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First published online August 17, 2007
Journal of Experimental Biology 210, 2999-3014 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.006007

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Temperature adaptation in two bivalve species from different thermal habitats: energetics and remodelling of membrane lipids

Fabrice Pernet^1,*, Réjean Tremblay², Luc Comeau³ and Helga Guderley⁴

¹ Institut de Recherche sur les Zones Côtières, 232B rue de l'Église, Shippagan, Nouveau-Brunswick, E8S 1J2, Canada
² Institut des Sciences de la Mer, 310 allée des Ursulines, Rimouski, Québec, G5L 3A1, Canada
³ Department of Fisheries and Oceans, Science Branch, Gulf Fisheries Centre, PO Box 5030, Moncton, New Brunswick, E1C 9B6, Canada
⁴ Département de Biologie, Université Laval, Québec, Québec, G1K 7P4, Canada

^* Author for correspondence (e-mail: fpernet{at}umcs.ca)

Accepted 18 June 2007

We compared lipid dynamics and the physiological responses of blue mussels Mytilus edulis, a cold-adapted species, and oysters Crassostrea virginica, a warmer-water species, during simulated overwintering and passage to spring conditions. To simulate overwintering, animals were held at 0°C, 4°C and 9°C for 3 months and then gradually brought to and maintained at 20°C for 5 weeks to simulate spring–summer conditions. Changes in lipid class and fatty acid composition were related to clearance rate and oxygen consumption.

We found major differences between species in triglyceride (TAG) metabolism during overwintering. Mussels used digestive gland TAG stores for energy metabolism or reproductive processes during the winter, whereas oysters did not accumulate large TAG stores prior to overwintering. Mussel TAG contained high levels of 20:5n-3 compared to levels in oysters and in the diet. This may help to counteract the effect of low temperature by reducing the melting point of TAG and thus increasing the availability of storage fats at low temperature. Mussels seemed better able to mobilise 20:5n-3 and 18:4n-3 than other fatty acids.

We also found that both bivalves underwent a major remodelling of membrane phospholipids. The unsaturation index decreased in the gills and digestive glands of both species during the early stages of warming, principally due to decreases in 22:6n-3 and 20:5n-3. In digestive glands, the unsaturation index did not increase with decreasing temperature beyond a threshold attained at 9°C whereas a perfect negative relationship was observed in gills, as predicted by homeoviscous adaptation. The presence of digestive enzymes and acids in the digestive gland microenvironment may lead to specific requirements for membrane stability. That oysters had lower metabolic rates than mussels coincides with a lower unsaturation index of their lipids, as predicted by Hulbert's theory of membranes as metabolic pacemakers. Both species showed increased 20:4n-6 levels in their tissues as temperature rose, suggesting an increasing availability of this fatty acid for eicosanoid biosynthesis during stress responses.

The contrast between the species in TAG dynamics and the similarity of their phospholipid remodelling emphasises the essential functional role of membrane phospholipid structure and the contrasting use of TAG by oysters and mussels during overwintering.

Key words: lipid, fatty acid, triglyceride, phospholipid, homeoviscous adaptation, mollusc, temperature adaptation, acclimation

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