Oil globule size in fish eggs: A matter of biome and reproductive strategy

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.

Buoyancy of post‐fertilised Dissostichus mawsoni eggs and implications for early life history

“…However, D . mawsoni eggs are large compared to other studied pelagic fish eggs (Baras et al, 2018; Barneche et al, 2018) and this gives the eggs higher ascent speeds than other pelagic eggs of similar buoyancy. Average ascent speeds of 2.7–3.6 mm s −1 clearly defines them at the upper range for pelagic eggs (Sundby & Kristiansen, 2015).…”

Survey on egg and fry production of giant gourami (Osphronemus goramy): Current rearing practices and recommendations for future research

“…The species can reproduce naturally in the rearing structures, allowing for relatively easy egg production on most small‐scale farms. There were no particular difficulties for first feeding during larval rearing, one of the main bottlenecks generally faced in fish culture (Hamre et al, ), because the larvae have large vitelline reserves and oil globule (Baras et al, ). The production of giant gourami fry was highly integrated in local agrosystems.…”

The Evolution of Alternative Buoyancy Mechanisms in Freshwater Fish Eggs

“…In the marine environment approximately 70% of teleost species produce pelagic eggs (Baras et al, 2018). The osmolality of marine eggs is lower than that of seawater, but higher than that of ovarian fluid (Finn et al, 2002;Wootton and Smith, 2015;Sørensen et al, 2016).…”

“…The evolution of effective egg hydration has been considered a major evolutionary innovation by teleost fishes that has allowed them to populate the open ocean (Wootton and Smith, 2015). While about 81% of marine fishes produce pelagic eggs with oil droplets, the average volume of these oil droplets is only 2% of total egg volume (Baras et al, 2018). In addition, many marine pelagic species lack oil droplets, while some species with demersal eggs have lipid contents equivalent to those of pelagic species.…”

“…For freshwater fishes, the osmolality of eggs is higher than that of freshwater, which means that egg buoyancy is less readily achieved by hydration alone. In this case, the lipid content of freshwater eggs has been assumed to play the primary role in achieving neutral or positive buoyancy (Wootton and Smith, 2015;Baras et al, 2018). Among 351 freshwater fishes for which data were available, the proportion of species with pelagic eggs was estimated to be 21% (Baras et al, 2018), including many species of economic importance, such as grass carp (Ctenopharyngodon idella) and bighead carp (Hypophthalmichthys nobilis) (FAO, 2018).…”