WAX ESTERS IN TROPICAL ZOOPLANKTON AND NEKTON AND THE GEOGRAPHICAL DISTRIBUTION OF WAX ESTERS IN MARINE COPEPODS1

Analyses of zooplankton and nekton from the central South Pacific showed that surface an d epipclagic zooplankton had small amounts of neutral lipids, mainly triglyccridcs, while deep-water tropical copepods had wax esters as the major lipid type, presumed to function as a rcscrvc srtoragc. The total lipid content and wax esters of tropical copepods captured at about 500 m were significantly lower than those of spccics from high latitudes. The median percentage of wax esters incrcascd progrcssivdy from tropical to polar latitudes in nine species of Cdiznus and five or six species of Euchaela. Deep-living tropical copepods had the same median percentage of wax esters as mcsopelagic and bathypelagic copepods from a subtropical station. Caridcan and penaeidcan decapods and euphausiids showed less than 5% wax esters. Gncrthophausia, a mcsopelagic mysid, stored wax cstcrs (32% of the lipid). A deep-water chactognath (Eukrohnia sp. ?) and a mesopolagic cranchiid squid had over 25% wax esters. Fish species not previously analyzed containing more than 10% wax esters included three myctophids and members of the As,troncsthidae, Cheilodipteridae, Gonostomatidae, Paralepididac, Scopelosauridae, and Stcrrioptychiclac. In previous studies of the occurrence and importance of reserve lipids in marinc zooplankton ( Lee et al. 1970, 1971, 1973) we have shown that copepods from the upper 500 m in polar and temperate latitudes had a higher lipid content than did copcpods from subtropical latitudes and that at an oceanic, subtropical station mcsopelagic and bathypclagic copepods (bw Carroll 1965; Gunstone 1967). We confirmed by direct mcal Travel funds and ship time were provided by the Marinc Life Research Group and National Science Foundation Grant GB 12413. This work was supported by National Science Foundation Grant Cl3 24834 and Marine Life Research Croup Gcncral Funds. surcments on zooplankton the hypothesis of Lewis ( 1967)) based on the observed increases in percentage by weight of oleic acid, that wax esters would be increasingly important in decpcr-dwelling pelagic animals. Nevenzcl et al. (1965, 1966) had shown that olcic acid is an important constitucnt fatty acid in wax cstcrs of the fishes Ruvettus pretiosus and Latimeria chalumnae. Morris (1971a) showed that in the tropical Atlantic the mesopelagic copepods Paraeuchaeta sarsi and Megacalanus princeps contained wax esters. Hcrc we report lipid data on fish and several dominant zooplankton groups collcctcd at a tropical station in the central South Pacific (24” S, 154” W) about 700 km southwest of Tahiti. Based on previous work ( Lee ct al. 1971)) WC hypo thcsizcd that tropical zooplankton from the near-surface waters would have little USC for lipid as a rcscrvc storage material because they have sufficient supplies of food and relatively small variations in the abiotic and biotic components of the cnvironmcnt; however, the deep-living zooplankton and nekton would tend to LIMNOLOGY AND OCEANOGRAPIIY 227 MARCH 1973, V. 18(Z) 228 RICHARD F. LEE AND JED HIROTA store lipid as a reserve because of the relatively lower concentrations of food and greater temporal and spatial variations in the rate of supply, Our objectives here arc fourfold: to determine whether the taxonomic and bathymetric patterns of the distribution of lipids in tropical zooplankton and nekton are similar to those previously observed at higher latitudes (over a third of the total ocean area lies between 20” N and S : Svordrup et al. 1942); to test the hypothesis that tropical epipclagic copepods have a lower average lipid content than those similar groups from higher latitudes; to determine whether the patterns of lipid distribution in copepods smaller than 2 mm in body length remain unchanged (all our previous data were for species larger than 2 mm); and to summarize existing data on the geographical distribution of total lipid content, wax ester, and triglyceride percentagcs in marine copepods. The chief scientist of the Aries III Expedition was J. A. McGowan, and his help in securing samples is greatly appreciated. We also thank R. J, LcBrasseur and J. D. Fulton for providing temperate water copepods; Arctic copepods were provided by H. Kobayashi and H. Fcrnandcz; and Antarctic copepods were provided by 0. Holm-Hansen. We thank A. A. Benson, in whose laboratory the lipid analyses were carried out, for his support and advice. WC gratefully acknowlcdgc the assistance of D. C, Judkins, T. Antczana, R. L. Wisner, and R. K. Johnson for their identification of dccapods, euphausiids, and fish. WC thank E. L. Vcnrick, J. A. McGowan, and A. W. Mantyla for kindly allowing us to use their unpublished data. J. C. Nevenzel made many helpful suggestions concerning analytical techniques. R. G. Ackman and I-1. P. Jeffrics reviewed the manuscript and improved it considerably with their comments. METHODS WC collected zooplankton and micronekton on Aries III Expedition, RV T. Washington ( March 1971)) using a 3-m Isaacs-Kidd midwater trawl cquippcd with a l-m net of 0.505-mm mesh attached to the end ring of the trawl. The samples were collected at a station in the central South Pacific (24” S, 154” W). A 2,500m time-depth recorder was used to detcrmine the depth interval sampled with the trawl (most sampled from O-l,000 m); however, the depth at which a given organism was captured is unknown because the trawl hasno closing device. We were unable to obtain a good vertical series of samples from known depth intervals with bongo nets, because of the high diversity and low densities of the plankton, A neuston net of 0.333-mm mesh was used to collect animals in the upper 20 cm of the sea surface. Organisms were sorted at sea under a dissecting microscope to divide them into homogeneous groups of species, stage, or sex as soon as possible after capture. WC placed the animals into four groups : copepods, crustaceans other than copepods, molluscs and chaetognaths, and fish. WC attempted to collect and analyze most of the groups of copepods taken in the series of trawls and neuston samples; they were identified according to Brodskii ( 1950 ) , Grice ( 1963)) Hiilsemann ( 1966)) Mori ( 1964)) Rose ( 1933)) and Sewell (1947). Lipids were extracted from live organisms in chloroform : methanol ( 2 : 1 v/v) aboard ship; all subsequent work was carried out Lmder nitrogen. Some individuals of each group were preserved for later identification ( except when the identification was easy, e.g. Gaussia princeps), and the remainder were used for lipid extraction, After weighing the lipids, they were separated by thin-layer chromatography, and the amounts of wax esters and triglycerides were determined spectrophotometrically by the procedure of Armenta ( 1964) using dichromate digestion and measurement at 350 rnp. Wax ester and triglyceride were then exprcsscd as percent of total lipid. Five replicate analyses of the lipid of several zooplankton speDISTRLBUTION OF WAX ESTERS IN PELAGIC FAUNA 229 Table 1. Lipid composition. of copepods collected during the Aries III expedition of the RV T. Washington. A is tipid per individual (mg); B is lipid (% drv weight); C is trigtyceride (% total lipid}; D is wax ester (OJO total lipid)