Settlement and recruitment of Acanthaster planci on the Great Barrier Reef: questions of process and scale

This paper briefly reviews information on settlement and recruitment of Acanthaster planci in the Great Barrier Reef (GBR) system, with emphasis on the recent phenomenon of population outbreaks of the starfish. Observations that recent outbreaks have occurred in two series of activity (1962-77 and 1979-91 +), each characterized by a southward wave of infestations in the central section of the GBR, and that outbreaks are not preceded by observable increases in densities of juveniles, have important implications for settlement and recruitment processes. The pattern of outbreaks indicates that primary outbreaks occur infrequently and unpredictably in the vicinity of 16 degrees S. However, it is not possible to assert that primary outbreaks have not occurred elsewhere in the GBR system, and isolated outbreaks at the southern end of the GBR may be primary events. Present data are insufficient to discern whether recruitment leading to primary outbreaks is by mass settlement of larvae or aggregation of adult starfish of various ages. In contrast, evidence indicates strongly that the majority of outbreaks in the system are secondary infestations as a result of water-borne transport and subsequent mass settlements of planktonic larvae seeded by other outbreak populations (and initially by populations undergoing primary outbreaks). The likelihood and pattern of secondary outbreaks is influenced by several parameters that operate at vastly different scales. These include passive transport of larvae by large-scale circulation patterns at scales of 10 to the power of 4-10 to the power of 6 m, which can largely account for the southward wave, and substratum selectivity by larvae at small scales (0-10 to the power of -3 m). The spatial distribution of important cues for larval settlement (coral rubble and the coralline alga Lithothamnium pseudosorum) suggests that mass settlements are more likely to occur in deep water at the base of reefs, where they are less likely to be observed. Several foci are defined for future research on settlement and recruitment processes. These include (1) identification of parameters influencing the spatial and temporal distribution of recruitment events that initiate primary outbreaks, (2) testing of hypotheses relating to dissipation of the southward movement of outbreaks at about 20 degrees S despite an abundance of reefs in the area supporting high coral cover, (3) determining the period of competency of larvae for settlement, (4) elucidating the behaviour of larvae (particularly late brachiolaria) in the water column, (5) further work to identify substrata inductive of larval settlement and to determine the nature of inducers, the spatial distribution of inductive substrata, and the effect of small-scale hydrodynamic processes in modifying the response of larvae to inductive substrata, and (6) testing of the hypothesis of deep-water recruitment.