The ecology of individual specimens Acanthaster planci in low density populations

Souter, David William (2007). The ecology of individual specimens Acanthaster planci in low density populations PhD Thesis, School of Integrative Biology, The University of Queensland.

       
Attached Files (Some files may be inaccessible until you login with your UQ eSpace credentials)
Name Description MIMEType Size Downloads
n01front_souter.pdf n01front_souter.pdf application/pdf 5.09MB 0
n02content_souter.pdf n02content_souter.pdf application/pdf 16.96MB 0
Author Souter, David William
Thesis Title The ecology of individual specimens Acanthaster planci in low density populations
School, Centre or Institute School of Integrative Biology
Institution The University of Queensland
Publication date 2007-01-01
Thesis type PhD Thesis
Supervisor Hamish McCallum
Collection year 2008
Subjects L
Abstract/Summary This is the first long-term study of A. planci based on repeated observations of recaptured specimens living in three low density populations at Lady Musgrave Island Reef, Heron Island Reef and One Tree Island Reef, situated in the southern Great Barrier Reef. This study describes the growth of adult specimens, their dietary preferences, their feeding rates and their movements over periods ranging from days to more than three years. These data enable the ecology and life history strategy of A. planci in low density populations to be examined in detail for the first time. In addition, the results of this study enable a true comparison of the behaviour of individuals in low density populations with those maintained in the laboratory or comprising outbreaking populations in order to assess the applicability of previously held hypotheses, notions and beliefs concerning this starfish. The mean density of starfish at each reef ranged between 93.7 starfish·km-2 at Heron Island Reef and 535.7 starfish·km-2 at Lady Musgrave Island Reef and was well below that ascribed to outbreaking populations. Each population exhibited adult-dominated, uni-modal size frequency distributions with starfish ranging in size from 27 cm to 59 cm in total diameter. The mean size of starfish sampled within each population did not change significantly over the duration of the study and was 41.9 cm, 44.5 cm and 50.9 cm at Heron Island, One Tree Island and Lady Musgrave Island Reefs respectively. These results indicate that both adult mortality and recruitment within low density populations was low and/or unpredictable. The growth of 59 starfish ranging in size from 28 cm to 59 cm in total diameter and recaptured between 1 and 5 times, was recorded over periods ranging between 65 and 1252 days. The growth of these individuals was extremely plastic with the rates of change in size between encounters ranging between –9.2 mm·month-1 and 18.4 mm·month-1 demonstrating that, in the field, A. planci normally exhibit periods of significant growth and shrinkage, which are interspersed with periods of stasis. Several starfish maintained rapid growth beyond 45 cm in total diameter and many demonstrated that specimens of A. planci have the capacity to grow significantly throughout their entire size range. Although the rate of growth tends to decline with increasing size, many of these starfish maintained large sizes for the duration of the study. This study showed that in habitats where food is not limiting, specimens of A. planci exhibit plastic asymptotic growth where the maximum size of a starfish is likely to be constrained by morphometry and physiological demands, but sub-maximal asymptotes can be temporarily imposed by the various environmental conditions experienced throughout the life of an individual starfish, particularly food availability. The achievement of large body sizes increases fecundity and confers greater resilience to predation and to periods of food limitation. Phenotypically plastic growth enhances the survival and fitness of individuals by allowing them to accommodate changes in environmental conditions throughout their lives. The mean coral consumption of each of 14 recaptured starfish at Lady Musgrave Island Reef ranged between 0 cm2·day-1 and 974 cm2·day-1, while at One Tree Island Reef the feeding rates of 20 starfish ranged between 4 cm2·day-1and 570 cm2·day-1. The majority of individuals did not show significant changes in their feeding rates between encounters or between seasons, although the great variation in the amounts of coral eaten by these starfish each day might have obscured any clear seasonal changes in the feeding rates of these starfish. At Lady Musgrave Island Reef, significant variation in the feeding rates of different individuals was recorded during 2 of 4 field trips and also within both summer and winter. Some of this variation can be attributed to the fact that larger starfish generally consumed more coral than smaller starfish at both reefs. The remaining variation was attributed to differences in the composition of the coral community within the specific habitats occupied by different starfish. Starfish occupying habitats in which preferred corals were abundant generally consumed a smaller area of coral per day suggesting that the return per unit effort foraging on preferred corals is greater, thus increasing the survival and