Asexual reproduction in starfish
Encyclopedia
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 which continues to live independently as a "comet", eventually growing a new disc and further arms. Only certain genera
of starfish are able to reproduce in these ways.
is confined to the genera Coscinasterias
, Stephanasterias and Sclerasterias
.
A dense population of Stephanasterias albula
was studied at North Lubec, Maine
. All the individuals were fairly small, with arm lengths not exceeding 18mm (0.8in), 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.
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 madreporite
s and the longest arm. This species typically reproduces sexually in the winter and by fission at other times of year. 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. The presence of multiple madreporites seems to be a prerequisite of fission. 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.
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 different genera, Hydrasterias and Sclerasterias. Juveniles with arms measuring between 8mm and 15mm (occasionally 20mm) 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.
suggested that in certain members of the Ophiuroidea, "a radiary division occurs in which cast off arms formed new rays and a disk". 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.
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
. In the same year, Monks showed experimentally that the "comets" developing from the severed arms of Linckia columbiae
could indeed grow into new individuals.
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.
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 1 inch (2.5 cm) 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
. 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.
When arms were severed into several lengths in the laboratory, it was found that those over 0.5 inch (1 cm) 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 0.5 inch (1 cm) 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 1cm (0.4in) long.
Autotomy
Autotomy or self amputation is the act whereby an animal severs one or more of its own appendages, usually as a self-defense mechanism designed to elude a predator's grasp...
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 which continues to live independently as a "comet", eventually growing a new disc and further arms. Only certain genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of starfish are able to reproduce in these ways.
Fission
Fissiparity in the starfish family AsteriidaeAsteriidae
Asteriidae is a family of Asteroidea in the order Forcipulatida.-Genera:Genera within the family Asteriidae include:* Ampheraster Fisher, 1923 * Anteliaster Fisher, 1923 * Aphanasterias * Asterias Linnaeus, 1758...
is confined to the genera Coscinasterias
Coscinasterias
Coscinasterias is a genus of sea stars of the family Asteriidae.-Species:* Coscinasterias acutispina* Coscinasterias calamaria - Eleven-arm sea star* Coscinasterias dubia Clark, 1909* Coscinasterias eupleca Fisher...
, Stephanasterias and Sclerasterias
Sclerasterias
Sclerasterias is a genus of starfish in the family Asteriidae. Adult individuals have five arms but small, immature individuals have six. This led to the giving of a separate generic name to the juveniles, Hydrasterias, before it was realised that only one genus was involved. These young...
.
A dense population of Stephanasterias albula
Stephanasterias albula
Stephanasterias albula is a species of starfish in the family Asteriidae. It is the only species in its genus, Stephanasterias, which was described by Verrill in 1871. It has a circumboreal distribution and is found in the north west Atlantic Ocean, the north east Pacific Ocean, the Barents Sea and...
was studied at North Lubec, Maine
Lubec, Maine
Lubec is a town in Washington County, Maine, United States. The population was 1,652 at the 2000 census. Lubec is the easternmost town in the contiguous United States . However, the Aleutian Islands in Alaska extend into the eastern hemisphere, and if territories are included, Point Udall in the...
. All the individuals were fairly small, with arm lengths not exceeding 18mm (0.8in), 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.
Another species, Coscinasterias tenuispina
Coscinasterias tenuispina
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.-Description:...
, 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 madreporite
Madreporite
The madreporite is a lightcolored calcerous opening used to filter water into the water vascular system of echinoderms. It acts like a pressure-equalizing valve. It is visible as a small red or yellow button-like structure, looking like a small wart, on the aboral surface of the central disk of a...
s and the longest arm. This species typically reproduces sexually in the winter and by fission at other times of year. 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. The presence of multiple madreporites seems to be a prerequisite of fission. 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.
In Sclerasterias
Sclerasterias
Sclerasterias is a genus of starfish in the family Asteriidae. Adult individuals have five arms but small, immature individuals have six. This led to the giving of a separate generic name to the juveniles, Hydrasterias, before it was realised that only one genus was involved. These young...
, 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 different genera, Hydrasterias and Sclerasterias. Juveniles with arms measuring between 8mm and 15mm (occasionally 20mm) 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.
History
Writing in 1872, LutkenChristian Frederik Lütken
Christian Frederik Lütken was a Danish naturalist, born 4 October 1827 in Sorø, died 1901.After a career in the Danish army until 1852, he decided to dedicate his life to natural history and left the army at the grade of first lieutenant. From 1856-1862, he was Private docent at the University of...
suggested that in certain members of the Ophiuroidea, "a radiary division occurs in which cast off arms formed new rays and a disk". Six years later Ernst Haeckel
Ernst Haeckel
The "European War" became known as "The Great War", and it was not until 1920, in the book "The First World War 1914-1918" by Charles à Court Repington, that the term "First World War" was used as the official name for the conflict.-Research:...
observed that members of the genus Ophidiaster (Linckia
Linckia
Linckia is a genus of sea star. They are found mainly in the Indo-Pacific region.They are known to be creatures with remarkable regenerative abilities, capable of defensive autotomy against predators...
) were prone to cast off arms and that new discs, arms, madreporites and mouths formed on the severed surface of these.
In 1904, Kellogg observed numerous severed arms on reefs at Apia, Samoa
Samoa
Samoa , officially the Independent State of Samoa, formerly known as Western Samoa is a country encompassing the western part of the Samoan Islands in the South Pacific Ocean. It became independent from New Zealand in 1962. The two main islands of Samoa are Upolu and one of the biggest islands in...
, 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
Autotomy
Autotomy or self amputation is the act whereby an animal severs one or more of its own appendages, usually as a self-defense mechanism designed to elude a predator's grasp...
. In the same year, Monks showed experimentally that the "comets" developing from the severed arms of Linckia columbiae
Linckia columbiae
Linckia columbiae is a species of starfish in the family Ophidiasteridae that is found off the coast of California and adjacent areas of the semi-tropical north east Pacific Ocean...
could indeed grow into new individuals.
Autotomy of arms
Linckia multiforaLinckia multifora
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 Dalmation Linckia, mottled Linckia, spotted Linckia, multicolor sea star and multi-pore sea star.-Description:Linckia multifora has a small...
and Linckia guildingi are two species of starfish found on Hawaii which were found to exhibit autotomy
Autotomy
Autotomy or self amputation is the act whereby an animal severs one or more of its own appendages, usually as a self-defense mechanism designed to elude a predator's grasp...
, 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
Water vascular system
The water vascular system is a hydraulic system used by echinoderms, such as sea stars and sea urchins, for locomotion, food and waste transportation, and respiration. The system is composed of canals connecting numerous tube feet...
they still exhibit normal behaviour patterns.
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 1 inch (2.5 cm) 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
Tube feet
Tube feet are the many small tubular projections found most famously on the oral face of a sea star's arms, but are characteristic of the water vascular system of the echinoderm phylum which also includes sea urchins, sand dollars and sea cucumbers and many other sea creatures.Tube feet function in...
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
Coscinasterias tenuispina
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.-Description:...
. 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.
When arms were severed into several lengths in the laboratory, it was found that those over 0.5 inch (1 cm) 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 0.5 inch (1 cm) 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 1cm (0.4in) long.