Devoto Lab

Simplified protocol for setting up and breeding zebrafish

Useful links:

Zebrafish (Danio rerio) are widely available from local pet stores for about a dollar each. All of the needed equipment, food, and supplies that are necessary for maintaining and breeding them can also be found there. A word about pet stores: there is usually a wealth of knowledge to be found at small local pet stores rather than the “big ones” such as Petco or Petsmart. Tell them what you are trying to do and usually they can be a big help.

Set up

Material needed:

Tap water that has been allowed to sit at room temperature for several hours is fine. The temperature of the water should be kept between 25ºC (77 ºF) and 29ºC (84 ºF). The pH should be kept at around 7.0. If there is any question about this, local pet stores will test your water’s pH for free, otherwise a small pH kit can be purchased.

The lighting should be kept on daily cycle of 14 hours of light and10 hours of dark for the fish to breed. If the aquarium is kept in a rather quiet dark place, an aquarium light on the top of the tank hooked up to a timer (from Home Depot or Lowes) should be fine. Otherwise, the tank can be covered with dark sheeting or covered in some way to protect against outside light.

Husbandry and breeding

Material needed:

The fish. Obviously, for breeding you want similar numbers of male and females. Female fish have a rounder, whiter belly than the males. Males are usually slimmer with a golden or salmon colored belly. This is where a knowledgeable person at a pet store is invaluable; usually you will find this person at a small local shop. It is often best to start with about 5 fish, let the tank get acclimated for a couple of weeks and then add more fish. Depending on the quality of the fish, you should probably count on getting the fish at least a month before you want to breed them, so that you have time to get them in optimal health.

Feeding. Zebrafish are very hardy, and will survive some neglect. However, if you want them to breed, they must be healthy and happy. This is more likely if they are given frequent small feedings instead of infrequent larger feedings. If possible, twice a day feed them the amount of food that they can fully consume in 5 minutes. All of the food can be purchased at the pet store. Flake food is fine, but fish will breed better if given live food. Brine shrimp larva or live tubifex worms work well. Crumbled hard-boiled egg yolk has also been reported to work well. Many pet stores sell adult brine shrimp on a regular basis, zebrafish LOVE to eat those. The week prior to breeding the fish should be fed live food (or hard boiled egg yolk).

Breeding. The night before you want eggs, put a single layer of marbles in the bottom of the breeding tank (the tank that we gave you is perfect for this, it takes about 200 marbles) and fill it with water that has sat at room temperature for a couple of hours. Put in about 10 fish, optimally you would like 5-6 females and 4-5 males. If your local pet supplier guaranteed you an equal mix of males and females (and you trust them) you can scoop out 10 fish at random and put them in the breeding tank.

Collecting Eggs. Egg-laying usually takes place during the first 30 minutes of daylight. The eggs are laid and fall to the bottom of the tank between the marbles where the adults cannot eat them (they will devour all of the eggs in minutes if allowed to do so) Look carefully between the marbles, give the tank agentle shake, eggs will look like clear spheres about 2-3mm in diameter. If you don’t see any, don’t worry as they are hard to see between the marbles.
After 30 minutes of daylight, take the fish out of the tank but keep them separated from the others for the time being. Take the marbles out of the tank: the best way is with a net. Strain the water through the tea strainer; the eggs should now be in the strainer. Invert the strainer over a Petri dish and use a squirt-bottle of water to squirt them into the dish. If there are no eggs, put the same fish back in the breeding tank over marbles, wait another 30 minutes, and repeat the process. This repeat process can only be done about 3-4 times. Once you are finished, put the fish back into the permanent system.

Cleaning Eggs. Use a pipet to remove any eggs that look cloudy white or black, these are dead or decaying eggs. Also remove any fish poop or other debris that is in with the eggs. Do not overcrowd your eggs, usually about a hundred eggs in the large (100mm diameter) Petri dish is appropriate. At least once a day, clean out dead embryos from every dish. Keep the embryos at a temperature between 24 (75ºF) and 32 (90ºF) for optimal development. Zebrafish will hatch from their chorions during the 3rd or 4th day after fertilzation.

By convention, scientists’ 'standard' temperature for zebrafish is 28.5ºC. The stages are named by how many hours at 28.5ºC after fertilization it takes to develop to a given stage. The 6h stage occurs 6 hours after fertilization only if the embryos are kept at exactly 28.5ºC, if the embryos are colder then it will take considerably longer to reach the 6h stage.

Good luck!