fitness of individuals by reducing the time spent foraging and exposed to predators. The dietary preferences of 43 individuals were investigated. The diets of 10 of these starfish were recorded on multiple occasions. Twenty-four genera of scleractinian coral were recorded in the diet of these starfish. When eaten, Acropora and Seriatopora were universally preferred, while Stylophora was usually preferred and Porites was generally not preferred. Although regularly preyed upon, Montipora was often consumed in similar proportions to its availability. Pocillopora was more acceptable to individuals at One Tree Island Reef than at Lady Musgrave Island Reef and not preferred by any starfish investigated at Heron Island Reef. The dietary preferences of those starfish observed repeatedly did not change significantly between field trips or seasons. Significant variation in preferences between individuals examined during the same trip was uncommon at Lady Musgrave Island Reef and Heron Island Reef but occurred during each field trip conducted at One Tree Island Reef. These results indicate that A. planci does exhibit a hierarchy of preferences for various corals but the stability of this hierarchy is moderated by the dietary experience of the individual and the availability of preferred coral genera, such that when preferred corals, particularly Acropora, are abundant, there is little variation in preferences between individuals or within individuals over time. As the abundance of preferred corals declines, starfish consume a broader range of corals. The foraging behaviour of A. planci is consistent with a time minimising strategy and appears to conform to the predictions of optimal foraging theory. The mean daily displacement of individuals sampled ranged between 0 m·day-1 and 6.8 m·day-1 at Lady Musgrave Island Reef and between 0.2 m·day-1 and 4 m·day-1 at One Tree Island Reef. At both reefs, most starfish did not exhibit significant variation in their daily displacements between field trips or between seasons, but the daily displacements of different individuals varied significantly within 6 of the 7 field trips conducted, and also within summer and winter. Individuals occupying habitats with greater cover of live hard coral tended to exhibit smaller daily displacements. ‘Homing’ to a refuge was common among starfish at Lady Musgrave Island Reef but rare in starfish at One Tree Island Reef. No relationship between starfish size and daily displacement or ‘homing’ frequency was found at either reef. The mean daily displacement of the population sampled at Lady Musgrave Island Reef was significantly greater during summer than during winter, but such seasonal variation was not apparent within the population at One Tree Island Reef. The mean daily displacement of all starfish sampled at One Tree Island Reef was significantly greater than those sampled at Lady Musgrave Island Reef. The daily movements of A. planci in populations of low density are highly plastic and appear to be influenced by the abundance of preferred corals providing food, predation pressure and the cover of live hard coral providing suitable refuges. Over periods ranging between 140 days and 1249 days, individual starfish were relatively stationary. The mean rate of displacement between encounters for 41 starfish recaptured at Lady Musgrave Island Reef was 70.6 m·year-1 and was 43.9 m·year-1 for the 8 starfish recaptured at One Tree Island Reef. At Lady Musgrave Island Reef, almost 30% of recaptures occurred within 10 m of an individual’s last recorded position and overall, greater than 85% of all recaptures occurred within 100 m. The furthest any starfish moved from its point of initial capture was 330 m in 734 days. Starfish that had traversed sandy habitats exhibited greater changes in displacement compared with those that had travelled across coral. Starfish located on reef slopes tended to navigate directional paths, which prevented retracing ground already covered, while the direction of movement of starfish located on reticulated or isolated patch reefs was more random. These results show that specimens of A. planci in populations of low density do not undergo long migrations and could potentially live their entire life within a small area of reef. A. planci demonstrate plastic phenotypic responses in growth and foraging behaviour to varying environmental conditions experienced throughout their life. These characteristics have evolved to counteract unpredictable recruitment while living in low densities by promoting survival and longevity of individual starfish. This allows A. planci to adopt an iteroparous strategy which enhances the fitness of individuals by enabling them adapt the level of reproductive effort to prevailing environmental conditions. Because the life history strategy of A. planci has evolved to ensure survival and longevity of individual starfish, the appearance and persistence of outbreaks is an inevitable consequence of circumstances that facilitate greater than usual fertilisation success. Identifying the mechanisms that might produce ‘unnaturally’ high fertilisation success will be important in determining the causes of primary outbreaks.

 
Citation counts: Google Scholar Search Google Scholar
Created: Fri, 24 Oct 2008, 21:00:24 EST