The life of a Crown-Of-Thorns Starfish Acanthaster planci (COTS) progresses through five main stages:
1. Egg > blastula > gastrula
A single large female COTS can produce 60 million eggs over a spawning season. This occurs during the warmer months of October to February in Australian waters. The eggs are released into the water, where they are fertilised by sperm released simultaneously by nearby males.
The eggs begin dividing after fertilisation, reaching the 8-cell stage within hours.
The ball of cells continues to divide, forming a blastula (hollow ball of cells), hatching as a free-swimming gastrula (three-layered structure) after approximately one day. Unfertilised eggs are not buoyant and will not remain viable for an extended period of time.
2. Bipinnaria – the first larval stage
The COTS bipinnaria stage begins around day 2. Bipinnaria are zooplankton. They use bands of hair-like cilia to propel themselves, but are incapable of swimming against a current. The cilia also help them feed on plankton. Bipinnaria larvae are approximately 0.5 – 0.8 mm long.
3. Brachiolaria – the second larval stage
COTS brachiolaria begin to emerge around day 5 and grow to around 1.0 – 1.5mm.
The larval stages are able to remain viable in the water column for several days or even weeks before they settle on reefs. Some larvae remain close to where they were spawned. Others are dispersed as zooplankton, dependent on factors such as local currents. See Hock et al (2014) Connectivity networks reveal the risks of crown-of-thorns starfish outbreak on the Great Barrier Reef.
After settling, brachiolaria larvae metamorphose into juvenile starfish. Initially they have only 5 rudimentary arms, but they rapidly develop more. Juveniles feed on crustose coralline algae for the first 6 months. Their colour helps camouflage them against the algae.
At around 6 months, they start feeding on coral polyps and are able to grow much faster.
COTS reach sexual maturity towards the end of their second year. At this stage they are typically around 20 cm in diameter. They continue to grow for another couple of years. Most get to around 25-35 cm in diameter. Some grow more than twice that size, depending on the availability of food (corals) and living conditions. COTS can survive without feeding for up to 9 months, however, they may shrink in size when starved, which can make it difficult to age them. They have organs of sight and smell and are able to move to new coral using the tiny tube feet under their arms.
COTS have been known to live for up to 8 years in an aquarium. Their life expectancy in the wild has not been scientifically established.
Editor’s Note: Zara-Louise Cowan is a JCU PhD Student. In 2015 she was the recipient of a LIRRF grant funded under the Ian Potter Foundation 50th Anniversary Commemorative Grants Scheme to undertake field research on COTS at the Australian Museum’s Lizard Island Research Station.