FEATURE
Microalgae Different types of algae provide vital nutrients to rotifers, copepods and larvae of finfish, shellfish and shrimp by Dr Eric Henry, PhD, Research Scientist, Reed Mariculture Inc
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icroalgae are the foundation supporting much of the marine food chain, and they are the natural food of filter-feeding organisms. Algae are therefore essential for production of live feeds for larviculture of finfish and shrimp. Thousands of species of microalgae are known to science. They present an enormous range of cell sizes, cell structures, biochemical constituents that determine their nutritional value and digestibility, and they also vary widely in ease of culturing. Microalgae can be very difficult, even impossible to identify to species based on light microscopy alone, and different strain isolates that appear identical may exhibit very different biochemical profiles or behavior in culture. Careful consideration is therefore necessary in order to select the most suitable strains for different larviculture applications. Although many microalgae strains have been tested as feeds, only about 20 are in widespread use. What follows can be no more than a brief overview of the how these algae are most commonly used in larviculture.
Rotifers
The rotifers used in aquaculture (species of the genus Brachionus) are capable of ingesting particles as small as bacteria and as large as 10–30 ¾m; larger rotifers are capable of ingesting correspondingly
larger particles. Bacteria are too small to provide a significant source of nutrients, with the possible exception of vitamins (e.g. B12). Rotifer digestive systems are equipped with a unique grinding apparatus (mastax) that can mechanically disrupt yeast or algae cells, enabling rotifers to thrive on algae that may not be digestible by other filter-feeders. Rotifers can ingest inert particles, and yeast has often been used as an inexpensive feed to maintain rotifer cultures. But the nutritional value of rotifers as live feed for larval fish or shrimp is determined by the quality of the feed used to produce the rotifers, and only algae feeds can support high-density rotifer populations with optimal nutritional profiles. Many of the algae commonly used in aquaculture have been used to successfully culture rotifers, at least for experimental studies, but by far the most commonly used algae are strains of Chlorella, Nannochloropsis, and Tetraselmis. These algae can be produced reliably and at relatively low cost, but none can produce rotifers with an optimal nutritional profile for many larvae. Their most conspicuous shortcoming is the essential HUFAs (Highly Unsaturated Fatty Acids) content, in particular EPA and DHA, which for many fish are particularly important for nervous system development. Chlorella in particular lacks HUFAs, although it can support excellent growth of rotifers. Nannochloropsis contains high levels of EPA, and Tetraselmis moderate levels; both can support excellent rotifer growth, but neither contains DHA. A Japanese com-
30 | INTERNATIONAL AQUAFEED | September-October 2015