3 8 ^*x>^pectively, appeared in the lipids of different tissues

j^bwever, recent experiments with marine flat fish like

and turbot^^ have revealed that these species %%%'k the ability to chain-elongate and desaturate oleic,

Zlzinoleic or a-linolenic acids, Owen fed groups

turbot with radioactive oleate, linoleate and «-linolenate sand showed that these were incorporated without modification %mto the tissue lipids. This confirmed that turbot

czaunot desaturate and chain-elongate C^g fatty acids. Marine fish other than flat fish also appear incapable of further desaturating and chain elongating cidLetary a-linolenic acid. Thus the red bream (chrysophrys

•^ajor) grew poorly on diets containing 3 - 4 % a-linolenic

a*j:id and converted food inefficiently whereas diets containing ;->ng— chain polyunsaturated fatty acids of the n-3 series at

level of 2% led to rapid growth and good feed conversion^^*^^

In a recent review, Cowey and Sargent^^ have spec-

J'j0'tad that these fish have lost or never acquired the to form polyunsaturated fatty acids from C^^g

because, being carnivores, they obtained plenty polyunsaturated acids in their food. Thus the ability convert, for example, 18:3 (n-3) to 22:6 (n-3) would have

—11 —

little selection value. They pointed out that the ability

to convert acids to polyunsaturated acids (in this case

18:2 (n.-6) to 20:4(ii-6) is also lacking in the extreme

carnivores cat (Felis cattus ) and lion (Panthera leo) .

It seems that here is an instance of biochemical evolution along parallel lines between divergent forms in quite

separate environments. It appears therefore, that in the case of a few fish, at least, the total requirement of polyunsaturated fatty acids are obtained preformed in the diet.

Recently, Crawford showed that the

fatty acids of the dolphin (Tursiops truncatus) bear a much closer resemblance to those of land animals than to those of other marine vertebrates. The food chain of the dolphin provides a vast preponderance of the n-3 fatty acids compared to n-6, while the land food chain provides a more even balance between the two series,

Yet, the results showed that the dolphin has a significantly lower n^-3/n,—6 ratio than that of marine vertebrates. The ancestors of the dolphin evolved on the land with the

terrestrial n-3/n-6 balance and it appears that the dolphin fatty acids reflect its evolutionary origin rather than its present environment.

We have seen that certain fish obtain their polyunsaturated fatty acids entirely from the diet, and in others the origin is more complex. To add to this complexity, aquatic animals unlike their terrestrial counterparts are intimately exposed to fluctuating

12-

'environmental conditions such as oxygen concentration, temperature, and pressure (depth). Such variables involve the modification and regulation of the lipid and component fatty acids of the animal,

(iii) Environmental Influences (a) Temperature

Temperature is a major environmental factor well established as causing changes in the fatty acid composition of fish lipids. In general, a decrease in environmental temperature induces an increased degree of unsaturation of the tissue fatty acids. This effect has been observed in goldfish muscle^^, brain^^*^^, and intestinal^^ lipids;

51 52 53

rainbow trout , as well as other fish ' . Although most of the experimental work in this field has been done on freshwater fish, the phenomenon also applies to marine

An increase in unsaturated fatty acids at low temperatures may result from an increase in the amounts of phospholipids rich in these acids. That this was not

49

the case was demonstrated by Root , who showed that

change in environmental temperature does not influence the amount of the major phospholipids found in goldfish brain.

Brenner investigating the effect of environmental

temperature on the activity of liver mircrosomal desaturases from Pimelodus maculatus found that fish kept at 15° had higher desaturation and elongation activity than those kept at 30°, This increase in desaturation activity.

13-

evoked at relatively low environmental temperatures, fully compensated for the decrease in specific reaction rate with falling temperature characteristic of enzyme- catalysed reactions. They were, however, unable to

correlate this data with the proportion of polyunsaturated acids present in the liver microsomes at the two temperatures, as there were actually less polyunsaturated acids at the

lower temperature. Similar discrepancies have been reported by other workers^^,

Poikilothermic animals, such as fish, do not have a constant body temperature. The adaptation of the physio— chemical properties of their membranes to ever-

changing temperatures, therefore, has considerable survival value^^. The increase in fatty acid unsaturation with

decrease in temperature may be a means of adjusting

membrane viscosity within the range necessary for metabolic processes^^. Roots^^ has suggested that the modification of fatty acid composition assists in the maintenance of proper membrane fluidity and permeability for efficient functioning of the nervous system,

(b ) Effect of Pressure (Depth)

Like temperature, the pressure too has an

influence on the fatty acid composition of the lipids from aquatic animals but at present, there is not sufficient data to allow a precise statement about its relationship to the fatty acid pattern. Living in deep sea has meant adaptation to darkness, low temperatures and special food

5 7

I

it was thought that the pressure has less significant effect on deep sea life than temperature^^. Subsequent studies^^*^^ have shown that the relationship between temperature and

pressure is indeed more complex. Despite the considerable work in deep sea biology and biochemistry neither the effect of hydrostatic pressure on biomembrane structure nor the membrane lipid fatty acid composition as a function of depth in marine water column is known^^.

