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Home › Ocean Life › Marine › Invertebrates › Echinoderms › Sea Stars › COTS

Facts about the Crown of Thorns Sea Star

[Crown of Thorns Starfish Phylum: Echinodermata] [Class: Asteroidea] [Family: Acanthasteridae]

The crown of thorns starfish (Acanthaster planci) is a large and venomous sea star that thrives best in the tropical or subtropical coral reef ecosystems.

But, one of the most interesting crown of thorns facts could be that this coral-eating marine invertebrate is one of the biggest starfish in the ocean.

Crown-of-thorns Starfish Range and Habitat

In fact, one of the most widespread of all coral reef starfish species is a corallivorous predator called the crown of thorn starfish.

They can be found 'infesting' reef communities throughout most of the tropical and subtropical latitudes of the Pacific and Indian Oceans.

Plus...

COTS also flourish in the Red Sea, the Great Barrier Reef, the coastline of East Africa, southern parts of Japan, and French Polynesia.

So, why is the crown of thorns starfish regarded as a serious problem?

The reason why it's a problem is linked with the reason why it's called crown-of-thorns starfish. This marine animal has a battery of long thorn-like venomous spines - shaped like a crown.

Simply put:

Its common name comes from a resemblance to the woven crown of thorns placed on the head of Jesus (according to the New Testament).

The spikes are stiff and sharp, and they cover almost all of the aboral surface (upper). However, the crown of thorn sea stars do most damage because they prey on various hard stony coral species - of the order Scleractinia.

Crown-of-thorns Preferred Habitats

The key areas where crown of thorns sea star are most prevalent are those with a superabundance of fast-growing hard (stony) corals, especially Acropora (e.g., staghorn coral) and Montipora (e.g., vase coral).

So, shallow reefs with warm, clear, nutrient-rich water will help support dense coral growth for many types of hard corals - their main food source.

In other words...

They usually inhabit reefs that are close to the surface or shallower than sixty metres below sea level. This "sunlit" zone is where the 4 types of coral reefs are most plentiful.

Pro Tip: When populations are excessive (unchecked by natural predators), the destruction caused by overfeeding on reef polyps may cause significant loss of coral formation, such as that seen in recent years on the Great Barrier Reef.

Crown of Thorns Starfish Description

Like most of the sea star phylum, the crowned starfish also has numerous arms, up to a maximum of twenty one (between 13 and 21).

The lurid colouring (usually brown, grey, purple, green, or red) and imposing features of some sea star species are unmistakable. They bend and twist their arms to smother the craggy contours of the delicate corals on which they feed.

The arms (called rays in marine biology terminology) are incredibly flexible and prehensile. Their "coronal" crown radiates outwards from a central disk, which is a soft part of the body used for feeding and digestion.

Crown of Thorns Starfish Size

Acanthaster planci is the biggest in the species, often growing to eighty (80) centimeters in diameter (30 inches).

Even so, the average size for a fully grown adult is closer to fifty (50) centimetres (18 inches).

Crown of Thorns Sting

So, how poisonous is the crown-of-thorns starfish and what happens if you get poked by this spiny creature?

In fact, the crown of thorns animal is a venomous marine invertebrate in the classification Asteroidea. Even so, a crown of thorns starfish sting injury, or puncture wound to the skin, from these bottom dwellers is more likely to be done by accident.

The damage occurs if the spine penetrates the skin, or from the gelatinous venom surrounding the needles. Some starfish spines are sharp and long enough (up to 6 centimetres) to pierce a diver's neoprene wetsuit or gloves.

Fun Fact: The crown of thorns cardinalfish (Siphamia fuscolineata) seeks refuge from its predators by sheltering among the venomous spines of the crown-of-thorns starfish (Acanthaster planci).

4 Types of Crown-of-Thorns Species

What Do Crown of Thorns Starfish Eat?

These predatory starfishes represent various types of sea stars that feed directly on coral reef polyps in their open environment.

The crown-of-thorn has an ability to invert its entire stomach organs through its mouth. They digest a thin layer of soft coral skeleton and suck in the nutrients.

As a consequence, some conservation groups have taken a pro-active approach to tackle the devastation caused to coral reefs by the diet of these invasive and destructive creatures.

For example...

Fisherfolk and concerned biologists instigated a cull in some areas. They also made attempts to eradicate the giant sea star (Pisaster giganteus) in other vulnerable regions by poisoning them.

However, some efforts failed because they are sea stars - meaning they are capable of limb regeneration. In some cases, they can regenerate from a chopped up animal that has been reduced to half of its normal size.

Fun Fact: Trials of an autonomous starfish-killing robot named "COTSBot" took place in September 2015 with the intention of reducing populations of crown of thorns on the Great Barrier Reef, northern Australia.

How Does Crown-of-thorns Reproduce?

Many sea stars reproduce through "synchronised" broadcast spawning. This kind of reproductive behaviour means the male and female of the species release eggs (up to 50 million eggs per season) and sperm at the same time into the water column.

As with most examples of echinoderms, the fertilised eggs eventually drift into tiny cracks and crevices of the reef and find relative sanctuary from hungry predators.

After fertilisation takes place, the embryos start to develop into free-swimming bipinnaria larvae that will drift with currents for several weeks or months. The larvae eventually settle on the ocean floor before they transform into juveniles.

In case you were wondering...

The species that reproduce asexually, either through fissiparity or fragmentation (e.g. Coscinasterias), often splits their body in half, and each separate half can regrow the missing parts.

Similarly, if a sea star loses one of its rays and the central disc, it can "clone itself" and grow into a whole new star fish through regeneration, a process that's more common in Linckia starfish.

Crown of Thorns Predators and Threats

One of the world's largest marine snails is Triton's trumpet (Charonia). A decline in their population led to a thriving opportunity for these predatory starfish.

As a matter of fact, only a handful of marine species specifically target the crown-of-thorns as a food source:

• Guard crabs (Trapezia)
• Harlequin shrimp (Hymenocera picta)
• Polychaete worms (annelids)
• Pufferfishes (e.g., Arothron hispidus)
• Triggerfishes (especially the Titan triggerfish)

The starfish's spines bend over and flatten if they're removed from the water. In fact, the body surface collapses, ruptures, and releases body fluid.

Nonetheless, they fully recover their tubular characteristics and crown-like shape if they're immersed back into the water alive.

Even though the crown of thorns starfish (Acanthaster planci) is not edible for human consumption, scientists are monitoring their environmental status as very high numbers in localised areas also threaten other types of marine wildlife.

Interesting Fact: The International Union for Conservation of Nature (IUCN) does not see any specific risk for the population status of crown of thorns starfish (Acanthaster planci).

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