Savigny's Brittle Star

The tropical brittle star, O. savignyi, is not considered endangered. This species has a global distribution in tropical and subtropical regions, and is very abundant.

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

A defense mechanism of O. savignyi is its use of autonomy (casting off of limbs) when escaping predators. It also is capable of regenerating them. Negative phototaxis and damage-release alarm signals are also evolved characteristics to escape predators. A fish may bite the brittle star into smaller pieces before consuming it, so the ability to lose and regenerate limbs is an advantage. This species is prey to crabs and shrimp.

Known Predators:

• Fish, Actinopterygii
• Shrimp, Decapoda
• Crabs, Decapoda

Ophiactis savignyi is radially symmetric as an adult, and is generally found with six arms, but can be found with 1-7 arms. Individuals with fewer arms are in the process of regeneration. Like other ophiuroids, O. savignyi has arms that are sharply demarcated from the central disk. The arms are jointed and flexible and are usually variegated with intermittent dark and light markings. The animal's color ranges from mixes of green, brown, white, yellow, and cream. The oral surface is lighter than the aboral surface. Australian members of this species have a brown disc with bright yellow arms, but the species is usually green or blue. The disc of the animal is 3.8-5 mm and darkly pigmented with rough-tipped spines on top of the disc. The arms of the animal range from 16.3 mm-20 mm and have small, rough spines running along them. The oral surface of the central disc contains one to three oral papilliae that are flat and scaly. In O. savignyi, fission causes species to have a variable number of arms; specimens that recently split typically have three large arms and three small regenerating ones.

Sexual reproduction results in planktonic ophiopluteus larvae that have bilateral symmetry. They are v-shaped and have a crystalline skeleton with curving ciliated bands for feeding. Ophiopluteus larvae are free swimming until they metamorphose into adults.

Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry ; radial symmetry

Sexual Dimorphism: sexes alike

No information on the lifespan was found.

Ophiactis savignyi is benthic and resides inshore, on continental shelves, and continental slopes. This brittle star shelters itself in various subtropical marine and tropical marine habitats, including rubble, algae, corals, sponges, reefs, mangrove areas, ship hulls, and sea grasses like turtle grass. Ophiactis savignyi is more frequent on sponges than algae. This species can inhabit sponges in densities up to 1,892 individuals per 100 grams of dried sponge; 3,000 individuals per liter have also been reported. The density of O. savignyi depends on space and food available. One species of sponge inhabited by this brittle star is Tedania ignis; an inhabited algae species is Sargassum cymosum.

Range depth: 1 to 518 m.

Average depth: 259 m.

Habitat Regions: tropical ; saltwater or marine

Aquatic Biomes: benthic ; reef ; coastal

Ophiactis savignyi can be found in tropical marine habitats around the globe. Humans may have contributed to the dispersal of O. savignyi, especially in the western and eastern populations around the Isthmus of Panama. The species was separated by the isthmus until humans opened it with the Panama Canal. Ophiactis savignyi is found in the Pacific, the Atlantic Oceans, the Persian Gulf, and the Mediterranean Sea. Its ranges also include the northern, western, and eastern coasts of South America, Hawaii, French Polynesia, the Gulf of Mexico, the Caribbean, and the south-eastern and south-western coasts of North America. The brittle star is also along the coast of Australia and Southeast Asia extending up into China's eastern coast. This species is considered the most common brittle star in the world.

Biogeographic Regions: nearctic (Native ); palearctic (Native ); oriental (Native ); ethiopian (Native ); neotropical (Native ); australian (Native ); indian ocean (Native ); atlantic ocean (Native ); pacific ocean (Native ); mediterranean sea (Native )

Other Geographic Terms: cosmopolitan

Ophiactis savignyi is detritophagous, i.e., it feeds mainly on small particles of detritus. The stomach typically contains foraminiferans, bryozoans, organic detritus, and small gastropods. This brittle star is a suspension feeder using its tube feet to catch small particles and moving them toward its mouth. Ophiactis savignyi is also characterized as a deposit feeder, cleaning the outer surface of its habitat and filtering food from the water. A large individual suspension feeds by raising its arms into the water column. Ophiactis savignyi will also situate itself at the base of the excurrent pore of a sponge to collect food particles.

Animal Foods: mollusks; aquatic crustaceans; other marine invertebrates; zooplankton

Plant Foods: bryophytes; phytoplankton

Other Foods: detritus

Foraging Behavior: filter-feeding

Primary Diet: carnivore (Eats non-insect arthropods, Molluscivore , Eats other marine invertebrates); herbivore (Algivore); detritivore

Ophiactis savignyi dominates sponge habitats and competes with other brittle stars for space. In the Caribbean it inhabits at least ten species of sponges. This brittle star also inhabits various algae. Numbers are greater in sponges than in algal turf, probably because sponges provide a better refuge against predators. Other than providing food for species of fish, shrimp, and crab, the species at times occludes the excurrent siphon of sponges, which is potentially harmful. Ophiactis savignyi acts as a detritivore and recycles dead plankton, bacteria, small crustaceans, and invertebrates.

