Fatty acid profiles of a benthic chondrichthyan: captive feeding trials and ecological applications

Author: Crystal Beckmann

Beckmann, Crystal, 2014 Fatty acid profiles of a benthic chondrichthyan: captive feeding trials and ecological applications, Flinders University, School of Biological Sciences

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Fatty acid analysis is a tool for dietary investigation that complements traditional stomach content analyses. There is little known about how sharks alter dietary fatty acids following incorporation into tissues, the stability and turn-over rates of fatty acid in tissues, and whether specific fatty acids are selectively retained within tissues. Four controlled feeding experiments were used to determine the extent to which the fatty acid composition of diet is reflected in the tissues of the Port Jackson shark Heterodontus portusjacksoni and to improve our understanding of which fatty acid biomarkers can be used to reliably distinguish prey types. The first experiment tested the fatty acid profile of muscle and liver tissues and used two groups of sharks fed exclusive diets of prawns or squid, and an unfed control group. The liver and muscle fatty acid profiles showed significant differences between the groups fed exclusive diets and unfed control sharks, suggesting that the extent of dietary change was strong enough that both tissues could be used as indicators of diet. The different diet fed to sharks could, however, only be distinguished based on the liver fatty acid profiles, with prawn-fed sharks comparatively higher in 18:1n-7, 22:5n-3, 20:0 and 18:1n-9, while squid-fed sharks were higher in 16:0 and 22:6n-3. The lack of differences in the muscle fatty acid profiles suggesting that diet was not different enough to cause a change in the muscle fatty acid in the duration of the experiment. The second experiment further investigated liver and muscle fatty acid profile dynamics during a dietary change and assessed the fatty acid profiles of sharks fed squid for six weeks followed by prawns for an additional six weeks. This experiment showed significant differences in the liver and muscle fatty acid profiles within three weeks of a dietary switch driven by 22:6n-3 in the liver and muscle and 16:1n-7 in the liver. Higher levels of dissimilarity were observed before and after the dietary change in the muscle tissue which may indicate that dietary fatty acids are preferentially used in the muscle following a dietary change. Changes in fatty acid profiles over time and different incorporation rates between tissues were also evident when sharks were fed exclusive diets of artificial fish or poultry oil pellets for a period of 18 weeks. The fatty acid profiles from the liver and blood serum of fish oil and poultry oil fed sharks were significantly different within 12 weeks while the muscle fatty acid profiles of fed sharks did not differ until week 18. The drivers of dissimilarity which aligned with dietary input were 14:0, 18:2n-6, 20:5n-3, 18:1n-9 and 22:6n-3 in the liver and blood serum. The fourth experiment used yolks sampled from viable egg cases and recently hatched neonates fed a known diet to determine whether the fatty acids present in high levels in yolks or in diets are reflected in the liver and muscle tissues of hatchlings. The fatty acid profiles of hatchling tissues were more similar to yolk than to diet, demonstrating the conservative transfer of fatty acid from egg yolks to hatchlings as well as the preferential retention of some fatty acids in the muscle and liver. Specifically, arachidonic acid (ARA, 20:4n-6) was preferentially retained likely as a result of eicosanoid production during growth; dietary docosapentaenoic acid (DPA, 22:5n-3) was not reflected in shark tissues and is likely catabolised for energy; docosahexaenoic acid (DHA, 22:6n-3) was reflected in tissues and was a good dietary indicator; and high proportions of saturated fatty acids were retained in the muscle at the expense of dietary polyunsaturated fatty acids. The application of fatty acid profile analysis to investigate dietary information of wild specimens was assessed through the comparisons of fatty acid profiles and stomach contents from three locations. Fatty acid profile analysis indicated significant differences between the three locations sampled, however, stomach content analysis did not show significant differences between the two closest locations (Gulf St Vincent and Spencer Gulf). The discrepancy in the results from the two methods highlighted the ability of fatty acid profiles to complement information obtained from stomach content analysis. For example, soft-bodied prey such as molluscs which have their shells crushed and are rapidly digested, were underrepresented in the stomach content analysis, but could be readily detected in 16:1n-7 levels. Fatty acid analysis is increasingly powerful as a tool in studies of trophic ecology in marine ecosystems. This biochemical technique has become useful in deciphering spatial and temporal variability in diets, identifying predation on key species, and providing dietary information which is not always obtainable using more traditional methods.

Keywords: fatty acids,port jackson sharks,feeding ecology,omega 3,stomach content,young-of-the-year,DHA,muscle,liver

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2014
School: School of Biological Sciences
Supervisor: Charlie Huveneers