4.5 Optimal Levels and Ratios of Dietary n -3 and
4.5.1.1. Embryos and Yolk Sac Larvae
The lipid and fatty acid compositions of fish eggs and lipid metabolism during embryonic and early yolk sac larval stages were described in the previous edition of this book (Sargentet al.,1989). Therefore, this section only briefly summarizes these areas, focusing on new data related mainly to similar studies on additional species which have furthered our under- standing of the role of lipids and fatty acids in the earliest life stages of marine fish.
Although the lipid content and lipid class composition of fish eggs vary considerably with species, in many marine fish the eggs have relatively low levels of lipid (usually <5% of the wet weight) which is predominantly polar lipids (60–90% of the total lipid on average) (Sargentet al., 1989).
This situation is found in eggs from herring (Clupea harengus), haddock (Melanogrammus aeglefinus), whiting (Merlagus merlangus), saithe (Pollachius virens) (Tocher and Sargent, 1984), cod (Gadus morhua) (Fraseret al.,1988), and halibut (Falk-Petersenet al.,1989). Gilthead sea bream (Mourente and Odriozola, 1990; Ronnestadet al., 1994), Senegal sole (Solea senagalensis) (Vazquez et al., 1994), common dentex (Dentex dentex) (Mourente et al., 1999), sea bass (Ronnestadet al.,1998), and turbot (Silversandet al.,1996) have higher levels (>50%) of neutral lipids, with the eggs from all these species having oil globules similar to those found in the relatively lipid–rich eggs of sand eel (Ammodytes lancea) and capelin (Mallotus villosus) (Tocher and Sargent, 1984). The polar lipids of marine fish eggs are dominated by phospholipids, particularly phosphatidylcholine (PtdCho), followed by phosphatidylethanolamine (PtdEtn), phosphatidylserine (PtdSer), and phosphatidylinositol (PtdIns). The neutral lipids are mainly triacylglycerols and smaller amounts of cholesterol, although some lipid–rich eggs have globules that are predominantly steryl and/or wax esters (Sargentet al., 1989).
The total lipid of eggs from most marine fish studied is rich inn-3 HUFA, usually present at higher percentages than in the total lipid of other tis- sues, probably reflecting in part the preponderance of phospholipids in eggs, which, like phospholipids in other tissues, are generally higher in HUFA than neutral lipids (Sargentet al.,1989; Rainuzzo, 1993). Therefore, the eggs of herring, haddock, cod, whiting, saithe, plaice, halibut, turbot,
Senegal sole, and common dentex all contain high levels of 20:5n-3 and 22:6n-3 (Tocher and Sargent, 1984; Falk-Petersen et al., 1989; Rainuzzo, 1993; Parrishet al.,1993; Vazquezet al.,1994; Evanset al.,1996; Silversand et al.,1996; Mourenteet al.,1999). The fatty acid compositions of egg total lipid vary with species, again reflecting partly differences in lipid class com- positions, but are relatively more resistant to dietary changes than the fatty compositions of other tissues (Sargentet al.,1989). For instance, the levels of 22:6n-3 and totaln-3 HUFA were almost identical between wild and captive turbot eggs, whereas the levels of 18:2n-6 and, to a lesser extent, 20:1n-9 were much higher in eggs from captive fish (Silversandet al.,1996). These data support the previously appreciated resistance of egg composition to dietary changes but, at the same time, show that egg fatty acid composition can be affected by diet as evinced by elevated 18:2n-6 (Silversandet al.,1996). The effects of broodstock diet on the fatty acid composition of marine fish eggs and its relationship to egg quality criteria are described in Section 4.5.1.4.
In addition, the fatty acid composition of halibut eggs has been shown to vary throughout the spawning season and between first-time spawners and repeat spawners (Evanset al.,1996; Parrishet al.,1993).
Previously, lipid metabolism during embryogenesis and early larval deve- lopment had been studied in relatively few species (see Sargentet al.,1989).
