Yolk is a source of nutrients for developing embryos and for early fish larval stages that lack the ability to feed actively. Thanks to abundant networks of capillaries, yolk sacs are responsible for larval respiration before the development of gills, and in some fish species they are also the site of primary hematopoiesis. Fish yolk sac shapes and sizes differ among species. Cold-water fish with long periods of embryonic and larval development have larger yolk sacs as compared to those of warm-water species. Yolk comprises free amino acids, proteins, lipids, and minerals that are taken up by developing organisms through the yolk syncytial layers. Free amino acids and proteins are utilized before lipids. Yolk nutrients are divided between growth and energy production. The efficiency of yolk utilization for growth is higher in embryos, and then decreases with the increase in larval locomotion. The rate of yolk resorption depends mainly on water temperature, but it can also be modified by other environmental factors such as the availability of exogenous food or water that is contaminated with toxic compounds.
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