By Paul B. Brown and Liu
Bo, Purdue University
Anne Mulcahy was chairperson of Xerox Corporation (2002-2009) who, as far as I know, has little to do with the nutritional needs of aquatic animals. However, early life history nutrition in aquatic animals is a critical time for aquaculturists and Mrs. Mulcahy’s quote is one to keep in mind. Early developmental periods in the lives of animals have profound impacts on growth, reproduction and health in later life. Our understanding of these early life history events and subsequent aquaculture production parameters is not well understood with fish, crustaceans or bivalves. This broad area of research might yield significant new approaches to raising aquatic species and understanding basic biology in the future.
When most aquatic animals hatch from their eggs, they are small. We are talking microscopically small (< 12 mm) in many species, and it is difficult to feed an animal that is that small. Their mouths (gape size) can ingest only small particles and it is difficult making small feed pellets. The most common method of mass-propagating small-larvae species is by providing food as live organisms, most often rotifers (Brachionus sp.), followed by a second live food organism that is significantly larger (Artemia sp.), then finally formulated diets. The live food feeding approach has become somewhat of the norm, but the first larval formulated diets vary considerably. The early life history feeding period with live organisms is typically 30-45 days for most species. Copepods remain an attractive alternative to rotifers as these are commonly the natural prey items for many small-larvae carnivores, but mass propagation remains a bit challenging.
It would be easy to say progress in this challenging area has been slow. Just over 100 years ago, biologists were debating what small aquatic organisms ate. The debate centered on dissolved organic matter vs. intact cellular organisms (bacteria, algae or zooplankton). The original debate has diminished-living organisms appear to be the important food source for larval animals. However, the nutritional questions remain. What nutrients, in what concentrations and in which forms promote weight gain and survival of small larvae? As an example, we chose a recent article on larval lobster nutrition to emphasize a few points.
Colleagues in Australia (Conlan et al. 2014), recently evaluated an experimental diet as food for larval spiny rock lobsters (Panulirus ornatus). (Note: larval lobsters undergo multiple molts and changes in physical appearance before metamorphosing into juvenile lobsters. There are a limited number of formulated diets available for this group of animals). The experimental approach was to use weight gain and chemical composition data from larval lobsters as indicators of nutritional adequacy in the various stages of the growth cycle. The proportions of protein, lipid and ash were high at the premoult stage, reflecting growth and nutrient accumulation over the intermoult period, and lower in the post-moult stage, reflecting the large uptake of water to facilitate subsequent growth. The inverse trend of high levels of protein, lipid and ash in premoult animals reflects growth, nutrient accumulation and exoskeleton hardening over the intermoult period. Generally, dietary deficiencies or excess of essential nutrients such as protein, lipid and trace elements during early larval stages may lead to high mortalities and quality problems for larviculture. Mortality in this study was less than 5%, so gross nutrient deficiencies do not appear problematic. In many aquatic animals, larvae have a simple digestive tract with the ability to digest live food organisms, but may not be able to digest macronutrients found in formulated diets. Based on weight gains, the ingredients used in the experimental diet appear digestible to the larval lobsters. The overarching complexity of amino acid nutrition in animals is depicted (Fig.1).
The authors also reported that polar lipid (also known as phospholipid) was the dominant lipid class just prior to and after molting. Triacylglycerol concentrations were low despite being the principal lipid class available in the formulated diet. Likewise, despite receiving high concentrations of eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) in the dietary lipid source, levels of these fatty acids were comparatively low in larval lobsters. This indicated the long-chain polyunsaturated fatty acids such as DHA and EPA had important physiological functions in larvae during these important stages of the molt cycle. The long-chain fatty acids EPA and DHA have been focal points in larval fish nutrition for many years. A summary of fatty acid metabolism in larval fish in shown (Fig. 2).
Biological and Analytical
The complexity of nutrition has been confounded in the Conlan et al. study by the moult stage of the animal. Not only are nutritional considerations complex, but the interaction of nutrition and specific developmental periods introduces another level of complexity.
The physical challenges of nutritional research with small larvae necessitate a few changes in experimental protocols. Nutrient concentrations in surviving larvae have become common measures used to define dietary adequacies. This approach remains largely at the macronutrient level due to analytical challenges associated with many of the micronutrients and the limited analytical platforms for quantifying multiple micronutrients in individual samples (fatty acid and amino acid analyses are the exception to this generalization). However, the analytical challenges are rapidly dissolving away; a topic that will be discussed in future issues.
The other interesting point in this article was the lack of information on the dietary formulation. The authors declared the formulation proprietary. There are so few high quality larval diets available for aquaculture that any significant advance could lead to gaining significant market share for the developers. Hopefully this need within aquaculture and the resulting economic opportunity for businesses will eventually result in readily available larval diets for the hundreds of species raised in aquaculture.
Dr. Paul Brown is Professor of Fisheries and Aquatic Sciences in the Department of Forestry and Natural Resources of Purdue University. Brown has served as Associate Editor for the Progressive Fish-Culturist and the Journal of the World Aquaculture Society, among many others.
Conceicao, L., C. Aragao and I. Ronnestad. 2011. Proteins. in: G.J. Holt, editor. Larval Fish Nutrition. Wiley and Sons, NY, pp. 83-116.
Conlan, J.A., P.L. Jones, G.M. Turchini, M.R. Hall and D.S. Francis. 2014. Changes in nutritional composition of captive early-mid stage Panulirus ornatus phyllosoma over ecdysis and larval development. Aquaculture 434:159-170.
Dr. Liu Bo, Visiting Scholar at Purdue University, is Associate Professor at the Chinese Academy of Fisheries Science, Freshwater Fisheries Research Center, Wuxi City, China.