The role of bacteria in the digestion of seaweed by the abalone Haliotis midae.
The interaction of microorganisms in the digestive tract of invertebrates such as crustaceans, echinoderms and molluscs is well documented. Microorganisms may have different effects on the nutrition of invertebrates. In many cases, the presence of bacteria within the digestive system has led to an improvement in the growth rate of the host animal. Studies have shown that bacteria are able to hydrolyse the complex polysaccharide components of plants which are then available for digestion by the host. In addition, bacteria themselves may be utilised as a rich source of nitrogen by invertebrates living on protein deficient diets.
Gaining an understanding of the biology of the abalone Haliotis midae is of utmost importance if abalone aquaculture in South Africa is to be commercially viable. One of the challenges in farming abalone is to improve the rate of growth of the animal in order to speed up the turnover of the produce. Although there are research groups in the USA attempting to clone the abalone growth hormone to produce fast-growing abalone, it will still be necessary to optimise the digestive processes of the animal to ensure efficient use of feed. H. midae ingests a variety of algae which have significantly different chemical structures and compositions. The question we are asking is: 'how do these animals digest the range of structural polysaccharides that they ingest?' While work is being carried out on the digestive physiology of abalone, the potential endosymbiotic role of bacteria has not yet received attention. A potential role in the nutrition of the host would be implicated if the bacteria (either resident or transient) were capable of utilising the ingested material in the gut. If enteric bacteria are important in the digestive physiology of the abalone, then the farming techniques currently being implemented will have to be re-evaluated.
(Zoology Dept, University of Cape Town, Rondebosch 7700, South Africa)
The stability of DHA in two Artemia strains following enrichment and subsequent starvation.
The low content in essential n-3 highly unsaturated fatty acids (n-3 HUFA) in Artemia has been solved through bioencapsulation with microparticulate diets rich in n-3 HUFA. Recent results have demonstrated that docosahexaenoic acid (DHA) is more important than eicosapentaenoic acid (EPA) for the physiological functions of marine finfish larvae, including survival, growth, and pigmentation success. However, contrary to other live feeds, the enrichment of Artemia with DHA is difficult because of the inherent catabolism of the latter fatty acid upon enrichment. A recent finding of the capability of some particular Chinese Artemia strains to accumulate DHA opens new perspectives to study the metabolic pathways of DHA in Artemia. The present study documents the metabolic fate of DHA in two strains of Artemia (Chinese strain ARC1188 and Artemia franciscana from Great Salt Lake, Utah; GSL strain) upon enrichment and consequent starvation at various temperatures. Two strains of Artemia (ARC1188 and GSL) were enriched with an emulsion containing 37.5% DHA and 6.4% EPA at a temperature of 28C and a salinity of 30 ppt, and subsequently transferred to starvation for 72 h at three different temperatures (6, 12 and 22C). After enrichment the DHA content and DHA/EPA ratio amounted to 41.15 and 42.78 mg.g-1 DW, and 1.16 and 2.09 for the GSL and ARC1188 strain, respectively. During starvation at 12C the DHA content in the GSL strain decreased steadily towards 2.89 mg.g-1 DW (DHA/EPA ratio of 0.4), whereas the ARC1188 retained a DHA content of 24.07 mg.g-1 DW after 72 h (DHA/EPA ratio of 1.69). The reduction of DHA in the Chinese strain was observed only during the first 12 h of starvation. The degradation of DHA in the GSL strain was increasing with increasing temperature, whereas this was less the case for the ARC1188 strain.
The availability of a DHA retaining (ARC1188) and a DHA degrading strain (GSL) offers an unique opportunity to further investigate the metabolic pathways of DHA and possibly reveal methods to improve the DHA enrichment in Artemia.
(SINTEF, Applied Chemistry, Center of Aquaculture, N-7034 Trondheim, Norway)
