Starfish Life cycle

Starfish are capable of both sexual and asexual reproduction. Most species are dioecious, with separate male and female individuals, but some are hermaphrodites. For example, the common species Asterina gibbosa is protandric, with individuals being born male, but later changing into females.

Male and female sea stars are not distinguishable from the outside; one needs to see the gonads or be lucky enough to catch them spawning. Each arm contains two gonads, which release gametes through openings called gonoducts, located on the central body between the arms.

Reproduction

Fertilization takes place externally, both male and female releasing their gametes into the environment. The resulting fertilized embryos form part of the zooplankton in most species. However, some species brood their eggs, either by simply sitting over them, or using specialised brooding baskets on their aboral surface. In at least one species (Leptasterias tenera), the eggs are actually brooded inside the pyloric stomach. In these brooding species, the eggs are relatively large, and supplied with yolk, and they generally develop directly into miniature starfish, without a larval stage. Brooding is especially common in polar and deep-sea species, environments less favourable for larvae.

Sea stars commonly reproduce by free-spawning: releasing their gametes into the water where they are fertilized by gametes from the opposite sex. To increase their chances of fertilization, sea stars probably gather in groups when they are ready to spawn, use environmental signals to coordinate timing (day length to indicate the correct time of the year, dawn or dusk to indicate the correct time of day), and may use chemical signals to indicate their readiness to each other.

Some species of sea star also reproduce asexually by fragmentation, often with part of an arm becoming detached and eventually developing into an independent individual sea star. This has led to some notoriety .Sea stars can be pests to fishermen who make their living on the capture of clams and other mollusks at sea as sea stars prey on these. The fishermen would think they had killed the sea stars by chopping them up and disposing of them at sea, but each fragment would regenerate into a complete adult, ultimately leading to their increased numbers until the issue was better understood .A sea-star arm can only regenerate into a whole new organism if some of the central disk of the sea star is part of the chopped off arm.

Larval development

Like all echinoderms, starfish are developmentally (embryologically) deuterostomes; a feature they share with chordates (including vertebrates), but not with most other invertebrates. Their embryo initially develops bilateral symmetry, again reflecting their likely common ancestry with chordates. Later development takes a very different path, however, as the developing star fish settles out of the zooplankton and develops the characteristic radial symmetry. As the organism grows, one side of the body grows more than the other, and eventually absorbs the smaller side. After that, the body is formed into five parts around a central axis. Then the echinoderm has radial symmetry.

The larvae of echinoderms are ciliated, free-swimming organisms. Fertilized eggs grow into bipinnaria and (in most cases) later into brachiolaria larvae, which either grow using a yolk or by catching and eating other plankton. In either case, they live as plankton, suspended in the water and swimming by using beating cilia. The larvae are bilaterally symmetric — unlike adults, they have a distinct left and right side. Eventually, they undergo a complete metamorphosis, settle to the bottom, and grow into adults.

Lifespan

The lifespan of starfish varies considerably between species, generally being longer in larger species. For example, Leptasterias hexactis (adult weight 2 grams) reaches sexual maturity in two years, and lives for about ten years in total, while Pisaster ochraceus (adult weight 80 grams) reaches maturity in five years, and may live to the age of 34.