Asexual reproduction in starfish takes place by fission or through autotomy of arms. In fission, the central disc breaks into two pieces and each portion then regenerates the missing parts. In autotomy, an arm is shed with part of the central disc attached, which continues to live independently as a "comet", eventually growing a new set of arms. Fragmentation occurs on star fishes.
Fissiparity in the starfish family Asteriidae is confined to the genera Coscinasterias , Stephanasterias and Sclerasterias . [1] Another family in which asexual reproduction by fission has independently arisen is the Asterinidae. [2] The life span is at least four years. [3]
A dense population of Stephanasterias albula was studied at North Lubec, Maine. All the individuals were fairly small, with arm lengths not exceeding 18 mm (0.71 in), but no juveniles were found, suggesting that there had been no recent larval recruitment and that this species may be obligately fissiparous. Fission seemed to take place only in the spring and summer and for any individual, occurred once a year or once every two years. [4]
Another species, Coscinasterias tenuispina , has a variable number of arms but is often found with 7 arms divided into dis-similar sized groups of 3 and 4. It is unclear why fission starts in any particular part of the disc rather than any other, but the origin seemed to bear some relation to the position of the madreporites and the longest arm. This species typically reproduces sexually in the winter and by fission at other times of year. [5] The undivided individual has 1 to 5 madreporites and at least one is found in each offspring. New arms usually appear in groups of 4 and are normally accompanied by the appearance of additional madreporites. [5] The presence of multiple madreporites seems to be a prerequisite of fission. [5] In Brazil, only male individuals have been found and fission takes place all the year round, though primarily in the winter. Fission seems to be correlated with certain stress factors such as particularly low tides, when many starfish may be exposed to the air. [6]
Nepanthia belcheri has a variable number of arms and divides by fission in a similar manner. It is a hermaphrodite, some individuals having gonads that function as testes and others gonads that function as ovaries. After fission, the gonads regress and individuals that previously had mature female gonads become masculinized, developing male-type gonads. Many larger individuals provide evidence from the varying lengths of their arms that they have divided by fission on several occasions. [2]
In Sclerasterias , fissiparity seems to be confined to very young individuals. In these, there is a transitory hexamerous symmetry in what is a normally a pentamerously symmetrical genus. The immature individuals with 6 arms appear so different in appearance from mature individuals with 5 arms that they were at one time considered to be two genera, Hydrasterias and Sclerasterias. Juveniles with arms measuring between 8 mm and 15 mm (occasionally 20 mm) are usually involved in fission and undergo multiple divisions. A sample of 36 young Sclerasterias euplecta of this size was examined. 9 had only 5 arms and did not show evidence of fissiparity while the remainder had 6 arms, usually 3 longer than the other 3, following prior fission. In another sample of juvenile Scierasterias heteropau , the arms were similarly arranged in groups of three and there were 4 madreporites, 2 on the original and 2 on the regenerated section. Active fissiparity seems to be correlated with 6 arms and 4 madreporites. At some stage in their development as yet unexplained, only 5 arms and one madreporite appear, and the ability to divide in this way is lost. [1]
Writing in 1872, Lutken suggested that in certain members of the Ophiuroidea, "a radiary division occurs in which cast off arms formed new rays and a disk". [7] Six years later Ernst Haeckel observed that members of the genus Ophidiaster ( Linckia ) were prone to cast off arms and that new discs, arms, madreporites and mouths formed on the severed surface of these. [8]
In 1904, Kellogg observed numerous severed arms on reefs at Apia, Samoa, noting that many were sprouting new arms and suggested that Linckia diplax and Linckia pacifica had the ability to generate new individuals in this way. He thought the arms might be shed by autotomy. [9] In the same year, Monks showed experimentally that the "comets" developing from the severed arms of Linckia columbiae could indeed grow into new individuals. [10]
Linckia multifora and Linckia guildingi are two species of starfish found on Hawaii which were found to exhibit autotomy, shedding one or more arms frequently. The arms are known as "comets" and can move about independently and each one can grow into a new individual. Though severed from the nervous system and the water vascular system they still exhibit normal behaviour patterns. [11]
In a study undertaken in Hawaii, it was found that the detachment of an arm was not a sudden event. Most fractures took place about 2.5 cm (1 in) from the disk and started with a small crack appearing on the lower surface of the arm. This spread laterally and upwards towards the dorsal surface. Then the tube feet on the arm and those on the body pulled the two parts of the animal in opposite directions until they parted. The process could take about an hour to complete. The damaged tissue healed in about 10 days and the animal grew a new arm over the course of several months. Breaks took place in various positions on the arm, though Crozier noted a particular breaking zone in Coscinasterias tenuispina . [5] The immediate cause of the autotomy is not always apparent. Of 50 specimens of Linckia multifora brought to the laboratory, 18 had shed one or more arms within 24 hours. The mortality rate of newly severed arms was high, many succumbing to bacterial infection while the wounds were fresh. Once the wound had healed, in about 10 days, survival was more likely. [11]
When arms were severed into several lengths in the laboratory, it was found that those over 1 cm (0.4 in) in length were capable of regenerating. These included the tips of the arms and the central sections with wounds at each end. It takes about 10 months to regenerate a new disk with arms 1 cm (0.4 in) in length. The first development in the regeneration cycle is the formation of a crescent-shaped ridge at the damaged end. Grooves begin to form and a mouth develops at the point from which they radiate. The arms start to form and tube feet begin to appear. As the arms grow the disc begins to develop and eventually a madreporite appears. This process lasts for some time, and about 10 months after separation, the comet has a half disc and 4 arms about 1 cm (0.4 in) long. [11]
An echinoderm is any deuterostomal animal of the phylum Echinodermata, which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larvae, as adults echinoderms are recognisable by their usually five-pointed radial symmetry, and are found on the sea bed at every ocean depth from the intertidal zone to the abyssal zone. The phylum contains about 7,600 living species, making it the second-largest group of deuterostomes after the chordates, as well as the largest marine-only phylum. The first definitive echinoderms appeared near the start of the Cambrian.
