27. What is the life cycle of the crown-of-thorns starfish?

Once fertilisation (see 26) has occurred the egg develops from an embryo into a larva which feeds on small unicellular algae (phytoplankton). During its planktonic (ie. floating and drifting in the ocean) period the larva proceeds through several developmental phases going from gastrula to bipinnaria and brachiolaria. Towards the end of the last stage the larva develops a large sack like structure (primordium) and begins searching for a suitable surface on which to settle.

After settlement the larva metamorphoses into a juvenile starfish. This process takes about 2 days. Initially the juvenile starfish has only 5 rudimentary arms but additional arms develop rapidly as the starfish begins to feed on encrusting algae. At the end of 6 months the starfish is about 1 cm in size and begins to feed on corals. The starfish becomes sexually mature at the end of its second year by which time it has grown to about 20 cm in diameter.

After 3-4 years (when the starfish is about 35 cm in size) it is thought to go into a senile phase where growth declines dramatically and reproduction is low. This phase has been identified only in the laboratory. It is not known how long starfish live however, they have been kept in aquaria for as long as 8 years. A diagram of the life cycle of the crown-of-thorns starfish is given in Figure 13.

Figure 13
Diagrammatic representation of the life cycle of the crown-of-thorns starfish. Drawing not to scale.


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