CRC REEF RESEARCH CENTRE TECHNICAL REPORT No. 17

Genetic Determination Of Sources Of Acanthaster Planci Recruitment.

John A.H. Benzie, Australian Institute of Marine Science
Mary Wakeford, Australian Institute of Marine Science

EXECUTIVE SUMMARY

The precise origins of outbreak populations of crown-of-thorns starfish (Acanthaster planci) are still unknown. Modelling studies, and inferences from known aspects of A. planci biology, suggest outbreaks first occur in the region between latitudes 14-16°S. Early genetic studies indicated that outbreak populations sampled from Townsville to the Swains region all came from one genetic source, presumably within this region.

The present study provides detailed information for the first time on the genetic structure of populations close to the beginning of an outbreak phase, sampling several populations from the presumed region of outbreak origin. The principal objective of the study was to determine sources of recruits to Acanthaster planci populations in an attempt to better define the nature of the origin of outbreak populations in the region of 14-16°S. This was to be achieved through interpretation of the genetic structure of those populations by specifically addressing the following questions:

1. Are recruits to those A. planci populations showing a recent increase in population size different in genetic composition, and hence derived from different sources?

2. Is the genetic composition of the present "outbreaks" the same as those described from the 1980s, implying both present and past outbreaks were derived from the same source area?

3. Are there differences in the genetic constitution of different age classes in the same population suggesting temporal variation in the source of recruits, and is this variation the same order of magnitude as any spatial variation observed?

Six populations of the crown-of-thorns starfish, Acanthaster planci, showing large increases in population size in 1994-95 were examined using 100-300 individuals collected from each population in Nov 1995 - Feb 1996. Nine allozyme loci used previously to determine population structure in the 1980s outbreak populations were analysed. The results showed:

1. no significant genetic differentiation among the recent outbreak populations for any age class (2-6 years old), consistent with the recruits to each population being derived from the same source in each year,

2. the 1996 samples clustered with the eight 1986 outbreak populations in a small part of the genetic space spanned by the 1986 non-outbreak populations, suggesting that the outbreak populations were derived from the same source in both 1986 and 1996 and,

3. no significant variation between age classes in each of the six populations suggesting that no change in the source of recruits to these reefs has occurred over the last 5-6 years.

The data showed no significant genetic differences between populations and indicated that all populations have derived their recruits from the same gene pool for the last five years. This does not necessarily mean that all populations arose from larvae from one source reef. In combination with data showing simultaneous increases in population size at several reefs (Engelhardt and Lassig (in press)) and hydrodynamic models indicating much of the region between 14°S and 16°S is highly connected, the genetic data indicate that A. planci found on several reefs in the region act as one panmictic population. The data are consistent with any one or more of a number of reef populations in this region contributing to the production of a pool of recruits that might build up over time until they contribute significantly to colonisation of reefs downstream and give rise to outbreaks in the Central and Southern GBR. The considerable similarity in the genetic constitution of the 1986 and 1996 sets of outbreak populations is consistent with both being derived from the same source area.

THIS PUBLICATION IS CITED AS:
Benzie, John A.H. & Wakeford, Mary (1997)
Genetic determination of sources of Acanthaster Planci recruitment.
Technical Report No. 17
Townsville; CRC Reef Research Centre Ltd, 31 pp.

ISBN 1 876054 39 5

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