hemieuryalidae; Ophiuridae; Amphiuridae; Ophiothricidae; Ophiactidae; Ophionereididae; Ophiocomidae; Ophichitonidae; Ophiodermatidae; Ophiolepididae.

Physical characteristics

Ophiuroids have long slender flexible arms that are sharply separated from the disk. The common name "brittle star" refers to the fact that the arms of many species are easily broken off. Locomotion involves the entire arm; movement is made possible by an internal skeleton that supports the arm. The skeletal portions of the arm are made of calcium carbonate and look like vertebrae; they are called vertebral ossicles. Five arms are usual, but a few species of ophiurid ophiuroids have six or seven, and euryalid ophiuroids may have as many as 20 major additional arms.

Species with branched arms are restricted to the families Gorgonocephalidae and Euryalidae and are called basket stars. The basket stars are the largest ophiuroids; Gorgonocephalus stimpsoni can measure up to 27.5 in (70 cm) in arm length with a disk diameter of 5.63 in (14.3 cm). The branching of the arms in basket stars is repeated in the formation of smaller and smaller units in a fernlike manner.

The upper surface of the disk in ophiurid ophiuroids is covered with a series of scales. In contrast, euryalid ophiuroids are characterized by the presence of a covering on the disk without any large plates underneath. The mouth on the lower surface of the disk is framed by five jaws bearing spinelike teeth and papillae. The mouth leads directly to the saclike stomach, which ends in a blind pouch since the organism lacks intestines and an anus.

The oral side of the disk contains the bursal slits on each side of the base of each arm. These slits are openings for the respiratory bursae, which are specialized sac- or pouchlike formations in the body wall that serve for gas exchange. The gonads are attached to the walls of the bursae; in a few species, however, the gonads extend into the coelom (body cavity) of the arms. The number of gonads varies.

Distribution

Ophiuroids range from the poles to the tropics and from intertidal to abyssal plains as well as in deep sea trenches. The deepest recordings were taken from the Bourgainville Trench at 26,270 ft (8,006 m).

Habitat

Ophiuroids are found in all marine habitats. There are burrowing ophiuroids, notably members of the family Amphiuridae, that live on muddy or sandy sea bottoms, as well as species that live either exposed or cryptic (hidden) on all types of ocean substrates. In addition, some ophiuroids live on or in association with such other organisms as algae, sponges, and corals. Many are filter-feeding species, such as the basket stars that occupy habitats swept by currents. Like echinoderms in general, ophiuroids cannot tolerate low levels of salt concentration in the water; a few species, however, including Ophiura albida and Ophiophragmus filegranus, can adapt to brackish water and live in estuaries with salt concentrations as low as 10 ppt (ocean water is usually 34–35 ppt).

Behavior

Many ophiuroids move by powerful arm strokes that lift their disks and thrust them forward. Some species may even swim short distances in this manner. Other species, particularly those with very long arms, move with a slithering type of motion that pulls them along the substrate. Because of this motion, they are sometimes referred to as "serpent stars" or "snake stars." Only a few species use their tube feet for moving about. Ophiuroids are good climbers; filter-feeding species may cling to rock outcrops or to such other filter feeders as gorgonians and corals in order to position themselves in stronger currents above the sea floor.

Ophiuroids show a range of responses to light intensity, ranging from rapid escape, feeding at night, or color change to behavior that indicates relative indifference to light. For example, Ophiocoma wendtii is a highly photosensitive species that changes color from dark brown during the day to banded gray and black during the night; it moves rapidly away from shadows by crawling into crevices. It has been shown in this species that the arm plates function as compound eyes, focusing light through nerve bundles on the dorsal arm plates.

The best-known adaptation to partial predation in motile animals is probably autotomy, or the self-amputation of appendages.

Ophiuroids may lose arms and even disks in some amphiurid species. The brittle star's ability to shed parts of its arms has to do with the mechanical properties of the ligaments between the vertebral ossicles of the arms. Stimulation of the nerves at an arm joint causes this particular tissue—mutable collagenous tissue or MCT—to disintegrate, and the arm breaks off. The lost body parts are subsequently regenerated. Regeneration of missing arms may take a few months in warm tropical water but more than a year in cool temperate water.

Several species of ophiuroids are bioluminescent. It has been shown that the nocturnally active Ophiopsila riisei and Ophiopsila californica deter predators by luminescent flashes from their arms, and that their bioluminescence also functions as an aposematic (warning) signal to discourage crustacean predators.

