Acanthaster planci
The Crown of Thorns Starfish is a large starfish, native to The Great Barrier Reef, growing up to 80cm wide (although commonly only half that size). They can have as many as 21 arms, 600 ovaries and are covered in hundreds of poisonous spines, up to 4cm long. In a single year, A. planci can produce up to 65 million eggs and eat 10 square metres of coral.
A. planci was responsible for 42% of coral death on The Great Barrier Reef between 1985 and 2012 according to the Australian Institute of Marine Science.1 Many people were lead to believe that it was an introduced species, however it is native to The Great Barrier Reef and it is actually responsible for increasing the diversity of the reef because it prefers to eat the fast growing corals, allowing slow corals chance to grow. It also clears areas to allow recruitment of new corals. Unfortunately, it is currently in plague proportions and AMPTO recently (November 2017) culled 35,000 of them in one, ten day trip to the reefs near Palm Island off Townsville, as well as 47,000 in the Swains Reefs in January (2017). Click here to read more about the threat Crown of Thorns poses to The Great Barrier Reef.8
Taxonomy
Domain: Eukaryotes
Kingdom: Animalia
Phylum: Echinodermata
Class: Asteroidea
Order: Valvatida
Family: Acanthasteridae
Genus: Acanthaster
Species: planci
Description
A large Starfish, growing up to 80cm wide, with as many as 21 arms. They are most easily identified by the hundreds of poisonous spines on their back. Their mouth is located on the underside, with the anus opening on the dorsal surface. Light-sensitive eyespots are present on the tips of the arms and they may range in colour from red-orange, to purple. Their skeletal structure consists of tiny structures called ossicles, which are made of magnesium calcite.6
Habitat
Acanthaster planci, is usually found on shallow coral reefs, hiding under coral structures such as Pocillopora and Acropora.5
Reproduction
Acanthaster planci reproduces by spawning in the summer months of November to February. They will crawl to a high point, arch their body and release sperm and eggs into the water through five pores, as they wriggle their arms and tube feet vigorously. They are capable of producing 65 million eggs in a spawning season.6
New Research (Feb 2019) also suggests that Crown of Thorns are capable of Larval Cloning. This means that they are capable of cloning themselves at the larval stage, further increasing their fecundity. Now that this has been discovered, it opens up possibilities in developing new methods to control populations9
Lifecycle
A. planci emerges from a fertilised egg as a pelagic (free swimming) larvae, feeding on phytoplankton for about 10-30 days. During this phase, they go through a series of changes, including a ciliated, gastrula phase, a bipinnaria (free swimming, bilaterally symmetrical) phase, and then a phase called, branchiolaria. The latter phase, is intermediate between the larval and adult stages. The larva sinks to the bottom and attach themselves to the substrate, beginning the metamorphosis process. Once the tube feet develop from the water vascular system and the skeleton forms in rings around the anus, the larva frees itself from the bottom.The juvenile starfish only has 5 arms, which increases into adulthood.They are believed to live for around 15-17 years, although this is yet to be confirmed.2,3,4,5
Diet & Predators
Acanthaster planci is a ferocious predator. During its larval form, it feeds on smaller plankton, however, once mature, they feed on hard corals, such as Pocillopora, Acropora, Pavona and Porites. They feed by spitting their stomachs out of their mouths and spreading it over the coral. They then release enzymes to break down the food and absorb the nutrients through the stomach wall. The Corwn of Thorsns Starfish is natural to The Great Barrier Reef but it can reach plague proportions where they digest up to 10 square metres of coral per year. The Australian Institute of Marine Science claims that The Crown of Thorns Starfish is responsible for 42% of coral death from 1985-2012.
Their main predators are The Giant Triton Shells (Charonia tritonis), although The Napoleon Maori Wrasse (Cheilinus undulatus), Starry Puffer (Arathron stellatus) and Titan Trigger Fish (Ballistoides viridescens) are also known to feed on Crown of Thorns.6,7
Conservation
Crown of Thorns is currently not under threat and in fact, it is in plague proportions. They have been allowed to proliferate for a number of reasons, such as declining water quality, which produces more phytoplankton – a favourite food of the larvae – and also the fact that their main predators, The Giant Triton Shells (Charonia tritonis), were hunted in the 60’s and 70’s to be sold to tourists. There are very few living Triton shells found on The Great Barrier Reef these days.7
