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LARVAL DISPERSAL AND SETTLEMENT
28. How far do larvae of the crown-of-thorns starfish travel? Where do they go? These questions relate to a general process which scientists refer to as larval dispersal. It is perhaps one of the most important of all ecological processes that occur in the ocean and yet it is often the most poorly understood. Some scientists have suggested that this process may play a key role in the occurrence of outbreaks of the crown-of-thorns starfish. A combination of the right physical (eg. favourable currents and winds) and biological conditions (eg. decreased predation, increased food) during dispersal may lead to a small increase in the survival of the enormous numbers of larvae which may be produced during spawning. As suggested previously (see 26) a single female starfish may produce many millions of eggs (eg. 60 million). Increased survival of just a small proportion of this amount (eg. 2%) could lead to an enormous number of adults being produced (eg. 2% of 60 million - 1.2 million adults) from only 1 female. Of course, this assumes that all the larvae that survive to settlement develop into adults and that none are lost to predators or die of starvation. Clearly, this is unlikely. There is very little information about the dispersal of larvae of the crown-of-thorns starfish because scientists have been unable to obtain useful data on their distribution and abundance. Technically, this is very difficult as the larvae are extremely small (ie. between 0.5-1.0 mm in diameter). Given the tremendous volume of the ocean and its ability to dilute and disperse material, finding larvae of the crown-of-thorns starfish is very much like trying to find a "needle in a haystack". There is a further complication. The larvae of several other species of starfish (eg. the pincushion starfish, Culcita novaeguineae), look very similar to those of the crown-of-thorns starfish making it impossible for biologists to distinguish between them even when they are collected using plankton nets. Scientists are presently trying to overcome this problem by developing immunological markers (monoclonal antibodies) which will attach only to the exterior of larvae of the crown-of-thorns starfish (see 39). This will enable the larvae of this starfish to be readily distinguished from those of other starfish. Until this is achieved it is unlikely that major advances in our understanding of the dispersal of larvae of the crown-of-thorns starfish will be made. It is generally believed that larvae of the crown-of-thorns starfish behave in a manner similar to those of other marine animals and are carried passively in the prevailing water currents. Despite not knowing where they occur in the water column (eg. at the surface or down near the bottom) the larvae are thought to be incapable of swimming against even the weakest currents. Mathematical models of the water currents within the central region of the Great Barrier Reef have indicated that it is possible for the larvae of crown-of-thorns starfish to be transported up to 100 km in a week. As the maximum lifetime of these larvae is thought to be around 3 weeks it is feasible that they could travel some 300 km before settling. Long distance dispersal of this type may not always occur as further research has demonstrated that certain physical conditions (eg. tides, water currents, winds) may lead to larvae being retained in some locations on reefs for considerable periods of time (ie. for up to 20 days). Consequently, it is possible that a significant proportion of larvae may not be dispersed downstream to other reefs but may in fact be retained within the reef in which they were released (ie. the parent reef). It has been suggested that the areas where larvae are retained may give rise to outbreaks of crown-of-thorns starfish. This is presently being investigated. In the past it has been assumed that larvae normally starve during their time in the plankton and that it is only during times of very high nutrient conditions (phytoplankton blooms) that a small proportion of them survive. This notion has become the basis of an hypothesis (ie. terrestrial run-off) put forward to account for the occurrence of outbreaks of the crown-of-thorns starfish (see 13). Recent studies however, have shown that the larvae of this starfish can develop to settlement in the absence of such blooms. This work has important implications for the dispersal of not only the crown-of-thorns starfish but also many other coral reef organisms.
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