Age determination and life-history characteristics of Acanthaster planci (L.) (Echinodermata: Asteroidea)

In the past, age classes in Acanthaster planci (L.) populations have been interpreted from modes in size frequency distributions. The relationship between size and age has continued to be used in studies despite increasing evidence of growth characteristics which are inconsistent with inherent assumptions. This approach was rationalised because the ability to determine age is fundamental to understanding the ecology and life history of this unique species, capable of developing massive outbreak populations and incurring widespread mortality of hard coral species. Therefore, the aims of the project were to develop a valid method of age determination and employ it to investigate the population dynamics, the morphometry of individual and skeletal growth and other life-history characteristics of several populations from the Western Pacific region. Valid age determination in echinoderms has been achieved almost exclusively with echinoid species through skeletochronometric techniques. Periodic growth rings are generally found in larger skeletal elements such as test plates, since the echinoderm skeleton consists of an open tridimensional network, the calcitic stereom. However, the Asteroidea characteristically develop a skeleton of smaller ossicles which allows for a wide range of flexible movement, for locomotion, climbing and food handling. An exception is A. planci which has large spines that rest on pedicels, rooted in the aboral body wall, that do not restrict its habits. The aboral spine ossicles of adult A. planci have a linear growth pattern unlike the mode of development previously reported for echinoids. Numerous growth lines, perpendicular to the long axis were evident in spine sections and confirmed with tetracycline staining, apparently caused by frequent growth episodes. Spine growth in adults is by elongation with addition of new stereom at the base, preserving the entire growth history. Broad pigment bands develop parallel to the growth lines and are visible on the ossicle surface after the removal of soft tissues. Therefore, it was hypothesised that spine pigment band counts (SPBC) can be used to determine age . in A. planci, commencing after sexual maturity, in the third (2+) year. At this time, body growth slows and spine ossicle growth changes from enlargement in three dimensions to a mode primarily of elongation. Therefore, one SPBC (light and dark band pair) = 3+ years, two SPBC = 4+ years, etc. A biosynthetic mechanism was proposed to explain the functional role of the pigment banding process. Field studies were conducted on Davies Reef, Central GBR, to validate the SPBC method. They consisted of mark\recapture exercises and collections of morphometric data for seasonal and longer-term growth analyses. The recapture rate for marked individuals was 3.5%. Twelve of thirteen recaptured individuals whose release periods were at least twelve months supported the validation of age classes 3+, 4+ and 5+ years. A further ten recaptures were obtained with release periods of less than twelve months, with incomplete band pair formation, also supporting the method. Further independent evidence comes from morphometric results, including: annual incremental growth in the SPBC classes; a significant increase in mean spine ossicle length over the 38 month study period; consistent estimates of the growth constant (K = 0.039mo.) between the recapture and morphometric analyses; and the coincidence of the timing of the outbreak from survey results with the estimated age of the first outbreak cohort. The outbreak population density on Davies Reef was approximately 420ha -1.• This is at the lower end of the scale of outbreak sizes, and consisted of four principal cohorts, estimated to have settled between 1983 and 1986. A significant reduction in population size over the study period, following a profound decline in coral cover, was caused by high mortality rates in the post-outbreak cohorts. Lower mean asymptotic body sizes in each successive cohort occurred as a response to the increasingly limited resources. A. planci can grow to well over 60cm in diameter and 4kg in wet weight, but more often exhibits lower ranges, well below maximum attainable size. The mode of growth varies between habitat-dependent, asymptotic growth (determinate) and plastic asymptotic growth (indeterminate). Therefore, determinate growth occurs when constraints are imposed on an underlying potential for indeterminate growth. Further physiological studies are required to describe precisely how A. planci reach very large body sizes under solely intrinsic resource limitation. Sexually dimorphic characteristics were found in the Davies Reef outbreak population, where male starfish had lower gonad weights, and longer lifespans, promoting high fertilization rates during the decline phase of outbreaks. Higher estimated reproductive effort and a seasonal oscillation in whole body diameter of 2 to 3cm occurred in the post-outbreak cohorts. Therefore, larger body sizes in the pre-outbreak cohorts allowed for storage of relatively greater energy reserves to offset fluctuations in body size and the energetic demands of reproduction, promoting iteroparity and longevity. When resources became limited in higher densities, body reserves were drawn upon more heavily in order to support the increased reproductive effort causing resorption of body wall and skeletal tissues, resulting in shrinkage and presumably reduced lifespan. Among the Western Pacific populations studied (Suva Reef, Guam and Davies Reef) reproductive tactics were described as "big-bang iteroparity" (Davies Reef and Suva Reef), approaching semelparity in higher density outbreaks, and iteroparous with a lower reproductive output (Guam). A life-history strategy of phenotypically polymorphic bet-hedging is proposed for A. planci, which varies according to sex, population density, the pattern of mortality from stress (decreased production), and disturbance (loss of biomass). Therefore, A. planci owes its success to the ability to vary its channelling of resources into the various functions of growth, somatic maintenance, protection and reproduction. To maintain this variable strategy between iteroparity and semelparity implies that periodic outbreaks of A. planci occur within regions under natural conditions. The immediate concerns of management agencies regarding the prediction of outbreaks should focus on the dynamics of expanding populations i.e. those leading to primary outbreaks. These issues can only be addressed through the implementation of long-term population studies, including the assessment of age structure, particularly in areas where primary outbreaks are suspected to occur