This popular fish, frequently sold commercially under the name Zebra Danio, has become an important vertebrate model organism in scientific research. The zebrafish owes much of this success to its hardiness, cold resistance, easiness to breed and variable omnivorous diet. These same qualities have allowed this species to thrive in the wild in several non-native habitats such as America and Australia.
Key benefits for the laboratory include the fact that they are photoperiodic breeders, have regenerative abilities and their transparent embryos can be harvested easily and manipulated from birth. Transparency is perhaps the most crucial of all these qualities, as this allows microscopic visualisation of cellular processes including blood-flow and wound healing.
The UK Home Office regulates the breeding and use of zebrafish at the BCZF by granting an establishment licence under which named persons, such as the Veterinary Surgeon and Named Animal Care and Welfare Officers (NACWOs), can work. Tight regulations and careful consideration of project proposals ensure only well-considered, ethically reviewed research is undertaken.
The fish are housed within our 3 re-circulating systems in a selection of original “homemade” glass and modern, purpose-designed plastic tanks. A balance between glass and plastic is optimal: the demands of today’s research dictate a preference for plastic over glass, whereas welfare and visualization considerations favour glass.
Modern plastics offer far better versatility - for example, multiple tank sizes from 0.8-8 litres, integral and variable inlet and outlets controlled by adjustable taps as well as a range of drainage baffles from baby mesh to adult size. Self-cleaning and autoclavability features of plastic tanks also facilitate research.
Water quality is, of course, of paramount importance for healthy zebrafish stocks and is monitored by in situ probes and testing water samples. Automated alarms warn staff if levels fall outside the ideal range and action is taken with 24-7 cover. The eurythermal nature of zebrafish, although reported to be as broad as 6-41oC, is limited to between 26-29.5oC within the facility - closer to their breeding range (28-29oC).
Despite the resilience of zebrafish to withstand fluctuations in water quality, research endeavours require stability and vigilant management by both experienced aquarium technicians and several professional service providers.
Laboratory zebrafish do not currently have a standardized feeding regime, although recent research (which has considered both commercial aquaculture and laboratory animal principles) makes sound recommendations based on scientific findings. Logistics, cost and technical experience all impact upon choices, at Sheffield we use a mixture of live and pre-prepared dried foods. Marine rotifers (Brachious plicatilis) are an increasingly popular live larval diet and the salt-water crustacean Artemia is still a popular adult food.
Zebrafish are distinctly diurnal; sleeping more during the night than the day; light suppresses sleep. A standard photoperiod of 14 hours on and 10 hours off is operated and breeding is initiated from “sunrise”. Breeding times can be manipulated to any cycle required using LED light boxes. This manipulation of the photoperiod is popular with developmental biologists, whom may require developmental stages usually only reached at unsociable hours.
As oviparous breeders, successful embryo production depends heavily on the presence of eggs in females. Factors that impact upon breeding success including diet, stress, aggression, water quality, age, genetics (in-breeding) and compatibility between sexes. Breeding occurs daily and eggs are usually removed by filial cannabalism.
Once a week, embryos are collected from matings by either “marbling” or pair-mating fish.
“Marbling” (the in-house name given to the breeding of shoals of fish), produces vast numbers of embryos in a variety of ways. This comes, however, at the cost of reduced parental accuracy and synchronized birth timings. Specific pairs of fish can be bred in pair-mating tanks, which are designed to divide individual pairs overnight, reducing the risk of fighting and fish death. During “sunrise” the fish are brought together for phased breeding giving fewer embryos but defined parentage and birth stages. Both strategies are advantageous in different research methods.
The zebrafish originally attracted the attention of biologists due to its potential as a developmental model for the principles of cellular and molecular biology. Genetic engineering was employed from the offset and developed to create alterations in normal biological pathways.
Sheffield now houses over 750 genetically manipulated strains, including visually detectable transgenic lines. Further validation of these fish as a model for human disease is the 70 per cent similarity between the human and zebrafish genome, which helped shift research from basic science towards medical discovery.
A significant shift in funding opportunities towards medical projects has encouraged this transition further and global attention has now turned to the zebrafish’s potential as a non-mammalian model of disease – to the extent that we are now using it to study Parkinson’s disease, motor neurone disease, wound healing, cardiovascular disease and arteriosclerosis, hearing and balance, optic response, hypoxia, aging, epilepsy and muscular dystrophy.
The BCZF has helped reveal potential drug discoveries for epilepsy, Duchenne muscular dystrophy, chronic obstructive pulmonary disease (COPD), asthma and Parkinson’s disease. These significant achievements are primarily due to the fact that zebrafish are ideally suited to high-throughput, small molecule screens.
Dr Allen explained more about the University's zebrafish research during her presentation at Aquaculture 2016, in Las Vegas, US.
Dr Allen said that thanks to the University's research, some major advances in zebrafish husbandry have been made:
- A method for high throughput PCR based genotyping of larval zebrafish tail biopsies.
- Tamoxifen treatments to switch gene expression on/off in cre/lox systems.
- In vivo physiological recording from the lateral line of juvenile zebrafish
- Understanding the roles of distributed-in-schizopherenial (DISCI) and phosphodiesterase 4B (PDE 4B) in anxiety and depression.
This article was taken from the February 2016 Sustainable Aquaculture Magazine. To sign up for the May 2016 edition, please click here.
