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
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
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
This species has no negative impact on humans or economic importance to them.
Ophiactis savignyi has no significant positive impact on humans.
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 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 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 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
No information on the lifespan was found.
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
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:
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.[2] 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.
The disc of O. savignyi is up to 5 millimetres (0.2 in) in diameter. The aboral (upper) surface is covered by large overlapping scales and bears a scattering of spines, especially round the edges of the disc. The (usually) six arms are long, slender and tapering, and are composed of many segments with joints between them. Each segment bears five or six thorny spines. The aboral surface of the disc is a pale greenish-brown colour and the large, triangular radial shields, close to the origins of the arms, are contrastingly darker. The oral (under) surface is cream-coloured.[2]
O. savignyi has a near cosmopolitan distribution in warm seas. It is present in the western Indo-Pacific region, the eastern Pacific Ocean, and on both sides of the Atlantic Ocean.[3] The populations in the Pacific and the Atlantic Oceans were only united when the Panama Canal was cut across the Isthmus of Panama in 1914.[2]
This brittle star occurs from the intertidal zone down to about 500 metres (1,640 ft). It is found in all the zones of reefs, in mangrove habitats, in seagrass meadows, among seaweeds and in contaminated waters, and is often found living inside sponges, possibly as a commensal.[3]
Ophiactis savignyi is a deposit feeder and a scavenger, feeding on the detritus that accumulates on the sea bed. It raises the tips of its arms to detect food particles, rolls any nutritious items into a ball and moves them to the mouth, manipulating them by means of the tube feet. Examination of the stomach contents showed the remains of bryozoans, foraminiferans and gastropod among the large quantities of detritus and grains of sand that this brittle star had ingested.[4]
Individual O. savignyi are either male or female, and they can reproduce either sexually or asexually. Sexual reproduction involves the liberation of sperm and eggs into the sea and the development and eventual settlement of planktonic larvae. Asexual reproduction is by fragmentation, each portion being the same sex as its parent. Small, immature individuals (disc diameter under 4 millimetres (0.16 in)) usually have six arms and can split themselves in two and then regenerate the missing parts of the disc and arms, often ending up with five arms. Most larger individuals have five arms and can also undergo fragmentation. The larger males do so more often than do the females, and this may account for the fact that there is an excess of males in the population.[5] After splitting, the brittle star may still be able to reproduce sexually but some fragments of the disc may have no gonads and thus be unable to spawn until regeneration is complete.[4][6]
In Taiwan, mature gametes occur at any time from March to December, but most of the population of O. savignyi spawn during May and June. Fission takes place at any time of year but mostly occurs between July and December. Sampling the brittle stars throughout the year resulted in finding that 48% of the individuals inspected were in the process of regeneration. The sex ratio in this locality is twenty-four males for every one female.[7]
In Hawaii, this brittle star is often to be found living in association with a sponge such as Lissodendoryx schmidti (previously Damiriana hawaiiana). Up to twenty individuals can be found clumped together in cavities in the base of the sponge, the area being so filled with the slime secreted by the sponge that the brittle stars can hardly move their arms. These brittle stars often seem to have broken or partly regenerated arms. It is not clear how they enter the sponge, nor how they feed or reproduce.[8]
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.
Ophiactis savignyi, ou Ophiure de Savigny, est une espèce d'ophiure de la famille Ophiactidae. Elle est présente dans les parties tropicales et subtropicales de l'ensemble des océans du monde. Il s'agit probablement de l'ophiure avec la distribution géographique la plus importante. Le disque central d'O. savignyi a un diamètre compris entre 5 et environ 10 mm ; elle possède généralement 6 bras.
Le disque central de l'espèce mesure généralement 5 mm de diamètre, parfois jusqu'à 10 mm. La surface dorsale est recouverte d'écailles et porte une multitude d'épines, surtout sur les bords du disques. Les bras, habituellement au nombre de six, sont longs, fins et effilés ; ils sont composés de plusieurs segments maintenus ensemble par des articulations. Chacun de ces segments porte cinq ou six épines. Les bras atteignent une quinzaine de centimètres. La surface dorsale du disque oscille entre le verdâtre et le marron pâle alors que les grands boucliers radiaux, les plaques protectrices situées sur les bords du disque, sont sensiblement plus sombres. La surface orale, ou inférieure, est de couleur crème[2],[3],[4].
Dessin holotype par Jules-César Savigny.