Algae that O. savignyi inhabit include Hypnea species. Sponges that O. savignyi inhabit include Haliclona species, Tedania ignis, Scopalina ruetzleri, and Amphimedon viridis.

Ecosystem Impact: biodegradation

Ophiactis savignyi has no significant positive impact on humans.

This species has no negative impact on humans or economic importance to them.

The newly fertilized eggs of O. savignyi turn into ophiopluteus larvae. The larvae have a crystalline skeleton, are bilateral, and free-swimming. The exact amount of time before metamorphosis into a radial, benthic adult is unknown, but it is estimated to be a month. Individuals produced by fission regenerate into two adult organisms.

Development - Life Cycle: metamorphosis

Ophiactis savignyi, like other ophiuroids, perceives the environment by chemosensors in its tube feet. This brittle star is also able to detect very dilute concentrations of amino acids, and vitamins, which allows it to detect food and predators. Ophiactis savignyi responds to damage-release alarm signals. A divergence in response time to chemical alarm signals occurs between algae and sponge dwelling individuals of the species. Individuals dwelling in an algae habitat respond to conspecific (interspecies) and heterospecific (among different species) signals. Sponge dwelling individuals have little response to heterospecific signals. Sponge dwelling O. savignyi are better protected from predators and do not have as much environmental pressure as their algae dwelling members. This species also exhibits negative phototaxis (move away from light), and senses light from light-sensitive cells in skeletal plates within its dermis.

Communication Channels: chemical

Other Communication Modes: pheromones

Perception Channels: visual ; tactile ; chemical

Sexual reproduction by O. savignyi involves broadcast spawning. Each sex scatters its gametes in the water column. During sexual reproduction gametes from different colonies presumably mix. Sexual reproduction is followed by asexual reproduction via fission. The majority of individuals lose the ability to reproduce sexually after splitting. Sexual reproduction, in summer and fall, is usually followed by asexual reproduction. The separation of these two breeding modes may be because after splitting, O. savignyi must regenerate its lost limbs, which takes away from its ability to produce eggs and sperm. In both sexes the gametes of one or both of the newly divided brittle stars are reabsorbed. When O. savignyi reaches a large enough size, at least a 3.0 mm disc, it begins to spawn sexually.

Mating System: polygynandrous (promiscuous)

Ophiactis savignyi reproduces either through asexual splitting or sexual broadcast spawning. Asexual reproduction occurs by voluntary splitting down the organism's central disc, producing two halves, which regenerate into two functioning organisms; this and predation accounts for odd number of limbs found in some individuals. In O. savignyi simultaneous asexual and sexual reproduction is also known to occur, but mature gonads are usually reabsorbed in one or both freshly split clones, making simultaneous sexual reproduction unlikely.

Sexual maturity of O. savignyi appears to depend on size. Ophiactis savignyi displays a difference in sex ratio that results in a higher proportion of males than females. This difference may be due to a greater investment in gonad mass by females. Males are more likely to retain their ability to reproduce after splitting than females, which may be the cause of the male biased sex ratio. Sexual spawning may result in long-distance colonization of sponges and algae by brittle stars and would also account for the unusual sex ratio; a single organism can generate an entire colony. In Harrington Sound, a sponge colony was found with an all male sex ratio that supports this hypothesis.

Breeding interval: Asexual and sexual year round, but sexual more frequent late summer to fall.

Key Reproductive Features: year-round breeding ; sexual ; asexual ; fertilization (External ); broadcast (group) spawning

Ophiactis savignyi does not have any parental investment. When large enough the organism reproduces sexually. Immature and mature organisms reproduce by splitting with both halves being autonomous.

Parental Investment: no parental involvement

Ophiactis savignyi is a species of brittle star in the family Ophiactidae, commonly known as Savigny's brittle star or the little brittle star. It occurs in the tropical and subtropical parts of all the world's oceans and is thought to be the brittle star with the most widespread distribution. It was first described by the German zoologists Johannes Peter Müller and Franz Hermann Troschel in 1842. The specific name honours the French zoologist Marie Jules César Savigny.

