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• W96282840 abstract "In the past, age classes in Acanthaster planci (L.) populations have been interpretedfrom modes in size frequency distributions. The relationship between size and agehas continued to be used in studies despite increasing evidence of growthcharacteristics which are inconsistent with inherent assumptions. This approach wasrationalised because the ability to determine age is fundamental to understanding theecology and life history of this unique species, capable of developing massiveoutbreak populations and incurring widespread mortality of hard coral species.Therefore, the aims of the project were to develop a valid method of agedetermination and employ it to investigate the population dynamics, the morphometryof individual and skeletal growth and other life-history characteristics of severalpopulations from the Western Pacific region.Valid age determination in echinoderms has been achieved almost exclusively withechinoid species through skeletochronometric techniques. Periodic growth rings aregenerally found in larger skeletal elements such as test plates, since the echinodermskeleton consists of an open tridimensional network, the calcitic stereom. However,the Asteroidea characteristically develop a skeleton of smaller ossicles which allowsfor 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 theaboral body wall, that do not restrict its habits.The aboral spine ossicles of adult A. planci have a linear growth pattern unlike themode of development previously reported for echinoids. Numerous growth lines,perpendicular to the long axis were evident in spine sections and confirmed withtetracycline staining, apparently caused by frequent growth episodes. Spine growthin adults is by elongation with addition of new stereom at the base, preserving theentire growth history. Broad pigment bands develop parallel to the growth lines andare visible on the ossicle surface after the removal of soft tissues. Therefore, it washypothesised 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 threedimensions to a mode primarily of elongation. Therefore, one SPBC (light and darkband pair) = 3+ years, two SPBC = 4+ years, etc. A biosynthetic mechanism wasproposed to explain the functional role of the pigment banding process.Field studies were conducted on Davies Reef, Central GBR, to validate the SPBCmethod. They consisted of markrecapture exercises and collections of morphometricdata for seasonal and longer-term growth analyses. The recapture rate for markedindividuals was 3.5%. Twelve of thirteen recaptured individuals whose releaseperiods 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 lessthan twelve months, with incomplete band pair formation, also supporting the method.Further independent evidence comes from morphometric results, including: annualincremental growth in the SPBC classes; a significant increase in mean spine ossiclelength over the 38 month study period; consistent estimates of the growth constant(K = 0.039mo.) between the recapture and morphometric analyses; and thecoincidence of the timing of the outbreak from survey results with the estimated ageof the first outbreak cohort.The outbreak population density on Davies Reef was approximately 420ha -1.• Thisis at the lower end of the scale of outbreak sizes, and consisted of four principalcohorts, estimated to have settled between 1983 and 1986. A significant reductionin 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 meanasymptotic body sizes in each successive cohort occurred as a response to theincreasingly limited resources.A. planci can grow to well over 60cm in diameter and 4kg in wet weight, but moreoften exhibits lower ranges, well below maximum attainable size. The mode ofgrowth varies between habitat-dependent, asymptotic growth (determinate) and plasticasymptotic growth (indeterminate). Therefore, determinate growth occurs whenconstraints are imposed on an underlying potential for indeterminate growth. Further physiological studies are required to describe precisely how A. planci reach very largebody sizes under solely intrinsic resource limitation.Sexually dimorphic characteristics were found in the Davies Reef outbreakpopulation, where male starfish had lower gonad weights, and longer lifespans,promoting high fertilization rates during the decline phase of outbreaks. Higherestimated reproductive effort and a seasonal oscillation in whole body diameter of 2to 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 offsetfluctuations in body size and the energetic demands of reproduction, promotingiteroparity and longevity. When resources became limited in higher densities, bodyreserves were drawn upon more heavily in order to support the increased reproductiveeffort causing resorption of body wall and skeletal tissues, resulting in shrinkage andpresumably 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 SuvaReef), approaching semelparity in higher density outbreaks, and iteroparous with alower reproductive output (Guam). A life-history strategy of phenotypicallypolymorphic bet-hedging is proposed for A. planci, which varies according to sex,population density, the pattern of mortality from stress (decreased production), anddisturbance (loss of biomass). Therefore, A. planci owes its success to the ability tovary its channelling of resources into the various functions of growth, somaticmaintenance, protection and reproduction. To maintain this variable strategy betweeniteroparity and semelparity implies that periodic outbreaks of A. planci occur withinregions under natural conditions. The immediate concerns of management agenciesregarding the prediction of outbreaks should focus on the dynamics of expandingpopulations i.e. those leading to primary outbreaks. These issues can only beaddressed through the implementation of long-term population studies, including theassessment of age structure, particularly in areas where primary outbreaks aresuspected to occur." @default.
• W96282840 created "2016-06-24" @default.
• W96282840 creator A5064377778 @default.
• W96282840 date "1994-01-01" @default.
• W96282840 modified "2023-09-27" @default.
• W96282840 title "Age determination and life-history characteristics of Acanthaster planci (L.) (Echinodermata: Asteroidea)" @default.
• W96282840 hasPublicationYear "1994" @default.
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