The crown-of-thorns starfish is one of only a few animals that feed on living coral tissue. The starfish is named for the dense covering of long, sharp spines on its upper surface. At low densities the crown-of-thorns starfish is a ‘normal’ part of the reef’s ecology. However, when the numbers of crown-of-thorns starfish on a reef increase to the point where they consume coral faster than it can grow, the starfish can dramatically reduce coral cover, resulting in a major disturbance to the whole system (see Environmental status - corals). This situation is commonly known as a crown-of-thorns starfish ‘outbreak’. Outbreaks of crown-of-thorns starfish have been a concern on the Great Barrier Reef (GBR) for more than 40 years. Research suggests that the outbreak ‘trigger point’ is around 30 mature crown-of-thorns starfish per hectare of coral reef that has average levels of coral cover. Once crown-of-thorns starfish densities exceed this threshold, the population will begin to consume coral faster than it can grow and is considered to be an outbreak population.
Patterns of crown-of-thorns outbreaks on the Great Barrier Reef
There have been three recorded series of crown-of-thorns starfish outbreaks on the Great Barrier Reef. However, since underwater exploration of the Great Barrier Reef only began in earnest with the advent of SCUBA equipment, it is possible that previous outbreaks have occurred but have not been recorded. The three observed series of outbreaks have followed the same general pattern, with outbreaks first recorded in the northern section of the Great Barrier Reef near Cairns. Over the following ten to fifteen years, the series of outbreaks slowly moved south with an increasing number of reefs affected by crown-of-thorns starfish outbreaks in successive years. Outbreaks tend to occur on mid-shelf reefs but can also occur on inner and outer shelf reefs. Previous series of outbreaks have dissipated in the Mackay region and outbreaks have not been reported in the Keppel Islands or Capricorn Bunker group reefs in the southern Great Barrier Reef. It is thought that the north to south spread of outbreak series is due to crown-of-thorns starfish larvae being transported from one reef to another by the south flowing East Australian Current.
There have also been crown-of-thorns starfish outbreaks observed on offshore reefs in the Swains region in the southern Great Barrier Reef. While these outbreaks do not appear to be directly linked to the north-south wave pattern observed in series of outbreaks in the northern Great Barrier Reef, the relationship between the two outbreak patterns is still unclear.
History of crown-of-thorns starfish outbreaks
There are varying amounts of information available on each of the three series of outbreaks, with the most complete monitoring records kept by the Australian Institute of Marine Science Long Term Monitoring Program (AIMS LTMP). A brief account of each outbreak follows, but more detailed accounts are summarised in Moran, 1986, and on the AIMS LTMP website.
1962 to 1976
The first ‘outbreak scale’ populations of crown-of-thorns starfish to be noticed and described were at Green Island and nearby reefs offshore from Cairns in 1962. By 1970, the series of outbreaks had reached reefs off Townsville, and by the mid 1970’s, had spread to reefs in the Whitsunday and Mackay region where it gradually petered out.
1979 to 1991
A second series of outbreaks, starting to the north of Cairns and spreading southwards, occurred between 1979 and 1991 (Moran, 1986; Sweatman et al, 1997, 2000, 2001). The second outbreak series affected approximately 17% of the coral reefs in the Great Barrier Reef, with 5% of reefs having severe outbreaks. Approximately 57% of reefs that experienced an outbreak suffered 30% to over 50% coral mortality over at least one-third of their perimeters. On average, this outbreak series caused a 3 to 4 fold increase in the amount of dead coral on affected reefs.
1993 to present
In 1993, the first stages of a third series of outbreaks was detected, once again in the region north of Cairns. Surveys of reefs offshore from Cairns during 1994-95 found that only two out of 27 surveyed reefs (7.4%) had reef-wide outbreaks. However, by 1996-97, seven out of 28 (25%) reefs had reef wide outbreaks, while another thirteen of these reefs had smaller outbreaks over part of their area (spot outbreaks), leaving only eight that were completely free from outbreaks. In addition, the proportion of observed crown-of-thorns starfish that were sexually mature increased every year, suggesting that the series of outbreaks could increase in severity and geographic range.
By 2001-2002, the series of outbreaks had moved further south with numbers of crown-of-thorns starfish decreasing on reefs off Cairns, but increasing in the central Great Barrier Reef. By 2003, the AIMS LTMP and divers conducting crown-of-thorns starfish control programs (see Response) were recording the highest numbers of starfish on reefs in the Townsville region. In the most recent surveys (February – March 2004) conducted by the AIMS LTMP, new outbreaks have been detected in the Cape Upstart sector and small numbers of crown-of-thorns starfish (below outbreak levels) have been recorded on two reefs in the Whitsunday sector. These observations represent a rise in the number of crown-of-thorns starfish observed in this sector compared with surveys in the past decade and indicate the continued southward spread of the current series of outbreaks. While there have been isolated, anecdotal reports of localised increases in crown-of-thorns starfish numbers at specific sites in the northern Great Barrier Reef, there are no indications of widespread increase in crown-of-thorns numbers in this region.
