(updated 8 May 2020)
The life of a Crown of-Thorns Starfish Acanthaster cf. solaris (CoTS) progresses through five main stages:
1. Egg > blastula > gastrula
A single large female CoTS can produce 100 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. Aquarium studies indicate they can reach the “competent to settle” stage (i.e., late brachiolaria) as early as 9 days after fertilisation, but development times vary depending on a number of factors, such as temperature and food availability. Pratchett et al. (2016) identified 17-22 days after fertilisation as the peak settlement window.
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.
On the Great Barrier Reef, the prevailing sea current runs south. CoTS outbreaks usually start in the northern sector – in the general vicinity of Lizard Island. Dispersal distance was thought to be largely determined by how long larval CoTS are able to survive while still retaining the capacity to settle, and how far the current can carry them in that period. The consensus estimate was around 30 to 50 days and tens or hundreds of kilometres. See Pratchett et al (2017) at 2.5. Subsequently Allen at al (2019) reported they are able to clone themselves in their larval stage. This could extend their dispersal range. See Larval cloning.
When they settle they are still very very small (~0.5mm) and difficult to find.
C0TS brachiolaria (second larval stage). Image © Zara-Louise Cowan
After settling, brachiolaria larvae metamorphose into juvenile starfish. By this stage they usually measure between 0.3mm and 0.8mm in aquarium studiees. 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: This post was provided by Dr Zara-Louise Cowan. 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. Jennifer Wilmes contributed additional information in 2020, along with a new post on Finding Baby CoTS