Abstract
Here, on the basis of published information upon over 800 species from 39 orders and 202 families, we analyse how the size of oil globules in fish eggs varies between reproductive guilds and environments with contrasting salinities. About 80% of marine fishes produce eggs with oil globules, the volume of which (VO) never exceeds 9% of the egg reserves and averages 2%. These proportions vary very little between marine fish species producing pelagic or demersal eggs. Fewer freshwater fishes (about 40%) produce eggs with oil globules, but their globules are much larger (mean VO of 10%) and their volumes vary considerably between reproductive guilds: VO is on average 3–10 times lower in freshwater species with demersal eggs than among freshwater pelagophils and aphrophils (i.e., bubble nesters), where it averages about 40% (range of 23%–69%) of the egg reserves. These results support the idea that oil globules serve to hydrostatic lift in fresh waters, but not or very little in marine waters, where egg buoyancy can be achieved at a lower energy cost through egg hydration. Advantages and constraints of oil globules in fish eggs are discussed in the light of species producing eggs with very large oil globules (VO > 20%): access to oxygen‐rich water layers that are indispensable for egg development vs. smaller size of larvae at first feeding, since the saturated lipids contained in oil globules essentially serve very little to tissue construction.