Variations in biometric and physiological parameters of Acanthaster planci (L.) (Echinodermata; Asteroidea) during the course of a high density outbreak

Kettle, Brett Thomas (1990) Variations in biometric and physiological parameters of Acanthaster planci (L.) (Echinodermata; Asteroidea) during the course of a high density outbreak. PhD thesis, James Cook University.

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The Crown-of-Thorns starfish, Acanthaster planci (L.), a large and voracious corallivore, has been the focus for a large number of scientific studies, few of which have examined aspects of starfish physiology or high density (outbreak) populations throughout the time course of a single outbreak event. This study sought to examine the variations in size, population density, morphology, respiration rates, fecundity and resource allocation, among starfish from a high density outbreak population throughout an outbreak event. The study sequentially examined three aspects: a) the effect of size per se; b) the effects of starvation on captive starfish; and c) changes occurring in an outbreak population in the field.

Starfish from the Great Barrier Reef have less skeletal material (one fifteenth to one thirtieth of whole wet weight over the size range 280g to 3000g) than starfish from Guam (generally one seventh of whole wet weight) and the proportion of skeletal material decreases markedly in larger starfish. Small starfish (less than -10g whole wet weight) allocate resources for growth of the body wall; but an increasing proportion of energy is accumulated in the caeca between weight -10g and -280g (the onset of sexual maturity), when it is diverted to gametogenesis. Starfish generally exhibited allometric growth - large starfish are thinner in the oral - aboral axis, perhaps being leaner as a consequence of low ration diets. Food limitation plays an important role in apparent levels of "sub lethal predation" - starving starfish apparently cannot maintain regeneration rates and therefore accumulate their injuries over longer periods. Fecundity increases relative to increasing whole wet weight such that small adults (220mm diameter) allocate -8% of their wet weight to reproduction and larger adults (-400mm diameter) allocate -20%. Fecundity therefore increases rapidly with size (=30 million eggs to -60 million eggs over the size range -2000g to - 3000g whole wet weight) and the total potential fecundity of a population is strongly dependent on the size of individual starfish. Large starfish, the rare "giants" in excess of 500mm diameter, conform to the same size versus oxygen consumption relationship as do smaller starfish. There is no suggestion of any genotypic differences in metabolic rate in large animals. Rather, starfish size is likely to be commonly limited by low-ration diets towards the end of an outbreak or when food availability is low relative to the maintenance requirement of individuals within the population.

Acanthaster planci is, by comparison to other asteroids, relatively intolerant to food-limitation, surviving enforced starvation for only approximately six months. Responses to starvation are strongly influenced by starfish size. Small starfish can show continued growth and reproductive partitioning while large starfish quickly shrink and die. There is some suggestion that particulate organic films may be utilised by starving starfish to offset maintenance costs during starvation. Resources are readily resorbed from the caeca during starvation, and, as starvation becomes more severe, from the body wall and stomach. Gonadal growth is maintained and, after six months of starvation, starfish showed (on average) a fourfold increase in gonadal indices. Reproductive effort is thus extremely high even under conditions of extreme food shortage. Changes in the macroscopic appearance of starved starfish follow an interpretable pattern that can be rapidly used in the field to assess the gross health of starfish populations.

A two year field study on Helix Reef commenced in the initial stages of a high density outbreak and continued until starfish populations were too sparse to permit sampling. During this time, the estimated total population climbed to -37000 starfish and declined to -4000 starfish, while coral cover decreased from -40% total live cover to -1% live cover. The greatest fluctuations in both parameters occurred in a period of approximately 6 months. The outbreak population probably consisted not of one relatively long-lived cohort but of several successive cohorts. Cohorts of smaller starfish showed growth while cohorts of larger starfish were shrinking and being lost from the population. Size-specific reproductive potential was equivalent in the first year (abundant food) and the second year (very little food). Acanthaster planci is capable of very high reproductive effort during starvation. The wild population demonstrated similar compartmental priority systems to starved captive starfish. Reproduction can proceed without resorption from other compartments when food is abundant. As food supplies decrease, energy is partitioned from the caeca; but, if food limitation becomes acute, the body wall and the stomach will both yield resources to allow the continuation of high reproductive effort or to meet maintenance requirements. Marked changes in underwater weight frequency distributions suggest that decreases in population density result from mortality of large starfish.

Reproductive effort (energy allocated to reproduction as a fraction of energy acquired) is a key to the A. planci life strategy. That A. planci can achieve similar levels of reproductive output when food-limited as it does when food is plentiful is indicative of high reproductive effort. Previous work (Lawrence, 1990) has suggested that the active foraging, long-lived and highly fecund traits of A. planci in environments of high resource availability and long duration constitute a "competitive" life strategy. This study suggests that A. planci in high density outbreaks is short-lived and has very high reproductive effort in short duration environments of low resource availability — the same life strategy that produces a competitive species in conditions of excess food produces a ruderal species when food limitation becomes extreme.

Item ID: 27496
Item Type: Thesis (PhD)
Keywords: crown-of-thorns starfish; Acanthaster planci; starfish physiology; reproductive effort; food shortage impacts
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Kettle, B.T., and Lucas, J.S. (1987) Biometric relationships between organ indices, fecundity, oxygen consumption and body size in Acanthaster planci (L.) (Echinodermata; Asteroidea). Bulletin of Marine Science, 41 (2). pp. 541-555.

Date Deposited: 25 Jun 2013 05:07
FoR Codes: 06 BIOLOGICAL SCIENCES > 0606 Physiology > 060601 Animal Physiology - Biophysics @ 70%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 20%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060303 Biological Adaptation @ 10%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 49%
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