Hexamerous species. Colour in life: green background, disc with more or less regular dark and light green markings, arms similarly banded, ventrally white with some green markings on arms (Humphreys, 1981). Arm length to 2 cm, disc diameter to 5 mm. With 2-7 arms. Colour variegated light and dark on disc, banded yellow to blue green on arms. Aboral disc scales distinct, with scattered spinelets. Radial shields large, contiguous ar the distal end. Oral shields large, adoral shields smaller, not meeting proximally. Aboral arm plates elliptical with rounded lateral margins, distal edge convex with a median lobe made prominent by a dark spot on either side. Six short arm spines, rugose and spinulated. Reproduces asexually by splitting down the centre (fissiparity), resulting in individuals with arms of uneven length and irregular disc. Sexual reproduction is know also. Habitat: on rocks, gorgonians, sponges, crevices, algae, turtle-grass beds and other seagrasses. General distribution: tropical Indo-Pacific (Kalk, 1958); circumtropical (Tortonese, 1980); deep range 0-500, tropical, circumglobal (Rowe & Gates, 1995>/a>); tropical Atlantic, Indian and Pacific Oceans (Sastry, 1991). Also distributed in Gilbert Islands, Tuamotus (Clark, 1954); SE Arabia, Persian Gulf, W India, Pakistan, Maldive area Ceylon, Bay of Bengal, East Indies, north Australia, Philippine, China, south Japan, South Pacific Is. and Hawaiian Is. (Clark & Rowe, 1971); Australia (Kalk (1958) and Rowe & Gates (1995)); Lakshadweep (India) in Sastry (1991); India (Andamans and Lakshadweep), tropical Atlantic, Indian and Pacific Oceans (Sastry, 1996). Ecology: benthic, inshore, continental shelf, continental slope (Rowe & Gates, 1995).

Known from seamounts and knolls

Six arms. Disk circular (dd = 1.3 mm). Covered by numerous imbricating scales. Some small spines on scales at margin of disk (Fig. 8g). Radial shields large, triangular, contiguous (Fig. 8g). Ventral interradus with some spines on the scales (Fig. 8h). Bursal slit enlarged. Oral shields oval (Fig. 8i). Adoral shields widened laterally. One oral papilla on each side of jaw angle (Fig. 8i). Apical papilla well developed. Dorsal arm plate wider than long, sometimes subdivided into two plates (Fig. 8j). Ventral arm plate octogonal. Six arm spines, with denticles along margin and at tip (Fig. 8l). Single tentacle scale semi-elliptical (Fig. 8l).

Disk rounded (dd = 1.1 to 5 mm) and covered by large imbricated scales and short scattered spines over and at the edge of disk. Radial shields large, triangular and joining distally (Fig. 3J). Ventral interradius with scales and scattered spines (Fig. 3K). Oral shields diamond shaped with rounded edges. Adoral shields small and not meeting proximally. Two flattened oral papillae in each side of the jaw (Fig. 3L). Five or six arms. Dorsal arm plates wider than long with rounded edges (Fig. 3H). Ventral arm plates quadrangular with rounded edges. Five to six short, rugose and spinulose arm spines. Single, rounded, lanceolated tentacle scale (Fig. 3I). Color of the disk olive greenish-brown, with darker radial shields (Fig. 3G). Ventral side cream (Fig. 3K).

Cosmopolitan, in warm waters throughout the western Indo-Pacific, eastern Pacific, including Malpelo Island off western coast of Colombia, and both sides of the Atlantic, including Ascension island in the South Atlantic. Western Atlantic from South Carolina, Bermuda, Mexican Caribbean, Honduras, and Brazil (Devaney 1974, Pawson 1978, Hendler et al. 1995, Durán-Gonzáles et al. 2005, Cohen-Rengifo et al. 2009). In Brazil from Amapá, Pará, Maranhão (Albuquerque 1986), Ceará (Lima-Verde 1969), Paraíba (Gondim et al. 2008), Pernambuco (Tommasi 1970), Alagoas (Miranda et al. 2012), Bahia (Alves and Cerqueira 2000), Abrolhos off southern Bahia (Tommasi 1970), Rio de Janeiro (Brito 1960), and São Paulo (Tommasi 1970). Intertidal to 518 m. Found at 10 m in this study.

Cosmopolitan. Indo-Pacific, Eastern Pacific and both sides of the Atlantic. In the Eastern Pacific, the species has been recorded in the Hawaiian Islands, Mexico, Costa Rica, Panama, Colombia, Galapagos Islands and Peru (Clark HL 1915, Nielsen 1932, Maluf 1988, Hendler et al. 1995, Alvarado and Fernández 2005, Neira and Cantera 2005). In Mexico from the Gulf of California, on the Pacific side of Baja California and Baja California Sur, Nayarit, Marías Islands, Jalisco, Colima, Revillagigedo Islands, Michoacán, Guerrero and Oaxaca (Brusca 1980, Honey-Escandón et al. 2008). Commonly found in shallow water up to 518 m (Hendler et al. 1995). In this study, Ophiactis savignyi was collected on coral reefs from Nayarit, Jalisco, Colima, Michoacán, Guerrero and Oaxaca; between 2.3 to 26 m depth.