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Introduction to the reef

What made the reef?

Dangers of the reef!

The Crown Of Thorns Starfish

The infamous crown-of-thorns starfish grows to over a foot across and has 10-20 arms. It is well known for its voracious appetite for live hard-corals. At various times it has been blamed for the killing of large portions of reefs in parts of the pacific ocean, including a large portion of the great barrier reef of Australia during the 1960's. It is so despised that many scuba clubs organize "starfish hunts" in which these starfish are rounded up in an effort to save reefs from destruction. These starfish should be handled carefully, since the long, sharp spines are mildly venomous and can inflict painful wounds (slow to heal, too, as many can attest!).

Crown of Thorns Seastars are not just found in Australia, but in Australia they are found on the Great Barrier Reef. These echinoderms (star shaped animals) can occur in plague proportions, eating all live coral in their path, and are drastically reducing the size and viability of Australia's Great Barrier Reef. The Crown-of-thorns are most active in summer and are the only known venomous sea star.

DESCRIPTION

World wide crown of Thorns are Multi coloured but those on the Great Barrier Reef are normally brown or reddish grey with red-tipped spines. These spines are long, sharp, poisonous and number in their Thousands and grow to a length of 4 to 5 cm (A thin layer of skin with venom glands envelopes the spines) They do not have a set number of arms, the number of arms ranging from 6 to 23 Their size is usually 25-35 cm in diameter, but some individuals can grow to 80 cm depending on availability of certain corals.

HABITAT & LOCATION

Found on Australias Great Barrier Reef, they obviously like to live on coral but the crown-of-thorns starfish prefers to live in sheltered areas such as lagoons, and in deeper water along reef fronts, where there is less disturbance by currents, tides, weather etc.

FEEDING

The crown-of-thorns seastar feeds on polyps. It spreads it's stomach out through the mouth over a lump of living coral, secreting digestive juices, kills the coral and then sucks in the resulting "soup". After feeding it moves on leaving a patch of white, or a coral skeleton. The Crown of Thorns seastar usually feed twice a day for several hours.
Small and juvenile Crown of Thorns starfish feed at night, so they can avoid predators which are more active during daylight. Depending on its size the Crown of Thorns can eat from 2 to 6 sq. meters of coral a year. That can be up to 180 times as much coral destroyed compared to its own size per year When times are bad, like many other starfish, it may survive without feeding for up to 9 months.

Preferred Coral
The Crown-of-thorns starfish will eat most types of coral but prefer the branching tubular and staghorn type corals called Acropora (which is fast growing). They do not tend to eat Brain corals or large, hemispherical (termed massive) corals (such as the coral Porites).

DEFENCE

The Crown of Thorns like all Echinoderms have hard plates in the skin, but the Crown-of-thorns starfish also has spines and these are covered with skin filled with poisonous glands. These contain toxic compounds called saponins which are a group of chemicals that are related to steroids and are thought to be a defence mechanism against other marine animals This same toxin is painful to humans who if they step on or touch a C of T experience intense pain (for up to several hours) and sometimes nausea and vomiting. The human's skin may turn a dark blue and swell up for a couple of days.

PREDATORS

The Crown of Thorns has a few predators:

1. the helmet shells;
2. the rare Giant triton shell, Charonia tritonis;
3. harlequin shrimp, which are found in pairs - they feed on sea stars and tear a hole in the skeleton to feed on the tissue;
4. The humphead Maori wrasse;
5. Puffer fish;
6. two species of trigger fish;
7. a shrimp and a worm.

But there are not enough of these predators to markedly reduce the numbers of the C of T and thus slow down the destruction of the Great Barrier Reef.

MOVEMENT

They do not swim (except very weakly at the Laval stage) Juvenile and adult crown-of-thorns starfish do not walk on their arms but rather on a large number of tiny tube feet which occur along grooves extending underneath each arm The tube feet are operated hydraulically using fluid within the cavity of the starfish.

BREEDING

Crown-of-thorns are prolific breeders. Large females can release many millions of eggs over the breeding season Starfish come together into groups in the summer breeding season (Nov to Jan)and Females shed eggs into the water which are fertilised by sperm simultaneously released from nearby males into the water. Therefore Fertilisation occurs outside the female's body. Spawning has been observed at most times of the day and night and there seems to be little lunar or tidal pattern to the timing of spawning.

Life Cycle

1. The fertilised eggs floats away. In this planktonic stage the larva develops a large sack like structure When it has grown to half a millimeter in size it settles on to a reef.
2. After settlement the larva metamorphoses (changes) 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.
3. At the end of 6 months the starfish is about 1 cm in size and begins to feed on corals. It is still vulnerable to predators and it hides, only coming out at night to feed.
4. sexually mature at the end of its second year by which time it has grown to about 20 cm in diameter.
5. By 3 to 4 years they are mature and stop growing.
6. Their life expectancy is not certain but Crown of Thorns have lived up to 8 years in an aquarium.

Unlike some species the crown-of-thorns starfish is not known to reproduce asexually. That is, an entire new starfish cannot form from just a small part (e.g. arm) of its body.

OUTBREAKS

Outbreaks are where there is a sudden explosion in numbers of Crown of Thorns due it is thought to favourable spawning conditions, and high survival rates of juvenile Crown of Thorns
They appear natural but some scientist point out that if the rate of destruction of the reef in recent years had been occurring for centuries before mans intrusion on the Great Barrier Reef, the reef would not exist today. Are we (man) responsible for these increased "outbreaks"? Lots of research is being done to determine the factors effecting outbreaks.