HUFAS and Phospholipids
Given that our focus needs to be providing limiting nutrients in enriched rotifers, it must be said that in the past Highly Unsaturated Fatty Acids (HUFA) lipids and especially in Lipid Membrane form have been very difficult and critical to manage nutrients.
Most marine fish larvae need very high levels of the essential fatty acids DHA and EPA and just the right amount of ARA. It is important to provide these critical levels without developing an excess of DPA or other less critical HUFAs. Further, it is very important that the majority of these fatty acids be Membrane Lipids (polar phospholipids and glycolipids and non-polar sterols), as opposed to triglyceride oils.
Why? Larval fish build tissue, and especially neural tissue, very rapidly. Fish cell membranes are mostly made up of polar phospholipids and glycolipids constructed from DHA and EPA, and non-polar sterols. These lipids are especially important for neural and eye development. Fish larvae must have an abundance of these lipids to grow well and without deformities. While larval fish can easily form their HUFA membrane lipids from other HUFA membrane lipids, they have a very limited ability to produce phospholipid and glycolipid HUFAs from triglyceride (oil) HUFAs. Further, marine larvae lack the ability to generate HUFAs (DHA, EPA and ARA) from more plentiful shorter-chain fatty acids.
Why? The details involve enzymes (elongase, desaturase, reductase and more). These enzymes are deficient in larvae that have evolved to consume phospholipid-rich live prey (the details make my head spin). Suffice it to say that HUFA phospholipids and other membrane HUFAs are the form of nutrition found in most species of marine larval fish eggs. That is what nature offers and that is what nature demands. Unfortunately, when rotifers are enriched with the wrong feeds or protocols, they can accumulate triglyceride (oil) HUFAs and still be quite deficient in membrane lipid HUFAs.
If you want to know more I would suggest starting with Sarget, Toucher and Bell, in Fish Nutrition edited by Halvar and Hardy. From there go on to Sargent et al. (1999), Aquaculture 179 217-229. Good Luck!
|Short Lipid Primer
Fatty acids are the building blocks that make up lipids. Fatty acids are defined first by their length. Thus EPA is C20:, (20 carbon chain) and DHA is C22: (22 carbon chain). Further, fatty acids can be saturated with hydrogen or unsaturated at different points along the carbon chain. The level of saturation determines their geometry and functionality. Saturated fatty acids are straight and tend to harden at moderate temperatures. Unsaturated fatty acids are kinked and tend to stay liquid at even low temperatures. The level of saturation and geometry of fatty acids affects their functionality in nearly all respects.
Enzymatic Lipid Transformation
Many organisms lack the ability to modify some fatty acids. These fatty acids are highly unsaturated long chain fatty acids (HUFAs) such as DHA and EPA, and are considered Essential Fatty Acids for marine fish – fatty acids that must be supplied though feed and cannot be synthesized by marine fish larvae.