The causes of buoyancy in eggs of marine teleosts

Abstract

Pelagic eggs and demersal eggs of teleosts both have osmotic concentrations similar to that of the maternal body fluids, less than half that of sea water. Pelagic eggs are buoyant because they contain such large quantities of this dilute aqueous fluid. While the demersal eggs of teleosts usually have a water content of 60–70%, the buoyant pelagic eggs of marine teleosts such as whiting, Norway pout, saithe, cod, haddock, turbot, dab, plaice, witch, long rough dab, halibut and sole typically have a very high water content (ca. 92 %) and a lipid content of 10–17% of egg dry weight. About 90% of the buoyancy of such eggs in sea water is caused by their high aqueous content, only about 10% being caused by lipid. The buoyant eggs of grenadier and ling have large oil globules and higher lipid contents, 27 and 35 % of dry weight respectively. Nevertheless, most of their buoyancy is provided by their high aqueous contents (89 and 81 % water). The high water content of pelagic eggs is brought about by a massive influx of water into the oocytes during meiotic maturation (ripening) after vitellogenesis but before ovulation. In cod and plaice, ripening is accompanied by a four- to five-fold increase in both water content and free amino-acids, and by a large influx of both potassium and sodium. In cod, free amino-acids contribute much more than these inorganic ions to the water influx and to the total osmotic concentration of the mature egg, but in plaice the relative contribution of inorganic ions approaches that of the free amino-acids.

Effect of biotic and abiotic factors on the biochemical composition of wild eggs and larvae of several fish species

“…Therefore, for these 2 species, changes in protein, carbohydrate and lipid content of the eggs were not observed as a mechanism to modify egg buoyancy. This result is in agreement with those that considered the function of lipid in the eggs as nutritive and not as hydrostatic (Craik & Harvey 1987). As vertical distribution of pelagic eggs is more influenced by wind-induced mixing than by egg buoyancy (Coombs et al 1985, Sundby 1991, to assure optimal buoyancy, it is probably enough to produce eggs of a density which allows the eggs to stay buoyant in the upper layers according to the prevailing range of salinity in the spawning areas, additional strategies such as changes in the biochemical composition of the eggs not being necessary.…”

“…As there is a spatial and temporal variability in both temperature and salinity, hence in egg buoyancy (Coombs et al 1985), any change in egg density directed toward achieving optimal buoyancy could benefit the recruitment processes. Egg buoyancy can vary according to egg water content (Craik & Harvey 1987) and chorion thickness (Kjesbu et al 1992). Differences in egg density can also occur as a result of changes in egg quality, with eggs' low specific weight relative to sea water when egg quality is poorer (Kjesbu et al 1992).…”

Chorion ultrastructure of some fish eggs from the South-East Atlantic

“…The egg of S. saurus scombroides, the chorion of which is thick, is large and has a narrow perivitelline space which makes the yolk-embryo mass large, thereby enhancing egg buoyancy by means of a high water content. This finding supports the statement of Craik and Harvey (1987) that high water content rather than lipid is the principal cause of buoyancy. The chorion of the small eggs of P. costatus, though bearing ornamentation, appears to be rather thin, thereby helping to reduce the relative density of each egg.…”

Parental effects in the European sardine Sardina pilchardus

“…Usually, a reduction in egg size is observed as the spawning season progresses and the quantity of food available increases (Hinckley 1990, McEvoy & McEvoy 1991, Baynes & Howell 1996. However, many authors maintain that egg size (or diameter) is not a good indicator of the quality of the spawn (Devauchelle & Coves 1988, Lagomarsino et al 1988, Nissling et al 1998, and that larval characteristics are determined by other variables such as size, condition and age of the adult females (Kristjánsson & Vøllestad 1996) or the biochemical composition of the egg (Craik & Harvey 1987, Devauchelle & Coves 1988, Srivastava & Brown 1991, MacKenzie et al 1996, Pickova et al 1997.…”