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FEATURE

Nutrition for the new laboratory rat by Peter H. Bowyer, Plymouth University, UK and Marc Tye, University of Minnesota, USA

Zebrafish (Danio rerio) are a small freshwater fish belonging to the cyprinid family (Spence, 2006). The species is native to warm water streams in the Ganges and Brahmaptura River basins located in India, Bangladesh, and Nepal (Barman, 1991; Laale, 1977). They are thought to be an annual species that breeds during the monsoon season, when food such as aquatic insects are most plentiful (Spence, 2006). Zebrafish are considered to be omnivorous having been observed feeding throughout the water column, from the surface to the benthos, on a varied diet (Spence et al 2008).

Z

ebrafish have and continue to be a popular aquarium fish thanks to their hardiness and low-cost but in recent years the species has become of interest as a model organism for biomedical, pharmaceutical, neurological, eco-toxicological and genetics research. So much so, that zebrafish are often coined as “the new laboratory rat�. Many biological characteristics have contributed to their popularity such as their high fecundity, short generation time, predictable spawning and low cost of maintenance. Furthermore, approximately 70 percent of the human genome is similar to that of the zebrafish, making it a viable model for human genetics research (Howe et al. 2013). Zebrafish are utilised throughout their life cycle but the early developmental stages are particularly attractive to researchers as, unlike mice, the animals produce an externally fertilised embryo that is transparent, allowing its embryonic development to be observed simply by placing it under a microscope. Today these fish are cultured in most major biomedical research facilities around the world including the United States (877 institutions), Germany (359), England (180), China (255), France (219), Spain (138), Taiwan (84), to name but a few (Kinth et al. 2013). Estimating the exact numbers of fish used is almost impossible but millions, if not hundreds of millions of zebrafish are now thought to be used in scientific research every year (Reed & Jennings, 2010).

In 2010, the Research Animal Department of the British RSPCA released figures detailing the number of scientific papers using zebrafish published over recent years on the PubMed Database (Reed & Jennings, 2010). Revisiting and elaborating upon these figures it is clear that exponential growth in the use of zebrafish for scientific purposes continues (Figure 1.). Optimal culture conditions such as water temperature and water chemistry values have been established for zebrafish, but our knowledge on nutrition requirements has drastically lagged behind. Many biomedical researchers are now asking for a standardised diet and open-formulations for this important research animal (Lawrence 2007, Penglase et al. 2012, Watts et al. 2012). This is not a new issue; a standardised diet for rodent models was established almost 40 years ago, followed by standardised diets for other models including guinea pigs, rabbits, primates, and swine. At present zebrafish facilities feed their stock a variety of different dry feeds, alongside live feeds. These include flake intended for use by the aquarium hobbyist, pellet for rearing larvae of marine fish and a select few commercially advertised zebrafish diets.

Zebrafish nutrition

Zebrafish nutrition remains very much in its infancy, being mostly limited to comparisons between commercially prepared feeds or against live feed. Formulating appropriate diets is paramount to guaranteeing zebrafish are nutritionally satisfied and thus a healthy model organism. At present poor nutrition and feeding practices has led to variability among results from human disease, pharmaceutical, toxicology, neurology and reproduction studies using zebrafish. Meeting individual amino-acid requirements ensures that growth of the animal is not compromised, but its importance extends to the consideration that deficiencies can be of detriment to immune and metabolic status. With some popular commercial zebrafish diets containing up to 60 percent crude protein levels, over-formulation is also of particular concern. Excessive supply of certain amino acids has been suggested to incur similar effects to deficiencies triggering stress responses, toxicity, interference with metabolic function and subsequently depressed growth (Choo, 1991). However, this excess supply of protein is most likely to be of detri-

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