Plasma non-esterified fatty acids of elasmobranchs: comparisons of temperate and tropical species and effects of environmental salinity

We investigated the influence of environments with different average temperatures and different salinities on plasma NEFA in elasmobranchs by comparing species from tropical vs. cold temperate marine waters, and tropical freshwater vs. tropical marine waters. The influence of the environment on plasma NEFA is significant, especially with regard to essential fatty acids (EFA) and the n-3/n-6 ratio. n-3/n-6 ratios in tropical marine elasmobranchs were lower by two-fold or more compared with temperate marine elasmobranchs, because of higher levels of arachidonic acid (AA, 20:4n-6) and docosatetraenoic acid (22:4n-6), and less docosahexaenoic acid (DHA, 22:6n-3), in the tropical species. These results are similar to those in earlier studies on lipids in teleosts. n-3/n-6 ratios and levels of EFA were similar between tropical freshwater and tropical marine elasmobranchs. This suggests that the observation in temperate waters that marine fishes have higher levels of n-3 fatty acids and n-3/n-6 ratios than freshwater fishes may not hold true in tropical waters, at least in elasmobranchs. It also suggests that plasma NEFA are little affected by freshwater vs. seawater adaptation in elasmobranchs. Likewise, we found that plasma NEFA composition and levels were not markedly affected by salinity acclimation (2 weeks) in the euryhaline stingray Himantura signifer. However, in contrast to our comparisons of freshwater-adapted vs. marine species, the level of n-3 fatty acids and the n-3/n-6 ratio were observed to significantly decrease, indicating a potential role of n-3 fatty acids in salinity acclimation in H. signifer.