Zoology - Theses
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ItemThe nutritional biology of the black lip abalone, H.rubra( 1991)Nutritional factors affecting the growth of the black lip abalone H. rubra were investigated. To determine the growth value of various diets, abalone were fed single species of algae and a mixed diet, and their growth and feed intake were measured and compared. Such dietary trials have been used extensively by researchers to assess diet quality. However in this study, a critical analysis of this method revealed that previous diets and disturbance may affect results for the first 50 days or more, and that growth rates may change over long periods. Thus, dietary trials should be modified to standardise diets prior to the trials, and should extend over periods of greater than 100 days, with regular measurements of growth. The highest growth rate was achieved on a mixed diet, with little variability over time. The food preferences of abalone for the test algae were determined. The growth of H. rubra was significantly correlated with food preference, in accordance with the optimal foraging model, which predicts that animals should prefer to consume foods that maximise the intake of nutrients that are limiting in their diet, resulting in optimal growth. Abalone preferred and grew most rapidly on the perennial red alga Jeannerettia. However, abalone were selective within the reds, so that preference cannot be predicted on the basis of the divisions of algae, as has previously been supposed. The brown algae tested were least preferred, and the hierarchy of preference was related to the phenolic contents of these algae. Food intake did not correlate with preference or growth, supporting the observation made by other authors that intake is influenced by an array of factors other than the nutritional quality of the food. In the case of H. rubra, intake is most likely influenced by anti-nutritional factors, such as tannins, toxins, toughness and fibre. For herbivores, nitrogen has been identified as one of the nutrients most likely to be limiting, although energy is considered important also. The intakes of digestible nitrogen and energy on each of the test algae were determined by measuring the amounts ingested, and lost in the faeces. The nitrogen content of the algae and the intakes of digestible nitrogen were correlated with both preference and growth, indicating the important role it plays in influencing growth and setting preference. The energy contents of the algae were similar compared with the highly variable nitrogen contents. Consequently, it is likely that differences in energy availability between algae do not play a role in determining preference or intake. The importance of energy availability in determining growth was investigated by estimating the maintenance energy requirements of abalone, and the energy cost of growth. From this, a calculation was made of the amount of metabolisable energy that was available on the various diets after the demands for maintenance had been met. The high variability in food intake resulted in large differences in intakes of digestible energy. The amounts available for growth appear: to indicate that the growth performance of abalone on some of the diets tested is limited by energy availability.