R. P. Evans á C. C. Parrish á P. Zhu J. A. Brown á P. J. Davis Changes in phospholipase A 2 activity and lipid content during early development of Atlantic halibut (Hippoglossus hippoglossus) Received: 9 September 1996 / Accepted: 8 September 1997 Abstract During early development in ®sh, phospho- lipase A 2 (EC 3.1.1.4) regulates membrane lipid modi- ®cations, which relates to changes in environmental conditions and provision of fatty acids required for metabolic energy substrates and prostaglandin biosyn- thesis. A method to analyze phospholipase A 2 in rat tissues has been modi®ed to measure its activity in em- bryonic Atlantic halibut (Hippoglossus hippoglossus L.). Egg and embryo samples were collected during the 1994 spawning season. Enzyme activity was undetectable at fertilization but in 10-d embryos was 230 pmol mg )1 h )1 (at 20 °C) and increased by 120% at hatch (17-d). Signi®cant alterations in the fatty acid composition of important phospholipids, phosphatidylcholine (PC) and phosphatidylethanolamine (PE), were also observed. The content of some critical polyunsaturated fatty acids, and the ratio of unsaturated/saturated fatty acids, declined signi®cantly over development. Acyl-chain re- structuring mediated through the activity of phospho- lipase A 2 , coupled with other observed lipid changes (signi®cant increases in the PC/PE ratio and cholesterol content), would produce a decreased ¯uidity of mem- branes during embryonic development, coinciding with the predicted upward movement of larvae in the water column. Arachidonic acid (20:4n-6) removed from PE could serve as a precursor for biosynthesis of 2-series prostaglandins, and eicosapentaenoic acid (20:5n-3) from PC is a likely source for other prostaglandin types. Despite removal of polyunsaturated fatty acids, there was an overall increase in lipid and fatty acid concen- tration, which can be attributed to amino acid catabo- lism during early developmental stages. Introduction The composition and content of egg lipids are important during early stages of ®sh development (Bruce et al. 1993). Poikilotherms alter phospholipids, which are es- sential structural components of cell membranes, to maintain phase state or ¯uidity despite changes in am- bient temperature. Homeoviscous adaptation describes this restructuring of phospholipid acyl chain moieties through adjustment of the unsaturated/saturated fatty acid ratio and the unsaturation index (mean number of double bonds in a fatty acid). The molecular species composition of phospholipids in marine ®sh tissues consists primarily of saturated fatty acids (SFA) in the sn-1 and polyunsaturated fatty acids (PUFA) in the sn-2 position (Bell and Dick 1991). Phospholipases A 2 (PLA 2 ) in various tissues are important in retailoring membrane lipids through cleavage of the sn-2 fatty acid moiety with subsequent re-acylation to produce new membrane phospholipids (Gurr and Harwood 1991). Enzyme me- diated thermal adaptations of membranes in poi- kilotherms have been extensively reviewed (Hazel 1989; Hazel and Williams 1990). Thermal adaptation may be important during the early life stages of Atlantic halibut (Hippoglossus hip- poglossus L.). In the North Atlantic, halibut eggs are spawned and fertilized at depths of 700 to 1000 m in late winter or early spring, and then ascend in the water column to the point of neutral buoyancy (Lùnning et al. 1982; Haug 1990; Mangor-Jensen and Waiwood 1995). Most halibut post-larvae have been found in the upper 5 to 50 m of the water column (Haug 1990). Atlantic halibut larvae hatch from eggs very early in their embryonic development, and exogenous feeding is delayed until 45 to 55 d post hatch when held at 4.5 °C (Lein and Holmefjord 1992; Holmefjord et al. 1993). Thus the eggs and embryos are essentially a ``closed system'' Marine Biology (1998) 130: 369±376 Ó Springer-Verlag 1998 Communicated by J.P. Grassle, New Brunswick R.P. Evans (&) á C.C. Parrish á P. Zhu á J.A. Brown Ocean Sciences Centre, Memorial University of Newfoundland, St. John's, Newfoundland, A1C 5S7, Canada R.P. Evans á P. Zhu á P.J. Davis Department of Biochemistry, Memorial University of Newfoundland, St. John's, Newfoundland, A1C 5S7, Canada