Use of the microalga Tetraselmis in shrimp culture
By Eric C Henry, PhD, Research Scientist, Reed Mariculture Inc
For many years Tetraselmis has been one of the microalgae most frequently recommended as a feed for early life stages of shrimp. Fortunately, Tetraselmis is easier to identify by light microscopy, at least to correct genus, than some other flagellates. This is because it is relatively large (8-12 µm), and very few other green flagellates bear four equal flagella.
Many Tetraselmis strains have proved to be easy to grow under hatchery conditions and are valued as feed because they contain significant levels of the essential Omega-3 fatty acid Eicosapentaenoic acid (EPA). Some strains contain significant levels of taurine, a sulfur amino acid that may have significant nutritional value for larval and juvenile shrimp. Taurine has been only rarely reported in microalgae, but in some Tetraselmis strains it may constitute as much as 10 percent of total amino acids.
Several studies have reported that Tetraselmis has value beyond providing essential nutritional components. Addition of Tetraselmis to a formula feed diet was found to lower several measures of oxidative activity and consequent stress in vannamei shrimp. Suppression of Vibrio bacteria by Tetraselmis has been shown in culture of shrimp, as well as Artemia and finfish.
The performance of larval and post-larval shrimp what have been fed Tetraselmis with larvae and fed other algae have been compared, either singly or in combination with Tetraselmis or other algae. By far, the most comprehensive study was by Ronquillo, who cultured Tetraselmis at six different salinities, nine pH levels, and two temperatures, as well as varying culture medium nutrients. Tetraselmis produced under this wide range of conditions was then fed to Metapenaeus ensis, Metapenaeopsis barbata, Trachypenaeus curvirostris, Penaeus chinensis, P. indicus, P. japonicus, P. latisulcatus, P. merguiensis, and P. semisulcatus.
Later studies focused on Penaeus monodon or Litopenaeus vannamei. In none of the trials did Tetraselmis serve well as the sole feed; this is not surprising because it lacks the Omega-3 fatty acid DHA, which has been shown to be required by some shrimp.
This finding is consistent with the general principle that no single species of algae is likely to provide the truly optimal diet for any animal, and indeed the combination of Tetraselmis with other algae almost always yielded improved performance, even for algae that could support development of shrimp as sole feeds.
Nevertheless, in some of the studies the Tetraselmis strain tested appeared to provide considerably more benefit than strains used in other studies. Some of the discrepancies in results could be due to use of different culture conditions in the production of the Tetraselmis used as feed; it has been shown that the performance of Artemia-fed Tetraselmis was significantly impaired when the algae was cultured with suboptimal nitrogen or phosphorus.
It is also likely that the different strains of Tetraselmis used in different studies possessed intrinsically different nutritional profiles, because it has been shown that different strains grown under the same conditions can have dramatically different biochemical compositions. For example, Tzovenis reported as much as a 3.8-fold differences in EPA content among five Tetraselmis strains isolated from the Ionian Sea, while taurine was no more than one percent of amino acids and was undetectable in three strains.
Wikfors analysed the essential fatty acid and sterol content of nine Tetraselmis strains, and they found as much as an eight-fold difference in EPA content among the strains, and a more than 200-fold difference in the content of certain sterols.
Sterol content of algae is often overlooked as a significant nutritional component, yet it has been shown that only certain sterols can support growth of Penaeus japonicus.
To date, most of the numerous strains of Tetraselmis that have been isolated have not been subjected to detailed biochemical or molecular genetic analysis, but limited studies indicate that the common conception of Tetraselmis based on light microscopy indeed represents a natural group of related organisms.
Despite this reassurance that all Tetraselmis strains are in fact probably related, the delimitation of species within the genus remains problematic. Some 36 species have been validly described in the taxonomic literature, yet the NCMA culture collection of 119 Tetraselmis strains assigns species names to only seven strains.
Many shrimp feeding studies have used novel or locally isolated Tetraselmis strains, while providing little or no characterisation of strain properties. Although these strains are usually assigned species names, the authors never state the criteria they used for these species" designations.
Such ambiguity in the identities of algal strains used for feeding studies is a very common and fundamental problem in aquacultural research. No matter how rigorous the experimental protocols of these studies, or how sophisticated the statistical analysis of the results, when algal strains are not unambiguously identified, and preferably made available to other workers, the results of the studies cannot provide definitive guidance.
Progress in development of optimized algal diets incorporating Tetraselmis will depend on more rigorous biochemical analyses and explicit identification of the strains that perform best.