However, in recent years the number of species studied has expanded greatly, to include dolphin fish (Coryphaena hippurus) (Ostrowski and Divakaran, 1991), plaice (Pleuronectes platessa) turbot (Rainuzzoet al.,1992), gilthead sea bream (Ronnestad et al., 1994), Senegal sole (Vazquez et al., 1994;
Mourente and Vazquez, 1996), cod (Fraseret al.,1988; Finnet al.1995), hal- ibut (Ronnestadet al.,1995), sea bass (Ronnestadet al.,1998), and common dentex (Mourenteet al.,1999). These studies establish that the utilization of lipids and fatty acids during embryonic and early larval development varies considerably between species. It had been reported previously that lipids were utilized as an energy source mainly after hatching in red sea bream and flounder (Pseudopleuronectes americanus) (Sargentet al.,1989), whereas, in Atlantic herring and cod, phospholipid was utilized during both embryo- genesis and to a greater extent during early larval development (Tocheret al., 1985a; Fraseret al.,1988). In dolphin fish, lipids were catabolized through- out the development period but to a greater extent during embryogenesis than larval development (Ostrowski and Divakaran, 1991). Furthermore, in both red sea bream and red drum, neutral lipids were the main lipids utilized, whereas polar lipids and specifically PtdCho were catabolized pri- marily in herring and cod, although neutral lipid utilization increased after hatching (Sargent et al.,1989). It was postulated that catabolism of phos- pholipids for energy may be a common characteristic of fish eggs that were rich in phospholipids (Sargentet al.,1989). This postulate continues to hold
since PtdCho was also catabolized primarily in the phospholipid-rich eggs of halibut and plaice, but not in turbot eggs where neutral lipids account for more than 50% of total lipid (Rainuzzoet al.,1992; Finnet al.,1995; Ronnes- tadet al.,1995). In contrast, in marine pelagic eggs that contain higher lipid levels, reflecting high levels of neutral lipid in oil globules or otherwise, such as from sea bream, sea bass, Senegal sole, and dentex, lipids are utilized primarily after hatching and mainly as neutral lipid, whether from the oil globule or otherwise (Ronnestadet al.,1994, 1998; Mourente and Vazquez, 1996; Mourenteet al.,1999). Therefore, in marine fish eggs it appears that, in general, lipid utilization occurs to a greater extent after hatching, par- ticularly in neutral lipid-rich eggs, possibly reflecting the greater energy demands of the mobile, free-swimming yolk sac larvae compared to the em- bryonic egg phase. In relation to specific lipid classes, two main patterns of utilization are apparent in marine fish, obviously directly related to egg lipid compositions. Phospholipid-rich eggs tend to utilize phospholipids, particularly PtdCho, whereas neutral lipid-rich eggs utilize primarily triacyl- glycerols and also steryl and wax esters where present. Another relatively common feature observed during development is the conservation and/or synthesis of PtdEtn, as reported in both the phospholipid-rich eggs of cod (Fraseret al.,1988), plaice, and halibut (Rainuzzoet al.,1992; Ronnestad et al.,1995) and the neutral lipid-rich eggs of turbot (Rainuzzoet al.,1992), Senegal sole (Mourente and Vazquez, 1996), and dentex (Mourenteet al., 1999). This results in a decrease in and normalization of the PtdCho:PtdEtn ratio as development proceeds, from the high values seen in most marine fish eggs to the values normally observed in fish tissues. This is particularly the case in phospholipid-rich eggs, dominated by PtdCho, where PtdCho is catabolized during embryogenesis.
Catabolism of lipids, whichever class, results in the release of free fatty acids, which can either be utilized for energy or reacylated back into lipid pools for other uses, which, during embryogenesis and early larval develop- ment, can be for the formation of rapidly developing larval tissues. Inn-3 HUFA-rich marine eggs, it is perhaps obvious that PUFA and HUFA will be catabolized for energy, particularly in phospholipid-rich eggs. Catabolism of HUFA, both 22:6n-3 and 20:5n-3, for energy, has been reported in cod (Finnet al.,1995), halibut (Ronnestadet al.,1995), Senegal sole (Vazquez et al.,1994; Mourente and Vazquez, 1996), and dentex (Mourente et al., 1999). In dentex, PUFA in both PtdCho and triacylglycerol were utilized, along with other fatty acids, generally in line with their order of abundance (Mourente et al., 1999). Similarly, in cod the fatty acids in PtdCho were catabolized nonselectively (Finnet al.,1995). However, in an early study in At- lantic herring, many of the PUFA liberated by the catabolism of PtdCho were selectively retained in the neutral lipid pool (Tocheret al.,1985b). Although
also utilized for energy, PUFA were relatively conserved in comparison with saturated and monounsaturated fatty acids during development of Senegal sole (Mourente and Vazquez, 1996). In cod, monounsaturated fatty acids in triacylglycerol were selectively catabolized in comparison with PUFA (Finn et al., 1995). In halibut, 22:6n-3 was a quantitatively important fuel, with almost 40% of the 22:6n-3 from PtdCho hydrolysis being catabolized, but with over 60% of the 22:6n-3 being selectively retained at the same time in PtdEtn (38%) and neutral lipids (23%) (Ronnestadet al.,1995). Fraser et al.(1988) had reported earlier that about 33% of the 22:6n-3 released dur- ing PtdCho catabolism in cod eggs was incorporated into TAG and steryl esters. Similar retention of 22:6n-3 in PtdEtn was observed in Senegal sole and dentex (Mourente and Vazquez, 1996; Mourente et al., 1999). Some selective retention of 20:5n-3 was also reported in halibut (Ronnestadet al., 1995) and 20:4n-6 was selectively retained during development of Senegal sole (Mourente and Vazquez 1996). Based on studies mainly with plaice, Rainuzzo (1993) suggested that utilization of HUFA, including 22:6n-3, oc- curred mainly in earlier stages of development when yolk was still present, whereas, later, in nonfeeding larvae, 22:6n-3 and 20:4n-6 were selectively re- tained in PtdEtn, at the expense of other fatty acids, including 20:5n-3, which were mainly catabolized. Thus, HUFA, as well as having well-established roles in membrane structure and function, can also serve as important energy sources during embryonic and early larval development of marine fish.