The feeding ecology of rabbitfish (Siganidae) at Green Island reef: ontogenetic and interspecific differences in diet, morphology and habitat utilisation

Pitt, Joanna Maria (1997) The feeding ecology of rabbitfish (Siganidae) at Green Island reef: ontogenetic and interspecific differences in diet, morphology and habitat utilisation. PhD thesis, James Cook University of North Queensland.

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The Siganidae are a taxonomically uniform yet behaviourally diverse family of herbivorous fishes. The intention of this study was to examine the interaction between diet, morphology and behaviour in this family by making ontogenetic and interspecific comparisons of species selected for their contrasting behavioural traits. The species chosen for the study were Siganus doliatus, S. fuscescens, S. punctatus and S. lineatus. Siganus doliatus and S. punctatus are considered pairing species, while S. fuscescens and S. lineatus are considered shoaling species.

Siganids showed significant dietary differentiation through ontogeny, and interspecific dietary differences in the adult phase. The juveniles fed selectively on turfing algae and animal material within the seagrass beds, but the diets of the adults were more representative of the relative abundances of items in their feeding areas. The diets of the juveniles and the adults of the pairing species can largely be explained in the context of the predictions that should apply to Type I herbivores based on algal chemistry and morphology and by the varying energetic requirements of different ontogenetic stages. The presence of chemically defended algae in the diets of the adults suggests that foods may be consumed on the basis of their nutritional value regardless of the effects of such defences, or that the adults of these species are minimally affected by the secondary chemistry of these algae. However, the dominance of seagrasses in the diets of adults of the shoaling species bears further investigation.

There are subtle morphological differences between the four species which have the potential to influence feeding ecology. The development of the alimentary tract in these species seems to be under the control of allometric growth functions, which persist after maturity. Relative gut length is consistent between species, and appears adapted to a general, rather than specific, herbivorous diet. There is the possibility that a functional threshold associated with size exists for bulk processing and/or absorption, and this may influence the ability of the larger species to maintain a positive energy balance from more refractory dietary items such as seagrass. The caeca are the only part of the gut where interspecific differences in relative absorptive area may be present, but this requires confirmation. Differences in stomach structure between the pairing and shoaling species are the most interesting aspect of this research, and may play a role in the digstion of seagrassses by the shoaling species. Aspects of the gape and head profile may affect the physical accessibility of food items to some species. Juvenile siganids used microhabitats within the seagrass beds in a nonrandom fashion, and were often closely associated with large Sargassum plants. Strong behavioural patterns appear to have developed in association with these distributions, and distributions appear to be related to predator avoidance behaviours. The role of predation here seems to be as the indirect cause of preferences for structurally complex microhabitats. Differences in the distributions of new and older recruits suggest that settlement to seagrass beds operates in a non-selective manner, and that the observed habitat distributions occur because of microhabitat selection in the first month or so post-settlement. In siganids, this strategy appears to be mediated by the large size of recruitment shoals, which provides some protection from predators during this intermediate period.

Adult siganids at Green Island Reef exhibit spatial partitioning of resources. The greatest differences in resource use occur between the pairing and shoaling species, but differences are also evident between the two shoaling species. Feeding is partitioned on two scales: by habitat (coral versus seagrass) between pairing and shoaling species, and by site within the shoaling species. However, no partitioning is apparent between the two pairing species. Space utilisation in the coral areas appears to be determined at a finer scale, and is influenced by species-specific activity patterns and the characteristics of the individual sites. The occurrence of distinct forays and apparently extensive resting periods in S. lineatus is an unusual feature in an herbivorous fish, and bears further investigation. Although it is possible to make generalisations regarding the distributions of the four study species, the structural features of the different study sites mean that each site is utilised in a slightly different way in order to fulfill the requirements of the each species. There was no evidence of competitive interactions between species. Rather, it appears that predation-motivated behaviour, in the context of the different social habits of the four study species, has led to activity patterns which maximise access to food resources while minimising the risk of predation.

When all aspects of resource use by these species are taken into consideration, an interesting pattern emerges. In the juvenile phase, which is associated with the seagrass beds, resource utilisation patterns are very similar between species. As a result of the total habitat separation, there is no overlap in resource use between juveniles and the adults of the pairing species. Even between juveniles and the adults of the shoaling species, total resource overlap is very low. Among the adults, the only large overlap is between the two pairing species; these species also have moderate overlaps with S. fuscescens. Adult S. lineatus have low overlap with all other adults.

Item ID: 33790
Item Type: Thesis (PhD)
Keywords: behavior; behaviour; diet; ecology; feeding; GBR; Great Barrier Reef; Green Island; rabbitfish; morphology; rabbitfishes; Siganidae
Date Deposited: 21 Jul 2015 05:11
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 33%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 34%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060202 Community Ecology (excl Invasive Species Ecology) @ 33%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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