The 2003 LTMP surveys showed that 15% of the surveyed reefs had outbreaks of crown-of-thorns starfish. This is higher than the number of reefs affected in the 1988 series, which resulted in widespread declines in coral cover on reefs in the central Great Barrier Reef (Figure 1). An updated list of the reefs with active outbreaks and more information on the north-south outbreak patterns can be found on the AIMS LTMP website.
Recovery from crown-of-thorns starfish outbreaks
While crown-of-thorns starfish outbreaks can cause dramatic declines in coral cover, coral reefs can and do recover from outbreaks. The rate of recovery will depend on factors such as the extent of coral decline, the type of coral remaining, coral larval supply and recruitment, and the influence of subsequent disturbance events such as storms. Affected reefs have been observed to recover to pre-outbreak levels of coral cover within 10-15 years after the outbreak, although recovery may take longer if slower growing corals have been depleted. For more information about coral reef recovery, see Environmental status – corals: variation on coral reefs.
While three series of outbreaks have been documented on the Great Barrier Reef, the absence of historical information on the timing of outbreaks prior to 1960 makes it difficult to assess whether the pattern and intensity of outbreaks has changed (Engelhardt et al, 2001). It is also difficult to determine if the length of time between series of outbreaks is changing, which would affect the extent of coral recovery between outbreaks. Nevertheless, if the frequency of outbreaks has increased due to human activities, it is possible that this could lead to a gradual decline in coral abundance and diversity, especially if slower growing corals are consumed, or reef recovery is hampered by events such as coral bleaching or poor water quality. Broad-scale monitoring of the Great Barrier Reef needs to be maintained in order to document the frequency and patterns of crown-of-thorns starfish outbreaks, and detect possible changes in coral condition and community structure (see Response).
Reproductive characteristics and links to outbreaks
The crown-of-thorns starfish has a very high reproductive capacity. Each female can produce millions of eggs in a breeding season which means that a very small increase in fertilisation success or larval survival rates can result in a great increase in the number of juvenile starfish settling on the reef after the spawning event. As a result, outbreaks can theoretically arise from situations where fertilisation success or larval survival is increased.
Spawning and fertilisation
Crown-of-thorns starfish are dioecious, with males and females reproducing sexually through broadcast spawning with peak spawning activity recorded during December in the central Great Barrier Reef (Babcock and Mundy 1993). The exact timing of spawning is not predictable and spawning has been observed during the day and at night, and at various stages of the tidal and lunar cycles.
As well as producing large numbers of eggs and despite being broadcast spawners, the eggs themselves have high fertilisation rates. Studies have demonstrated fertilisation rates of more than 70% at a distance as great as 8m downstream from a single spawning male and successful fertilisation has been observed between spawning adults up to 100m apart. The large volumes of sperm released by male starfish are the primary cause of the high rates of fertilisation derived from widely spaced individuals. Under natural conditions this means that fertilisation rates can be quite high even though individuals may be spaced quite widely apart.
Reproductive output is also related to the availability of food. When there is little food available, adult females reabsorb their body wall and skeletal tissues, which reduces their size, lifespan and overall reproductive capacity (Stump 1993). However, when food is plentiful, females are able to take advantage of the favourable conditions and produce more eggs (Stump 1993).
Larval development may be influenced by environmental factors, and small variations in temperature and salinity may have significant effects on larval development and recruitment success (Lucas, 1982; Johnson and Babcock, 1994). This implies that crown-of-thorns starfish populations may be influenced by climatic events. Research has also demonstrated that the survival of crown-of-thorns starfish larvae may be closely linked to the availability of food and thus may be correlated with the amount of phytoplankton in the water (Lucas, 1982).
Spawning, recruitment and outbreaks
Another important factor to consider is whether outbreaks arise from a single spawning event, or the combination of several consecutive spawning seasons. Unfortunately, counting the numbers of crown-of-thorns starfish of a specific size present in a population cannot be used to identify recruitment pulses from previous years as starfish size may be dependant on the quantity and quality of the available food (Stump 1993).
Nevertheless, analysis of annual pigment bands in the starfish spines can be reliably used to determine the age of sexually mature crown-of-thorns starfish (Stump and Lucas 1990; Stump 1993). These studies suggest that outbreaks arise from several consecutive recruitment events and not from one single pulse. Research has also found no evidence of genetic differentiation between age classes present in outbreak populations. This suggests that outbreak populations are derived from one genetic source, with no change in the source of recruits (Benzie and Wakeford, 1997).
The crown-of-thorns starfish is an organism that can be viewed from two different perspectives. Firstly, that it is a component of the natural coral reef ecosystem and should be considered using the condition, pressure and response model as it relates to the starfish itself. Secondly, the crown-of-thorns starfish can be considered as a direct pressure on hard corals (because it feeds on them) and an indirect pressure on other reef organisms (such as many fishes and invertebrates) that are reliant on hard corals for food or shelter (see Environmental status – corals and Environmental status – fishes).
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