The ecological role of sea hares (Opisthobranchia: Anaspidea) within tropical intertidal habitats

Clarke, Cathryn Lynne (2004) The ecological role of sea hares (Opisthobranchia: Anaspidea) within tropical intertidal habitats. Masters (Research) thesis, James Cook University.

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Abstract

Herbivory in temperate marine ecosystems has been the focus of widespread research attention. In comparison, little is known about tropical herbivores and the role they play in controlling the abundance of marine macrophytes. This study investigates the role of one group of invertebrate herbivores, the sea hares. Their role as herbivores in tropical intertidal habitats was examined at four study sites situated near Townsville, Queensland, Australia. The roles these animals play as herbivores were investigated by examining the variation in density and feeding specificity. The variation in density was documented by surveying the distribution and abundance of sea hares and their food macrophytes for one year. In addition, the population dynamics of population irruptions of sea hares at a single site were examined in greater detail. There was extreme variation in the densities reached by each of the five local species recorded. Aplysia dactylomela, Aplysia extraordinaria, and Petalifera petalifera remained at low abundance throughout the sampling period. In contrast, Bursatella leachii and Stylocheilus striatus were found in extremely high density within the seagrass beds of Shelly Beach in association with a bloom of the cyanobacteria Calothrix crustacea. The population dynamics of Bursatella leachii showed that at this location, the population underwent continuous recruitment in contrast to the results of previous studies. The relationships between each sea hare species and their preferred host macrophytes were species specific. The density of Aplysia dactylomela was positively correlated with a limited number of red algae species, but the clearest relationship was with the red macrophytic group (Rhodophyta), a finding consistent with previous feeding preferences experiments for this species. Aplysia extraordinaria density was correlated with several of the available red algae species however because of the low density, its feeding ecology remains unclear. Petalifera petalifera was found to maintain a very strict association with the brown calcified alga, Padina tenuis. The temporal distribution of all sea hares was restricted to the winter months, a relationship that may be related to environmental limitations, such as ultraviolet radiation. The abundances of B. leachii and S. striatus could not be quantitatively related to any of the algal species recorded because cyanobacteria bloom abundance was not estimated. The feeding specificity of Bursatella leachii was more closely examined using two-way choice preference testing. This species preferred to consume the green alga, Enteromorpha sp. to the cyanobacterium Calothrix crustacea, the brown alga Sargassum sp., and the red alga Pterocladia pinnata. There was no difference in the feeding hierarchy obtained by examining edibility or attractiveness. Feeding specificity of the six local sea hares was also investigated by examining the morphology of the radula feeding organ. Scanning electron microscopy was used to examine the fine details of the radula teeth. Three radula teeth types were created based on the sea hare radulae examined: simple, bilobed and denticulate. These radula types correspond to the feeding preferences exhibited by each sea hare. Sea hares with complex radulae were more likely to be highly specialised feeders while those with simple radulae were more likely to be generalists. A theoretical framework was developed, based on these radula types, which can now be used to make testable predictions about feeding preferences of sea hare species with unknown feeding specificity. The results of this study demonstrate that the density and feeding specificity of sea hares are species specific. Therefore care should be taken in extrapolating information from studies based on a small number of sea hare species to the entire sea hare group. Three of the sea hare species examined within (A. dactylomela, A. extraordinaria and P. petalifera) are not likely to play major roles as herbivores in their habitats as a result of their low densities. However, Bursatella leachii and Stylocheilus striatus would exert a strong influence on their seagrass habitats as a because of their high density and feeding specialisation on cyanobacteria. Sea hares may play an increased role in habitats, such as shallow intertidal ecosystems, unavailable to the primary herbivores.

Item ID: 1083
Item Type: Thesis (Masters (Research))
Keywords: Sea hares, Tropical intertidal habitats, Density, Feeding specificity, Marine macrophytes, Population irruptions, Aplysia dactylomela, Aplysia extraordinaria, Petalifera petalifera, Bursatella leachii, Stylocheilus striatus, Two-way choice preference testing, Radula teeth types, Feeding specialisation on cyanobacteria
Date Deposited: 13 Oct 2006
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 0%
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