Feeding ecology and diet

Most ophiuroid species are able to feed on more than one type of food and use more than one feeding method. Researchers recognize two major feeding groups, however. There is a carnivorous group of predators and carrion feeders that usually have short arm spines and tube feet, and seize their food in one of their arm loops. Species in the second major group feed mainly on small particles from the substrate or suspended in the water. The arm spines and tube feet in this group are relatively long, and are the principal food-trapping organs. It has also been shown that some species, such as the amphiurid Microphiopholis gracillima, are able to take up dissolved organic material from the water. Basket stars, however, appear to be specialized predatory suspension feeders that mainly take zooplankton, or live animal food, from the water column. Basket stars capture their prey by a rapid flexing movement of the arm that encircles the prey. The prey is then gripped by tiny sharp hooks arranged along the distant part of the arms.

Reproductive biology

Most ophiuroids have separate sexes. When the gametes are mature they are discharged into the bursae and expelled into the water through the bursal slits. The eggs are usually small with a low yolk content and a diameter of 0.0039–0.07 in (0.1–0.18 mm). Following fertilization, the eggs develop into a ciliated blastula which then matures further into the characteristic ophiopluteus larvae with arms, a mouth, and an anus. The ophiopluteus larva is planktonic; the larval stage

lasts two to five weeks, during which the larvae feed on such microscopic particles as diatoms, dinoflagellates and other small flagellates. Subsequently the larval body goes through a remarkable metamorphosis into a young brittle star. The body flattens, the larval gut closes, and the anus disappears.

Other species develop via a vittelaria larva, which originates from eggs with a high yolk content. This larva does not feed; it develops into a young brittle star within approximately 12 days. There are also species that undergo direct development; the larval stages are suppressed and a juvenile brittle star emerges directly from the gastrula stage.

There are approximately 60 hermaphroditic species of ophiuroids that practice brood care. The eggs are retained and developed in the maternal body; in most cases, the bursae serve as brood chambers in which the young brittle stars also may be fed by the mother, as in Amphipholis squamata.

Lastly, there are about 45 species of brittle stars with a mixed life history that includes both sexual reproduction and clonal proliferation by fission. These brittle stars have six arms, and spontaneously divide into two halves. Each half then subsequently regenerates three new arms to form two new complete individuals. Although asexual species are uncommon in this class, they can be very successful, range widely, and occur in high densities in certain habitats. For most of these species, the asexual mode is associated with small body size, while sexual maturity is usually attained after an individual reaches a certain size.

Conservation status

Such conservation organizations as the IUCN do not list any species of ophiuroids as endangered.

Significance to humans

Ophiuroids are of no direct significance to humans except for purposes of scientific research.

Species accounts

List of Species

Snake star
Northern basket star
Dwarf brittle star
Amphiura filiformis
Tropical brittle star
Common brittle star
Ophiura ophiura

Snake star

Astrobrachion constrictum

order

Euryalida

family

Asteroschematidae

taxonomy

Astrobrachion constrictum Farquhar, 1900.

other common names

None known.

physical characteristics

A large brittle star with a disk as much as 0.9 in (23 mm) in diameter. The arms are long, about 13.7 in (350 mm) and broken near the tip. A soft, smooth skin covers both the disk and the arms. The spines are shorter than the arm width; the distal spines are flattened, turning into compound hooks with 2–4 lateral teeth. Colors range from uniform bright yellow or deep red to almost black. Dark red or black individuals may have white spots or stripes.

distribution

The species is found throughout New Zealand, southeastern Australia, and New Caledonia.

habitat

Astrobrachion constrictum lives in a mutualistic relationship with the black coral Anthipates spp. These species are normally found together in relatively deep water (164–590 ft or 50–180 m) along the continental shelf but also in shallower water (up to 33 ft or 10 m deep) in the fjords of southwestern New Zealand. The black corals attach themselves to hard substrates and the snake stars coil around the branches of the coral.

behavior

Astrobrachion constrictum is a nocturnal feeder. During the day it remains tightly coiled around its host. The relationship with the black coral is probably mutualistic; in the process of feeding by gathering mucus from the coral, the snake star also protects its host from organisms and detritus that would otherwise smother the colony. Snake stars show a strong preference for black corals; if they are removed, they move relatively quickly toward a black coral colony, probably by sensing chemical signals from the coral.

feeding ecology and diet

Snake stars feed during the night by stretching 2–3 arms out in the water column to snare drifting plankton with their tube feet and arm spines. They also wipe the branches of the coral in order to feed on the plankton and detritus collected on the coral.

reproductive biology

A distinctive feature of this family is the position of the gonads within the arms. One shallow-water study of the fjords of southern New Zealand reported that these brittle stars have a prolonged reproductive period, possibly even continuous, as found in many deep-sea ophiuroids. The oocytes are relatively large (0.015–0.019 in or 400–500 µm), indicating that development is lecitotrophic (larvae do not feed), either direct or abbreviated. A larval stage, however, is not known.

conservation status

Not listed by the IUCN.

significance to humans

None known.


