8. Lipids and Essential Fatty Acids in Aquatic Food Webs
195
methods and intercalibration measures is important for the scientific progress.
Moreover, rigorously detailing of the exact units used to express published fatty
acid values is very important to avoid misunderstandings.
Research in mariculture has contributed significantly to the physiological understanding of EFA deficiency in aquatic animals, including the general symptoms of deficiency, and to the establishment of methods to assess EFA deficiency.
If used with some care, these methods and the general base of knowledge are
relevant for freshwater organisms as well.
EFAs are needed for growth, efficient membrane transport, and synthesis of
prostaglandins that regulate metabolism. Fecundity and the number of viable
offspring are especially sensitive to EFA deficiency. This may be because EFA
deficiency is critical during stages when neural tissues, including eyes and brain,
are developing. Other ultimate symptoms or effects of EFA deficiency are reduced
growth rate, enhanced mortality, reduced viability to environmental stress, and
inadequate behavior.
Identification of the important physiological role of DHA is another contribution from mariculture that is important for freshwater studies. Marine species
show two distinct patterns of DHA metabolism; some species exhibit lower
catabolism of DHA than of all other fatty acids during starvation, whereas other
species tend to catabolize DHA much faster than all other fatty acids. It is postulated that these different patterns in DHA metabolism characterize species evolutionarily adapted to low and high temperatures, respectively. This hypothesis
should be further elaborated both for marine and freshwater animals.
The species metabolic flexibility to elongate and desaturate short EFA is decisive for their dietary EFA requirements. The dietary EFA requirements of a
species will, for example, be lower if it is capable to synthesize DHA from
18:3ro3.1t is an interesting question if there is a systematic difference in the ability
to elongate and desaturate short EFA moieties between freshwater and marine
species. Otherwise, there is no reason to believe that EFA requirements of freshwater fish are different from that of marine fish species.
The relationship between dietary fatty acid composition and animal fatty acid
composition has been very well described in mariculture research through use of
model organisms of algae, zooplankton, and fish. It is important that studies of
EFA dynamics in aquatic animals must be based on the understanding that TAG
and WE primarily serve as sources of metabolic energy, whereas PLs, rich in
PUFA, are structural lipids important for functions such as cellular transports,
enzymatic reactions, and prostaglandin synthesis. The fatty acid composition of
TAG is mainly determined by the dietary fatty acids, whereas the composition of
PL is controlled by enzymes that select more strongly for PUFA (i.e., partly
genetic control). It is in this regard important to recognize that very lean specimens will exhibit total fatty acid composition, which reflects the composition of
their PL, whereas fat individuals will exhibit compositions close to their TG or
WE. This is frequently overlooked in ecological studies. General compartment
models with structural and storage lipids included, sensitivity to considerations of
the species-specific evolutionary adaptations to temperature, and the strategy of
195
methods and intercalibration measures is important for the scientific progress.
Moreover, rigorously detailing of the exact units used to express published fatty
acid values is very important to avoid misunderstandings.
Research in mariculture has contributed significantly to the physiological understanding of EFA deficiency in aquatic animals, including the general symptoms of deficiency, and to the establishment of methods to assess EFA deficiency.
If used with some care, these methods and the general base of knowledge are
relevant for freshwater organisms as well.
EFAs are needed for growth, efficient membrane transport, and synthesis of
prostaglandins that regulate metabolism. Fecundity and the number of viable
offspring are especially sensitive to EFA deficiency. This may be because EFA
deficiency is critical during stages when neural tissues, including eyes and brain,
are developing. Other ultimate symptoms or effects of EFA deficiency are reduced
growth rate, enhanced mortality, reduced viability to environmental stress, and
inadequate behavior.
Identification of the important physiological role of DHA is another contribution from mariculture that is important for freshwater studies. Marine species
show two distinct patterns of DHA metabolism; some species exhibit lower
catabolism of DHA than of all other fatty acids during starvation, whereas other
species tend to catabolize DHA much faster than all other fatty acids. It is postulated that these different patterns in DHA metabolism characterize species evolutionarily adapted to low and high temperatures, respectively. This hypothesis
should be further elaborated both for marine and freshwater animals.
The species metabolic flexibility to elongate and desaturate short EFA is decisive for their dietary EFA requirements. The dietary EFA requirements of a
species will, for example, be lower if it is capable to synthesize DHA from
18:3ro3.1t is an interesting question if there is a systematic difference in the ability
to elongate and desaturate short EFA moieties between freshwater and marine
species. Otherwise, there is no reason to believe that EFA requirements of freshwater fish are different from that of marine fish species.
The relationship between dietary fatty acid composition and animal fatty acid
composition has been very well described in mariculture research through use of
model organisms of algae, zooplankton, and fish. It is important that studies of
EFA dynamics in aquatic animals must be based on the understanding that TAG
and WE primarily serve as sources of metabolic energy, whereas PLs, rich in
PUFA, are structural lipids important for functions such as cellular transports,
enzymatic reactions, and prostaglandin synthesis. The fatty acid composition of
TAG is mainly determined by the dietary fatty acids, whereas the composition of
PL is controlled by enzymes that select more strongly for PUFA (i.e., partly
genetic control). It is in this regard important to recognize that very lean specimens will exhibit total fatty acid composition, which reflects the composition of
their PL, whereas fat individuals will exhibit compositions close to their TG or
WE. This is frequently overlooked in ecological studies. General compartment
models with structural and storage lipids included, sensitivity to considerations of
the species-specific evolutionary adaptations to temperature, and the strategy of