SHORT COMMUNICATION Lipid sac area as a proxy for individual lipid content of arctic calanoid copepods DANIELVOGEDES 1,2 *, ØYSTEINVARPE 1,4 , JANNE E. SØREIDE 1 , MARTIN GRAEVE 3 , JØRGEN BERGE 1,5 AND STIG FALK- PETERSEN 4 1 UNIVERSITY CENTRE IN SVALBARD, PB 156, 9171 LONGYEARBYEN, NORWAY , 2 UNIVERSITY OF TROMSØ, 9037 TROMSØ, NORWAY , 3 ALFRED WEGENER INSTITUTE, AM HANDELSHAFEN 12, 27570 BREMERHAVEN, GERMANY , 4 NORWEGIAN POLAR INSTITUTE, POLAR ENVIRONMENTAL CENTRE, 9296 TROMSØ, NORWAY AND 5 AKVAPLAN-NIVA, POLAR ENVIRONMENTAL CENTRE, 9296 TROMSØ, NORWAY *CORRESPONDING AUTHOR: danielv@unis.no or daniel.vogedes@gmx.de Received January 14, 2010; accepted in principle April 25, 2010; accepted for publication May 20, 2010 Corresponding editor: Roger Harris We present an accurate, fast, simple and non-destructive photographic method to estimate wax ester and lipid content in single individuals of the calanoid copepod genus Calanus and test this method against gas-chromatographic lipid measurements. KEYWORDS: Calanus; lipid sac; energy; image analysis; wax ester; lipid content; gas chromatography In the Arctic, the three co-occurring calanoid copepods Calanus glacialis, C. finmarchicus and C. hyperboreus are the key trophic links between primary producers and higher trophic levels (Lee et al., 2006). Calanus spp. convert low-energy carbohydrates and proteins, but also dietary fatty acids from their micro algal diet into high-energy wax esters, making up to 70% lipids of their dry mass (Falk-Petersen et al., 2009). Lipids are mainly stored in a lipid sac which can fill more than 80% of the body cavity in older copepodite stages (Miller et al., 1998; Sargent and Falk-Petersen, 1988; Miller et al., 2000; Lee et al., 2006). Due to their abundance and extensive energy storage, Calanus spp. form an important source of food for many predators, e.g. herring (Clupea harengus)(Varpe et al., 2005), basking sharks (Cetorhinus maximus)(Sims et al., 2003) and little auks (Alle alle)(Steen et al., 2007). The lipid sac content of a single specimen is closely related to the total energy reserves of that individual, and there is high intra-specific variability in lipid sac size depending on developmental stage and time of the year (Arts and Evans, 1991; Søreide et al., 2008). Importantly, behavioral and life-history strategies of many copepods are state-dependent with individual energy reserves being a key state (Fiksen and Carlotti, 1998; Hays et al., 2001; Varpe et al., 2009; Søreide et al., 2010). Consequently, an accurate, easy, cheap and fast method to determine the lipid content of copepods is of great interest. There has been a range of attempts to measure total lipid contents in copepods using digital imaging (video, still image), chemical analysis and visually determined indices, with pioneering work such as that of Sushkina (Sushkina, 1961) and Petipa (Petipa, 1964). A review of these methods can be found in Lee et al.(Lee et al., 2006). Here we have measured both the lipid sac area as well as the wax ester and total lipid content of indi- vidual copepods by digital image analyses and gas chromatography, respectively. This allows for a corre- lation between area and lipids to be estimated. Our doi:10.1093/plankt/fbq068, available online at www.plankt.oxfordjournals.org. Advance Access publication June 21,2010 # The Author 2010. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/ licenses/by-nc/2.5), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. JOURNAL OF PLANKTON RESEARCH j VOLUME 32 j NUMBER 10 j PAGES 1471 – 1477 j 2010 Downloaded from https://academic.oup.com/plankt/article-abstract/32/10/1471/1441286 by guest on 09 February 2018