Northern basket star

Gorgonocephalus arcticus

order

Euryalida

family

Gorgonocephalidae

taxonomy

Gorgonocephalus arcticus Leach, 1819.

other common names

German: Gorgonenhaupt; Norwegian: Medusahode.

physical characteristics

A large basket star with a disk 4.01 in (102 mm) in diameter, with highly branched long (14.1 in or 360 mm) arms. The disk is pentagonal and bare, leathery with five pairs of ridges radiating from the center to the sides of the arms. The five sturdy arms divide into two branches near the disk and then redivide equally five or more times. The arm joints have short hooked spines. Color varies from yellowish brown to darker brown with lighter arms.

distribution

North Atlantic, from the arctic region to Cape Cod in the northeastern United States.

habitat

On rocky bottoms swept by currents, often clinging to gorgonians at a depth of 164–4,921 ft (5–1,500 m).

behavior

Northern basket stars are adapted to live in strong ocean currents and seek out positions high up in the water column in order to spread out their feeding fans in the form of a concave dish facing the current. They can hold out their arms in a stiff position for long periods of time. There is morphological evidence that the mutable collagenous tissues (MCT) of basket stars may be important in maintaining their stiff fans. The use of MCT lowers the rate of energy consumption in comparison to using muscles for the same purpose. In contrast to shallow-water basket stars that are strictly nocturnal, Gorgonocephalus arcticus uses its feeding fan during the day.

feeding ecology and diet

Gorgonocephalus arcticus is a predatory suspension feeder that captures zooplankton, primarily such macroscopic crustaceans as krill, by rapidly coiling and bending its arms. Rings of sharp hooks on the arms assist the basket star in capturing food by sticking to the prey.

reproductive biology

Not much is known about the reproduction of gorgonocephalids. The genus has a large number of gonads, as many as 1000 per individual. The closely related Gorgonocephalus caputmedusae lays its eggs free in water; however, a larval form is not known. Juveniles of Gorgonocephalus spp. are also known to live on the soft coral Gersemia rubiformis.

conservation status

Not listed by the IUCN.

significance to humans

None known.


Dwarf brittle star

Amphipholis squamata

order

Ophiurida

family

Amphiuridae

taxonomy

Amphipholis squamata Delle Chiaje, 1828, Mediterranean, Naples, Italy.

other common names

English: Scaly brittle star, long-armed brittle star; French: Ophiure écailleuse; German: Schuppiger Schlangenstern; Norwegian: Overgslangestjerne.

physical characteristics

The disk is very small, attaining a maximum diameter of only 0.19 in (5 mm); it is bluish or gray in color, circular in shape, and covered with scales. The arms are relatively short and thin. The radial plates are half-moon shaped and conspicuous. Dwarf brittle stars come in a range of colors from orange, dark brown or beige to black and gray. Previously these color varieties were thought to belong to sibling species, but genetic analyses do not support this hypothesis.

distribution

Amphipholis squamata has a world-wide distribution; it is found in all oceans of the world, including subarctic and subantarctic waters. This pattern of distribution is interesting; one would expect the organism to be restricted to certain regions since it is a brooder and supposedly a poor disperser. Genetic analyses indicate, however, that sporadic long-distance dispersal does in fact occur, probably through passive transport (drifting or rafting on macroalgae).

habitat

Dwarf brittle stars are found in the mid-and lower littoral zones among algae, bryozoans, and similar organisms; and sublittorally in waters several hundred meters deep. These brittle stars live mainly under stones but also on sandy surfaces.

behavior

Amphipholis squamata is a good climber that also uses its tube feet when it moves. The arms are extremely flexible in a vertical

direction. If the brittle star is dislodged from its substrate, it coils its arms over its disk and sinks rapidly to the bottom, probably to avoid being exposed to predators. When disturbed it produces light that appears as spots along each arm. Its bioluminescence is attributed to specific photocytes under the control of ganglia, or groups of nerve cells.