_Figure_11_f.jpg)
_01.jpg)
Spécimen observé dans son environnement à Singapour.
O. savignyi est un omnivore opportuniste à tendance charognarde : l'ophiure se nourrit des détritus déposés sur le substrat. Elle repère la nourriture à l'aide de ses bras articulés, puis forme une boule de détritus qu'elle porte à sa bouche en la manipulant à l'aide du podion. Le régime alimentaire de l'espèce se compose principalement d'ectoproctes, de foraminifères et de gastéropodes ; l'ophiure ingère également une quantité importante de sable[5].
La reproduction peut être sexuée ou asexuée. Dans le premier cas un nuage de semence comprenant des gamètes mâles et femelles est libéré, cela permet le développement d'une larve planctonique qui deviendra une ophiure après plusieurs stades larvaires. La reproduction asexuée suppose une fragmentation de l'ophiure : le nouveau spécimen ainsi crée est du même sexe que le spécimen d'origine. Les jeunes spécimens possèdent généralement six bras : ils ont la capacité de se séparer en deux avant de régénérer la partie manquante du disque et des bras. À l'issue de ce processus, les bras sont généralement au nombre de cinq ; le nouveau spécimen peut ensuite se reproduire sexuellement si certaines parties du disque central ne sont pas dépourvues de gonades, auquel cas cela n'est pas possible avant la fin de la régénération[5],[6]. Les mâles ont plus souvent recours à la fragmentation, ce qui pourrait expliquer la surreprésentation des mâles par rapport aux femelles[7]. À Taïwan, un recensement a établi qu'il y avait 24 mâles pour une femelle[8].
À Hawaï, l'espèce interagit avec l'éponge Lissodendoryx schmidti, probablement dans une relation de commensalisme. Cette éponge abrite jusqu'à une vingtaine d'ophiures qui parviennent à peine à se mouvoir tant elles sont resserrées. Ces ophiures présentent souvent des bras coupés ou en cours de régénération. Les observations ne permettent pas de savoir précisément comment ces spécimens parviennent à entrer à l'intérieur de l'éponge, ni comment elles se nourrissent[9]. À Singapour, l'espèce est observée en relation avec l'éponge Spheciospongia vagabunda[10].
L'espèce est décrite par les zoologistes allemands Johannes Peter Müller et Franz Hermann Troschel en 1842. Elle est d'abord placée dans le genre Ophiolepis avant de rejoindre le genre Ophiactis quand celui-ci est mis en place par le naturaliste danois Christian Frederik Lütken en 1856. L'épithète spécifique est choisie en l'honneur du zoologiste française Jules-César Savigny[1].
La répartition d'O. savignyi est presque cosmopolite, c'est-à-dire que l'espèce peut se rencontrer dans toutes les régions du monde. Ainsi, cette ophiure est présente dans le bassin Indo-Pacifique Ouest, dans l'est de l'océan Pacifique et des deux côtés de l'océan Atlantique[11] ; les populations présentes dans le Pacifique et l'Atlantique se sont mélangées après l'achèvement du canal de Panama, au niveau de l'isthme de Panama en 1914[3].
L'ophiure se rencontre depuis la zone intertidale jusqu'à environ 500 m de profondeur. L'habitat de l'espèce est particulièrement varié : elle apprécie les récifs, la mangrove, les herbiers marins, les fonds colonisés par le varech et même les eaux polluées. O. savignyi s'observe régulièrement à l'intérieur d'éponges, ce qui suggère probablement une interaction de commensalisme : l'hôte fournit une partie de sa nourriture au commensal[11].
Ophiactis savignyi, ou Ophiure de Savigny, est une espèce d'ophiure de la famille Ophiactidae. Elle est présente dans les parties tropicales et subtropicales de l'ensemble des océans du monde. Il s'agit probablement de l'ophiure avec la distribution géographique la plus importante. Le disque central d'O. savignyi a un diamètre compris entre 5 et environ 10 mm ; elle possède généralement 6 bras.
Ophiactis savignyi is een slangster uit de familie Ophiactidae.
De wetenschappelijke naam van de soort werd in 1842 gepubliceerd door Johannes Peter Müller & Franz Hermann Troschel.
Ophiactis savignyi is een slangster uit de familie Ophiactidae.
De wetenschappelijke naam van de soort werd in 1842 gepubliceerd door Johannes Peter Müller & Franz Hermann Troschel.
.jpg)
