We are often (and should be) concerned about how much food some of our more finicky or delicate aquarium animals are eating—or if they are eating at all. Too often, however, inadequate attention is paid to food quality. Without doubt, diet has an enormous impact on the health, appearance and even behavior of our marine aquarium livestock. In addition to providing essential vitamins and minerals, any captive diet must be carefully balanced as to include the right amount of those basic food components: proteins, carbohydrates and lipids (fats). For proper growth, development and reproduction, these components are required in slightly different proportions by different species. Getting the right amounts—and kinds—of fats in an animal diet is especially important. Based on an animal’s normal fat intake (that is, its natural diet), the ability to utilize dietary fats varies considerably.
All animals need lipids in the diet in order to build tissues and to meet their metabolic energy needs. The major types of lipids include steroids, phospholipids and neutral fats. The neutral (or “true”) fats are rather large compounds that are usually made up of many smaller subunits. Neutral fats are triglycerides. Triglycerides are organic compounds that are composed of a glycerol (i.e. alcohol) and three fatty acid molecules. A carboxyl group (which is what makes a fatty acid an acid) is attached to one end of each fatty acid molecule; however, fatty acids are hydrophobic (non-water soluble) due to the nonpolar C-H bonds in their hydrocarbon skeletons.
In triglycerides, fatty acids are long-chain compounds. The chains of most naturally occurring fatty acids are unbranched and are composed of an even number of carbon atoms. Their carbon chains may vary in length, but are generally 14-24 (and most often 16 or 18) carbons long. When digested by an animal, fatty acids are degraded through sequential removal of 2-carbon chunks. Because fatty acids in triglycerides are easily oxidized in metabolic processes, they are usually potent sources of energy; there is approximately twice as much stored energy in an ounce of fat as there is in an ounce of polysaccharide such as starch. But one property that makes a triglyceride (or any fat, for that matter) especially easy for an animal to digest and metabolize is a low melting point.
The terms “saturated fatty acid” and “unsaturated fatty acid” are used to denote the nutritional properties of fats. Saturated fats have no double bonds in the carbon chains, and so as many hydrogens as possible may be bonded to them. In other words, each carbon in the chain contains a pair of hydrogen atoms; these fats are “saturated” with hydrogen. Saturated fats, which tend to be solid at room temperature, are usually derived from fat reserves in the bodies of animals. Unsaturated fats, which tend to be liquid at room temperature, are usually derived from plant material (e.g. phytoplankton). Two or more carbon atoms in these compounds are connected by double bonds. These double bonds result in the removal of hydrogen from the hydrocarbon chain. Thus, because they are not saturated with hydrogen, the carbons are free to bond with other atoms. Fatty acid chains are referred to as monounsaturated if they contain one double bond; those that contain more than one double bond are referred to as polyunsaturated.
The presence of each double bond causes a kink in the carbon chain. These kinks prevent unsaturated fat molecules from tightly packing, which is the cause of their lower melting point compared to the straight-chained saturated fats. Occurring as liquids at room temperature, these fats are referred to as oils.
Though unsaturated fats contain just a little bit less stored energy compared to saturated fats, they are incredibly important for the feeding animal’s cellular structure. Cell membranes that have a high content of either monounsaturated or polyunsaturated fatty acids have increased membrane fluidity at a given temperature. Different degrees of cell fluidity vary in their permeability to various ions (e.g. H+ and Na+). Thus, by controlling the fatty acid content in their cell membranes, animals can optimize the efficiency of metabolic processes at the cellular level. This higher fluidity becomes much more important at lower temperatures, and so is of greater consequence for fish and invertebrate species that live in deeper waters or at higher latitudes (this explains why seafood that originates from colder waters is richer in certain fatty acids).
Animals cannot synthesize certain fatty acids that are required for normal bodily function and so must obtain them through their diet. These substances are referred to as essential fatty acids (EFAs). EFAs are produced pretty much exclusively by plants, but may be passed up the food chain from animal to animal. Omega fatty acids are among the most important EFAs.
These fatty acids are so-named because they denote the location of a double bond counting from the tail end of a carbon chain. “Omega” is used because omega (ω) is at the end of the Greek alphabet, and the position of a double bond is measured from the end of the chain. Hence, ω-3 (or n-3) fatty acids are double bonded at the third carbon atom from the end of their carbon chains.
Marine phytoplankton are a major source of omega-3 fatty acids. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are both omega-3 fatty acids that are commonly found in marine oils. EPA and DHA are important components in the diet of virtually all marine animal species. They not only provide a powerful energy source and essential material for certain tissues but play a role in various vital biological processes (e.g. inflammation). Therefore, the supplementation of these important lipids can have a significant positive influence on the overall health of animals in an aquarium system; this may be so if the animal obtains them directly by consuming the algae, or even indirectly by consuming creatures that have consumed the algae (e.g. copepods).
Investigators (Brown et al, 1997) found that microalgal feeds widely vary in their nutritional content. Comparing 40 different species of microalgae that are used in aquacultural applications, lipid content was found to range from 7 to 23% dry weight. Because fatty acids such as EPA and DHA are synthesized in different amounts and proportions by different phytoplankton species, it is important to use a microalgal feed that contains a well-balanced blend of algae (see Mixed Microalgal Feeds for a Balanced Nutrition). Chlorophytes (i.e. green algae) are a good source of carbohydrates and other nutrients, but are rather low in polyunsaturated fatty acid (PUFA) content. For this reason, certain chrysophytes (i.e. golden-brown algae and diatoms) should be included in the blend. For example, a blend of the golden-brown algae Nannochloropsis (which is rich in EPA) and Isochrysis (which is rich in DHA) can strike such a balance. Diatoms too are an especially good source of lipids, and are particularly rich in PUFAs (5-35% of the total fatty acid content). Carefully selected mixes that contain the ideal combination of chlorophytes and chrysophytes (such as OceanMagik) are the best means of meeting a marine animal’s dietary needs.
Clearly, not all fats are the same. And, not only should certain fats be present in an animal’s diet, but they should be properly balanced. After all, just as it is with everything from water chemistry to species compatibility, diet (and especially lipid content) should be properly addressed in order to reach that perfect—natural—balance. Successfully doing so will certainly go a long way in maintaining a healthy, beautiful, natural aquarium system.
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