feeding ecology and diet

Amphipholis squamata feeds on deposits left on sediment and particles suspended in the water. It uses its tube feet to wipe off particles from its sticky spines. The stomach content includes a variety of such items as unicellular algae, small gastropods, foraminiferans and amphipod limbs. Amphipholis has sometimes been observed to feed on dead fish. It can absorb dissolved free amino acids from sea water, primarily through symbiosis with bacteria living under its cuticle.

reproductive biology

The dwarf brittle star is a simultaneous hermaphroditic viviparous brooder that can brood several embryos at different stages of development in each bursa at the same time. The eggs are small (0.0039 in or 100 µm), which suggests that the larvae must obtain nutrition from the parent within the bursae. Breeding occurs throughout the year. It has also been shown in the laboratory that Amphipholis can reproduce in isolation. This finding suggests that it can reproduce by self-fertilization or possibly by parthenogenesis.

conservation status

Not listed by the IUCN.

significance to humans

None known.


No common name

Amphiura filiformis

order

Ophiurida

family

Amphiuridae

taxonomy

Amphiura filiformis Müller, 1776, Scandinavia, probably southern Norway.

other common names

None known.

physical characteristics

Disk up to 0.39 in (10 mm) in diameter, reddish gray in color, with very long fine arms. The dorsal side is covered with fine scales but the underside is bare.

distribution

Northeastern Atlantic from Iceland and western Norway to the Iberian peninsula; Mediterranean.

habitat

Buries itself in fine muddy silt or sand at depths of 15–3,600 ft (5–1,200 m). Very common in the North Sea; populations may be as dense as 4,000 individuals per 10.8 ft2(1 m2).

behavior

When Amphiura is burrowing, it uses its tube feet to move material away from itself and virtually sinks into the substrate.

The animal then uses wavelike movements of its arms and digging movements of the tube feet to hollow out a burrow lined with mucus and equipped with channels to the surface of the sediment. For ventilation, it waves one or more of its arms to allow oxygen-rich water to enter the burrow. Amphiura filiformis is selectively pursued by some flatfishes like dab (Limanda limanda) and frequently sheds its arms when attacked. It is able to sense the presence of predators attacking other members of its species in the surrounding water and will then pull its arms back into the burrow.

feeding ecology and diet

Amphiura filiformis is primarily a passive suspension feeder that collects small organic particles on its long sticky tube feet, which are covered with numerous papillae. In moving water, it raises its arms about an inch (2.5 cm) above the seafloor. The particles that it collects are packed together with mucus into a bolus (soft rounded mass) and carried along its arm to its mouth beneath the sediment surface. This species may also collect particles from the surface of the sediment or feed on larger dead organisms.

reproductive biology

Amphiura filiformis has separate sexes and a seasonal developmental pattern with a spawning period that extends from late spring to late summer. When spawning, it emerges from its burrow and uses its arms to lift its disk about 0.39 in (1 cm) above the bottom. Its life history includes an ophiopluteus larva.

conservation status

Not listed by the IUCN.

significance to humans

None known.


Tropical brittle star

Ophiactis savignyi

order

Ophiurida

family

Ophiactidae

taxonomy

Ophiactis savignyi Müller and Troschel, 1842, Egypt, presumably Red Sea.

other common names

English: Savigny's brittle star.

physical characteristics

A very small brittle star with a disk only 0.039–0.11 in (1–3 mm) in diameter. It usually has six arms, but since it is fissiparous some individuals that have not completed regeneration may have as few as two arms. Ophiactis has a conspicuous color pattern of green with darker markings and large bare radial shields.

distribution

Found in all tropical seas. Studies of its genetic pattern indicate that it has recently spread to the Atlantic Ocean from the Indo-Pacific, probably in ballast water or in fouling communities coming through the Panama Canal on ships.

habitat

Ophiactis savignyi is often associated with algal turfs and sponges but also hides in reef crevices.

behavior

Ophiactis savignyi is cryptic and colonizes mainly algae or sponges; it probably receives shelter from predators among these organisms. It grows larger when it lives among sponges in comparison to algal turf, and may benefit from particles gathered by the sponge colonies from the water they take in. Ophiactis can reach high densities, sometimes more than 280 per deciliter of sponge.

feeding ecology and diet

The diet and feeding patterns of Ophiactis savignyi have not been studied as of 2003. Ophiactids, however, belong to a group that feeds mainly on small particles from the substrate or suspended in the water.

reproductive biology

Ophiactis practices both sexual and asexual reproduction by fission. During fission, the mutable collagenous tissue of the disk softens, forms a groove, and tears irregularly across the disk. The two halves then regenerate into two new six-armed individuals. A single clone can in this way colonize a sponge or patch of alga. Several investigated populations seem to be maintained by asexual reproduction only. Sexual reproduction takes place when the brittle star has reached a size larger than 0.11 in (3 mm). The larva is a typical planktotrophic ophiopluteus.

conservation status

Not listed by the IUCN.

significance to humans

None known.


