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On the determination of zooplankton lipid content and the occurrence of gelatinous copepods

Abstract

Wax esters extracted and purified from the copepod Calanus pacificus elicit a different response in quantitative thin-layer chromatography-flame ionization detection (TLC-FID) than simpler, saturated wax esters available from commercial courses and commonly used for instrument calibration. Failure to use native copepod wax esters in calibration procedures results in underestimates of 18-32% in wax ester content, depending on the mass of lipid loaded on chromarods. In contrast to wax esters, tricylglcerols from the copepod Eucalanus californicus, as well as phospholipids from E. californicus and C. californicus and C. pacificus, elicited no significant difference in detector response in comparisons with commercially available standard compounds. Extraction of copepod lipids in solutions of chloroform:methanol results int he release of non-lipid material. Therefore, indirect gravimetric measured of lipid content that rely on calculation of lipid mass by difference after 'de-fatting' copepods greatly overestimate the true lipid content. Lipid-specific assays with appropriate calibration standards are essential for the accurate quantification of the lipid content of planktonic animals. The water content differs appreciably among co-occurring species of copepods, averaging 82-84% in Rhin-calanus nasutus, C. pacificus and Metridia pacifica, but 93% of body mass in E. californicus. The occurrence of gelatinous tissues in some planktonic copepod species underscores the need to take account of structural and life history diversity among taxa. Care must be taken in the selection of the independent variable used to scale zooplankton lipid content and other structural and metabolic relationships.

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