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea. Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish are also known as asteroids due to being in the class Asteroidea. About 1,900 species of starfish live on the seabed in all the world's oceans, from warm, tropical zones to frigid, polar regions. They are found from the intertidal zone down to abyssal depths, at 6,000 m (20,000 ft) below the surface.
Brittle stars, serpent stars, or ophiuroids are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomotion. The ophiuroids generally have five long, slender, whip-like arms which may reach up to 60 cm (24 in) in length on the largest specimens.
Linckia laevigata is a species of sea star in the shallow waters of tropical Indo-Pacific.
The Ophidiasteridae are a family of sea stars with about 30 genera. Occurring both in the Indo-Pacific and Atlantic Oceans, ophidiasterids are greatest in diversity in the Indo-Pacific. Many of the genera in this family exhibit brilliant colors and patterns, which sometimes can be attributed to aposematism and crypsis to protect themselves from predators. Some ophidiasterids possess remarkable powers of regeneration, enabling them to either reproduce asexually or to survive serious damage made by predators or forces of nature. Some species belonging to Linckia, Ophidiaster and Phataria shed single arms that regenerate the disc and the remaining rays to form a complete individual. Some of these also reproduce asexually by parthenogenesis.
Coscinasterias calamaria, or the eleven-armed sea star, is a starfish in the family Asteriidae. It was thought to be endemic to southern Australia and New Zealand but has since been documented as occurring in the Cape Peninsula as well. It is found around low tide levels and deeper, under rocks and wandering over seaweed in pools.
Linckia guildingi, also called the common comet star, Guilding's sea star or the green Linckia, is a species of sea star reported from the shallow waters of the tropical Pacific Ocean, Indian Ocean, Atlantic Ocean and the Caribbean Sea.
Linckia multifora is a variously colored starfish in the family Ophidiasteridae that is found in the Indian Ocean and Red Sea. Its common names include the Dalmatian Linckia, mottled Linckia, spotted Linckia, multicolor sea star and multi-pore sea star.
Coscinasterias tenuispina is a starfish in the family Asteriidae. It is sometimes called the blue spiny starfish or the white starfish. It occurs in shallow waters in the Atlantic Ocean and the Mediterranean Sea.
Linckia columbiae is a species of starfish in the family Ophidiasteridae. It is found in the East Pacific where it ranges from California (USA) to northwest Peru, including offshore islands such as the Galápagos. Common names include fragile star, Pacific comet sea star and variable sea star.
Nepanthia belcheri is a species of starfish in the family Asterinidae. It is found in shallow water in Southeast Asia and northeastern Australia. It is an unusual species in that it can reproduce sexually or can split in two by fission to form two new individuals. As a result, it has varying numbers of arms, and Hubert Lyman Clark, writing in 1938, stated, "It is a literal truth that no two of the 56 specimens at hand, nearly all from Lord Howe Island, are exactly alike in number, size and form of arms".
The Brisingidae are a family of starfish found only in the deep sea. They inhabit both the Atlantic and Pacific Oceans at abyssal depths, and also occur in the Southern Ocean and around Antarctica at slightly shallower depths.
The Freyellidae are a family of deep-sea-dwelling starfish. It is one of two families in the order Brisingida. The majority of species in this family are found in Antarctic waters and near Australia. Other species have been found near New Zealand and the United States.
Aquilonastra conandae is a species of starfish from the family Asterinidae found near the Mascarene Islands in the Indian Ocean. It is known for its asexual reproduction and is fissiparous. It is a small starfish, discrete and camouflaged, and occurs in coral reefs in the surf zone of large waves. The species was described in 2006 by Australian marine biologists P. Mark O'Loughlin and Francis Winston Edric Rowe, and gets its name from Chantal Conand.
Aquilonastra burtoni is a species of small sea star from the family Asterinidae from the Red Sea which has colonised the eastern Mediterranean by Lessepsian migration through the Suez Canal, although the Mediterranean populations are clonal reproducing through fissiparous asexual reproduction. It was originally described in 1840 by the English zoologist and philatelist John Edward Gray.
Neoferdina cumingi, also known as Cuming's sea star, is a species of starfish in the family Goniasteridae. It is native to the tropical Indo-Pacific region.
Echinaster luzonicus, the Luzon sea star, is a species of starfish in the family Echinasteridae, found in shallow parts of the western Indo-Pacific region. It sometimes lives symbiotically with a copepod or a comb jelly, and is prone to shed its arms, which then regenerate into new individuals.
Henricia sexradiata is a species of starfish in the family Echinasteridae. It is native to the western Atlantic Ocean and the Gulf of Mexico.
Coscinasterias muricata is a species of starfish in the family Asteriidae. It is a large 11-armed starfish and occurs in shallow waters in the temperate western Indo-Pacific region.
Starfish, or sea stars, are radially symmetrical, star-shaped organisms of the phylum Echinodermata and the class Asteroidea. Aside from their distinguished shape, starfish are most recognized for their remarkable ability to regenerate, or regrow, arms and, in some cases, entire bodies. While most species require the central body to be intact in order to regenerate arms, a few tropical species can grow an entirely new starfish from just a portion of a severed limb. Starfish regeneration across species follows a common three-phase model and can take up to a year or longer to complete. Though regeneration is used to recover limbs eaten or removed by predators, starfish are also capable of autotomizing and regenerating limbs to evade predators and reproduce.