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

Abstract

Wax esters extracted and purified from the copepod CaJanus pacificus elicit a different response in quantitative thin-layer chromatography-flame ionization detection (TLC-FID) than simpler, saturated wax esters available from commercial sources and commonly used for instrument calibration. Failure to use native copepod wax esters in calibration procedures results in underesti- mates of 18-32% in wax ester content, depending on the mass of lipid loaded on chromarods. In con- trast to wax esters, triacylglycerols from the copepod Eucalanus califomicus, as well as phospholipids from E.califomicus and C. pacificus, elicited no significant difference in detector response in com- parisons with commercially available standard compounds. Extraction of copepod lipids in solutions of chloroform:methanol results in the release of non-lipid material. Therefore, indirect gravimetric measures of lipid content that rely on calculation of lipid mass by difference after 'de-fatting' cope- pods 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 Rhn- calanus nasutus, C.pacificus and Melridia pacifica, but 93% of body mass in E.califomicus. The occur- rence 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 inde- pendent variable used to scale zooplankton lipid content and other structural and metabolic relation- ships.