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**The title, authors, and abstract for this completion report are provided below. For a copy of the completion report, please contact the GLFC via e-mail or via telephone at 734-662-3209** The effects of food quantity/quality on growth rates and nutritive condition of the opossum shrimp Mysis relicta and the amphipod Diporeia
Abstracts for draft manuscripts: Abstract# 1 Changes in Lipid Content and Fatty Acid Composition during Fasting in Cornerstone Macroinvertebrate Species (Mysis relicta and Diporeia Spp.) in the Laurentian Great Lakes. Christian Schlechtriem, Michael. T. Arts and Ora E. Johannsson Fatty acids are potential trophic markers to trace feeding relationships in aquatic ecosystems primarily because lipid reserves of organisms broadly reflect dietary sources of lipid and can therefore provide information on the availability of key fatty acids in the food web. However, the use of fatty acids for such applications may be constrained by the degree to which the fatty acid composition of organisms is obscured by factors other than straightforward uptake from the diet. Thus, we studied the effect of long term fasting, under controlled laboratory conditions, on the lipid content and fatty acid composition of field-caught Mysis relicta and Diporeia spp. Periods of three to six weeks are required to induce clear effects of fasting in M. relicta and greater than six weeks in Diporeia. From a lipid perspective, both species seem to be highly adapted to periods of food shortage. The fatty acid composition in fasting M. relicta and Diporeia spp. changed with decreasing lipid contents. Relative proportions of docosahexaenoic acid (DHA) and arachidonic acid (ARA) increased during fasting. Both fatty acids are highly conserved reflecting their role as key fatty acids in both macroinvertebrate species. DHA proportions in the total fatty acid composition of fasting M. relicta were significantly higher in comparison to field-caught animals. M. relicta with high proportions of DHA (>25%) and with low lipid contents (<15% of dry weight) can be clearly identified as fasting animals. Such thresholds will help to improve the validity of fatty acids in trophic studies. The identification of comparable thresholds for fasting Diporeia requires further study.
Abstract #2 Effect of Fasting under Different Temperature Conditions on Nucleic Acid Ratios in the Opossum Shrimp Mysis relicta: a Calibration Approach. Christian Schlechtriem, Ora E. Johannsson, Kelly L. Bowen, Richard W. Smith and Michael. T. Arts The opossum shrimp Mysis relicta is an important component of the diet of benthivorous and planktivorous fish common in the Great Lakes. The invasion of the Great Lakes by exotic invertebrates (Bythotrephes longimanus, Cercopagis pengoi, Dreissena polymorpha and D. bugensis) has altered the base and intermediate levels of the food webs. Information about the condition of M. relicta is required to reveal how aquatic invaders exert disruptive effects on the food webs. Biochemical indices based on nucleic acid ratios have been shown to be suitable proxies for the growth and condition of aquatic organisms. These indices are affected by multiple factors, such as food level, temperature, body size, sex/life stage, maturation and moult stage and need to be calibrated before field data can be interpreted on a quantitative basis. In this study we investigated the effect of fasting under different temperature conditions on the RNA/DNA, RNA/protein, protein/DNA and % proteins in M. relicta. In two controlled laboratory experiments at 3ºC and 8ºC, juvenile M. relicta were exposed to fasting conditions for 11 and 21 d. Several effects of time and temperature on the condition indices of fasting M. relicta were observed, however, only RNA, RNA/DNA and % protein are suitable as indices of metabolism and condition in fasting animals. The response time of RNA and % protein to fasting ranged from 2-3 days at 8ºC to between 4 and 11 days at 3ºC. Juvenile M. relicta with RNA/DNA ratios <1.5-1.8 were clearly identified as fasting animals.
Absrtact #3 Effect of different diets on the fatty acid profiles and Nucleic Acid Ratios of the benthic amphipod Diporeia spp: a Calibration Approach. Martin Kainz , Michael T. Arts and Ora E. Johannsson The benthic amphipod Diporeia spp. is a crucial organism within the benthivorous food web of the Great Lakes as it supplies essential nutrients to its consumers including fish. Through their putative impacts on Diporeia, the presence of exotic invertebrates has altered the abundance and therefore the biochemical quality of fish diets within Great Lake’s food webs. Investigating the effect of biochemical diet composition on somatic growth condition and survival of Diporeia is fundamental to understanding such benthic food web responses of exotic food web disturbances. The presence of specific dietary polyunsaturated fatty acids (PUFA) in the diet have been linked to enhanced somatic growth, reproduction and survival of aquatic invertebrates and fish. In a controlled laboratory experiment (30 d at 8ºC), we investigated how different algal PUFA concentrations of; a) the green alga Ankistrodesmus falcatus, b) a naturally-occurring diatom assemblage, c) a non-toxic cyanobacterium Microcystis spp. (strain 632), and d) no diet (fasting) affected somatic growth condition and survival of this benthic key amphipod. Dietary total PUFA concentrations were significantly (p<0.001) highest in A. falcatus (32.5±0.6 mg/g dry weight) than in diatoms (17.2±0.1 mg/g dry weight) and Microcystis spp. (1.4±0.1 mg/g dry weight); however, only diatoms contained the somatically highly desired PUFA eicosapentaenoic (EPA) and docosahexaenoic (DHA) acid. After 30 d we observed highest Diporeia survival (70%) for fasting animals, 67% for A. falcatus diet, 60% for diatom diet, but 0% survival when feeding on Microcystis. Although the dietary PUFA concentrations differed significantly from each other, there was no significant difference of PUFA concentrations in Diporeia among these different treatments (p=0.23). After 30 d we measured the highest body weight of Diporeia feeding on A. falcatus (1.1 mg dry weight) and the lowest in fasting Diporeia (0.7 mg dry weight). Applying nucleic acid ratios as proxies for somatic growth conditions of Diporeia, we found highest RNA/DNA ratios (used as a proxy of metabolic/synthetic activity), protein/DNA ratios ( a measure of cellular metabolic activity), and RNA/protein ratios (a measure of the cellular capacity for protein synthesis ) in Diporeia feeding on A. falcatus and diatoms, and lowest in fasting animals. Our results suggest that high dietary total, rather than specific PUFA concentrations are involved in enhanced somatic growth of Diporeia. Finally, we argue that high concentrations of such biochemically-considered dietary high quality PUFA have a direct positive impact on somatic growth conditions of the benthic amphipod Diporeia. |