Crown of Thorn Starfish - COTS (Acanthaster planci) Drupella snail  Black Banding Disease  White Banding Disease  Coral Bleaching  Coral Breakages Coral Malformations Filamentous Algae or Turf Algae  Algal Blooms

 Crown of Thorn Starfish - COTS (Acanthaster planci)

 

The Crown of Thorn Starfish (COTS) is regarded as a natural and ecologically important member of a coral reef community.   Adult starfish can reach diameters of 40cm or more and have numerous arms, ranging from 7 to 23. COTS are covered by numerous long sharp poisonous spines and should be handled with the utmost care if removal programs on reef sites of commercial or scientific importance are necessary.    COTS have an extraordinary life cycle history and reach sexual maturity in 2-3 years. The female can produce over 100 million eggs in a single breeding season. If COTS are within one metre of each other, up to 90% of the female's eggs can be successfully fertilised during a simultaneous spawning event. The larvae are planktonic and feed on phytoplankton and dissolved organic matter (DOM). The larvae then transform into juvenile starfish and settle on a reef. Initially the small COTS will consume various coralline algae, only and later in life swith to live coral tissue andn polyps.

 

COTS outbreaks

If large numbers of larvae encounter favourable conditions they may settle en-masse and result in the sudden occurrence of large numbers of COTS on extensive coral reef areas (some management agencies define active outbreaks as areas with more than 30 adult COTS per hectare). These events are called COTS outbreaks. COTS outbreaks are a natural occurence on reef systems and have been documented on numerous reefs in the Indo-Pacific, including the Maldives, Indonesia, Australia's Great Barrier Reef (GBR), Vanuatu and Fiji. 

However, increased nutrients levels because of coastal development, direct and indirect effects of tourism, agricultural practices and exploitation of natural predators may assist larval and juvenile COTS to rapidly outgrow this most most vulnerable phase of the COTS life cycle. Once settled into their reef habitat COTS are well camouflaged and covered with toxic spines. This makes them hard to find and even more difficult to eat. Man-made influences may be an important factor contributing to the reduced interval between outbreak events as curently observed.  On the GBR,  for example, COTS outbreaks are now recorded every 3-5 years, compared to a cycle of 8-10 years during the 1980's.
 

When ample coral genera are present on a reef, COTS seem to prefer these fast-growing corals as their main food source. Because of this preference COTS may have a positive long-term effect on healthy reef systems by weeding out the fast growing coral species, thereby creating much sought after space for slow-growing massive coral species. Thus, an occassional COTS outbreak may have a similar effect on a reef as a cyclone.  However, on reefs under chronic stresses such as nutrient loading, temperature elevations and continuous breakages, a serious acute stress such as an COTS outbreak may cause long-term damage and permanent changes in the coral reef community.

 Drupella snail

 

This rather inconspicuous snail has been responsible for extensive coral reef damage and is a relatively common inhabitant of coral reefs throughout the Indo-Pacific. Drupella uses specialised mouth parts or a radula and feeds on living coral tissue. It leaves white scars on affected corals, similar to a small to medium Crown-of-Thorn-Starfish feeding scar.  Two species of Drupella, (Drupella cornus and Drupella robusta) have been identified as known coral-eaters, although a total of five species have been discovered within the Drupella genus.  Adults are identified by a robust snail shell (5 cm long) and are covered with small cones or spikes.  The shell is completely overgrown with calcareous algae. This gives the animal a rather rough and corrugated look with a deep purple colour.  Juvenile shells are white and vary in size from 0.5 to 1cm.

 

Drupella feeding behaviour

The feeding behaviour of Drupella makes it a rather difficult to see whilst snorkelling or diving on a coral reef. The most conspicuous signs are usually the white feeding scars. A quick inspection at the base of the affected coral colony may reveal clumps of Drupella snails feeding or hiding deep within the coral colony. Typical coral favourites of Drupella are the Acropora family such as staghorn and plate coral colonies.

Concerns in Australia were initially raised in Western Australia where massive population "outbreaks" caused extensive coral damage to Ningaloo Reef during the 1980's. Within the Great Barrier Reef the distribution of Drupella can be categorised at a persistent low level.

 Black Banding Disease

 

Recently it has been discovered that this disease is actually an array of microorganisms,  very similar to microbial mats found in sulphur rich marine floor habitats such as deep sea vents.  It is the exposure of coral to this sulphur enriched micro- environment that kills the tissue.  Black banding can destroy coral tissue at a rate of over 2 cm per day and may result in the death of the entire coral colony.   A coral colony affected by Black banding disease will exhibit a distinctive black junction between the living coral tissue and the recently dead white skeleton. This black band itself is usually only 3-4 mm across, but may be over a meter long in large colonies.
Branching forms of coral usually have several branches affected simultaneously. Numerous corals are affected by black banding, ranging from the abundant plate and staghorn (Acropora) corals to the massive brain (Goniastrea) and boulder (Porites) coral species.   Black banding has been linked to pollution such as increased nutrification levels due to sewage disposal. However, more observations and studies are needed to identify the causes on this serious coral disease.

 White Banding Disease

 

A coral colony with White banding disease will show a distinct white band at the border between the living coral and the dead skeleton. Sometimes sections of the band will be brownish in appearance. This disease tends to be more frequently observed during summer on Acroprid corals, and studies have shown it is linked to high water temperatures. White banding destroys coral tissue at a rate of several mm per day.   Recently it has been shown that there are two forms of White banding disease - Type I and Type II. Type I exibits a clear line between the healthy and affected coral tissue, whereas in Type II such a markation is less clear and bleaching may be present.  Although the cause of white banding disease remains unknown, this is the only coral disease that has been responsible for changing coral communities on a regional scale.

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