Common brittle star

Ophiothrix fragilis

order

Ophiurida

family

Ophiothricidae

taxonomy

Ophiothrix fragilis Abildgaard, 1789, Denmark.

other common names

Dutch: Gewone brokkelster; German: Zerbrechlicher; Schlangenstern; Norwegian: Håstjerne.

physical characteristics

A large brittle star with a disk up to 0.78 in (20 mm) in diameter. The arms are long, colorfully banded and very bristly with seven glassy spines on each segment. Its color varies; it may be patterned in red, yellow, orange, violet, gray or brown.

distribution

Widely distributed in the eastern Atlantic from northern Norway to the Cape of Good Hope; also common in the Mediterranean.

habitat

Occurs from the lower shore to about 1,640 ft (500 m) on hard sea bottoms swept by currents; or beneath boulders, in crevices, or on substrates among sessile organisms. It may also form dense groups or aggregations (brittle star beds) on bare sand or shell sediments, with as many as 2000 individuals per square meter in habitats with few predators.

behavior

Predation pressure controls whether Ophiothrix fragilis hides in crevices or under rocks; that is, whether it is cryptic or lives in the open on the substrate. The formation of groups or aggregations on bare substrates helps individuals to maintain their position in strong currents. Under these conditions individual brittle stars interlock their supporting arms while lowering their disks onto the substrate. Lowering frictional resistance to the current helps to keep them stable. Juveniles seem to prefer to settle on or seek out adults; they can be seen clinging to the spines and bodies of adult individuals. They also settle on such other suspension feeders as sponges.

feeding ecology and diet

Ophiothrix fragilis is mainly a passive suspension feeder; like Amphiura filiformis, it collects organic particles on long and sticky tube feet covered with numerous papillae. It raises its arms about 3 in (7–8 cm) above the substrate; the collected particles are formed into a bolus and carried along the arm to the mouth.

reproductive biology

The sexes are separate, with a seasonal developmental pattern. The ophiopluteus larva can be found in the northwestern Mediterranean from March to October, with the peak settlement period in June. The time from fertilization to metamorphosis is about 26 days. The ophiopluteus has unusually long arms, indicating that it is capable of dispersing over long distances.

conservation status

Not listed by the IUCN.

significance to humans

None known.


No common name

Ophiura ophiura

order

Ophiurida

family

Ophiuridae

taxonomy

Ophiura ophiura Linnaeus, 1758 "the sea," (probably North Sea).

other common names

French: Ophiure commune; German: Gemusterter Schlangenstern; Norwegian: grå slangestjerne.

physical characteristics

Active brittle star with a large disk as much as 1.38 in (35 mm) in diameter and relatively short arms. The dorsal side of the disk is gray-brown or sandy orange and patterned with plates. The arm combs are well developed, with 20–30 papillae in each comb.

distribution

Northeastern Atlantic from northern Norway to Madeira; Mediterranean.

habitat

Lives on the surface of soft ocean bottoms at depths of 6.5–660 ft (2–200 m).

behavior

Ophiura ophiura moves rapidly in a rowing motion when it is disturbed. Occasionally it burrows shallowly in sediment. At rest, it assumes a position with its arms curved upwards at their tips, possibly to help it detect food from chemical cues in the water current.

feeding ecology and diet

Ophiura ophiura is an active omnivorous predatory brittle star that seizes its prey in one of its arm loops and then pounces on the food. It ingests a variety of small organisms that live on sediment, including mollusks, echinoderms, crustaceans and polychaetes; in addition it feeds on sediment, detritus, and benthic diatoms.

reproductive biology

Ophiura ophiura has separate sexes. Its developmental pattern is seasonal, with a long spawning period that lasts from spring to late summer. Its life history includes an ophiopluteus larva.

conservation status

Not listed by the IUCN.

significance to humans

None known.


Resources

Books

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Periodicals

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Mattias Sköld, PhD

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Ophiuroidea (Brittle and Basket Stars)

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