Asexual reproduction in starfish

Fission

Fissiparity in the starfish family Asteriidae is confined to the genera Coscinasterias, Stephanasterias and Sclerasterias. Another family in which asexual reproduction by fission has independently arisen is the Asterinidae. The life span is at least four years.

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.

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. 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.

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.

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.