One of the few attempts to correlate fatty

acid composition with depth in the ocean is that of Lewis^^, He analysed fatty acids of a variety of fish species from different depths down to 4400 m and found that concentration of medium-chain saturated acids (16:0 and 18:0) and long-

chain unsaturated acids and decreased with depth,

whereas the levels of oleic acid (18:1) increased with depth. These findings were supported by the recent work

6 3

of a Japanese group , who examined the polyenoic fatty acid contents of liver neutral lipids in six species of fish. These observations must reflect to some extent at least,

the known presence of wax esters rich in oleic acid in certain midwater species^^.

(c) Influence of Sex and Season

We have seen that fatty acid composition of fish lipids is influenced by environmental factors such as the temperature and pressure. We have also seen that it is dependent on the dietary intake of the animal. Most fish, especially those living in the sea, experience wide

-15

the plankton at different times of the year, he would find contrasts between spring, summer, autumn and winter in the sea almost as striking as those in the vegetation on the land. These seasonal changes in plankton have a profound effect on the lives of many fish.

As the environmental factors vary depending on the season, it is to be expected that the fatty acid com­ position also changes accordingly. Seasonal changes

are extremely complicated however, as the effects embrace not only the environmental influences but also those of maturity and spawning^^. To quote Jacquot^^ "The

significance of the seasonal variations is complex and it is almost impossible to distinguish surely between the effects of the many factors which play a part",

Nevertheless, several papers have documented changes in fatty acid composition in relation to season,

67

DeWitt observed that the liver lipids of the cod

(Gadus callavias) show an increase in polyunsaturated and monoenoic acids during the winter and summer months.

An abrupt decline in these acids took place during the March spawning season, and no variation in saturated fatty acids was evident through the annual cycle. Jangaard and

coworkers investigating the seasonal changes that

occurred in the fatty acids of cod (Gadus morhua), found that the fatty acids from the flesh showed no significant changes with respect to either sex or season. However, the hepatic lipids of female fish showed increasing amounts of 20;1 and 22:1 acids in late summer and fall, while other acids did not vary significantly. The male fish on the

“16-

other hand did not exhibit similar seasonal variations^

Gruger examining two mullet-oil samples taken

in the months of July and December demonstrated the

striking differences that are possible for a single species caught during different seasnons. The fish were caught in the same general area but the December samples contained half as much 16:2 and four times as much 22:6 as was found in the July samples.

Unlike most terrestrial animals, the majority of fish experience severe depletion of lipids and other body constitu#ents for a part of every year in their lives. This depletion may be due either to scarcity of food at certain periods of the year or be the effect of maturation and spawning. The production of eggs or sperm always depletes a fish but in certain fish such as salmon, ascending the rivers to spawn, the condition may be exaggerated by a

6 5

concurrent abstention from food , Such depletion can effect marked changes in the fatty acid composition of the lipid. For example, Lovern^^ observed that the fatty acids of the body lipid from mature Clupea harengus are more

saturated than that of fish at an earlier stage of maturation. Presumably this is caused by preferential utilization of unsaturated acids, though a change in body size or the nature of the diet could also be important,

70

Iverson having determined the fatty acid profiles of prime

and spent salmon concluded that the long-chain monounsaturated acids are preferentially utilized by the migrating salmon.

The real situation, however, may be more complex than that 71

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pronounced difference in liver triglyceride synthesis between ocean pre-spawning and river post-spawning pink salmon (Qnchorhynchus gorbuscha), After entering fresh­

water and spawning these fish had virtually lost the ability to synthesize triglyceride in the liver but the ability

to synthesize cholesterol esters had increased, (B) The Minor Acids

(i) Methyl-branched acids

Three main types of methyl-branched acids occur in fish lipids and each will be considered separately,

(a) Iso- and anteiso-acids

Iso and anteiso acids carry a methyl group on the penultimate and on the antepenultimate position of the

carbon chain respectively and constitute by far the most 72 widely distributed group of branched-chain fatty acids The Cg compound iso valeric acid belongs uo this group

73 74