Zebra Danio
Danio rerio

Updated during November, 2023.

Warning! This article includes personal opinions and speculations!

The zebra danio is also called zebrafish. The scientific name is Danio rerio, but some sources still use the synonym Brachydanio rerio. This black and white horizontally striped agile fish, usually, swims in a school/shoal, close beneath the water surface. It is easy to keep and grows to about six cm (about 2.4 inches) standard length.

There is also a spotted breedingform, that is usually sold under the common name leopard danio, or "frankei". When the spotted form first hit the market, many aquarists thought that it was a new species.

The zebra danio is native to India and prefers slightly cool water, of about 18 to 24°C. Regular water changes to keep up the water quality is much appretiated. Well fitting coverglass, above the aquarium, is highly recommend, because the zebra danio likes to jump and may jump out of the aquarium, if it gets the chance.

The scientific name was changed in 2001, from Brachydanio rerio to Danio rerio, by Fang Fang, Kullander, in the doctoral dissertation:

Phylogeny and Species Diversity of the South and Southeast Asian Cyprinid Genus Danio Hamilton (Teleostei, Cyprinidae). Ph.D. Thesis-Stockholm University, Department of Zoology. p. 1-26. Sweden, Stockholm University. Department of Zoology.

The zebra danio is one of the most studied laboratory species in the medical embryo research field. There are many medical studies, using the zebra danio,, relating to diabetes, brain tumours, cardiovascular disease, Alzheimer's, ALS and various other ailments. The zebra danio has been a great help in water quality studies. Another scientific project is/was about mapping the whole genome (the genetic code) for the zebra danio.

I have bred the zebra danio. It is very easy to breed them in a shoal. Because the adults eat the eggs, if they find them, the aquarist may want to take some precaution to protect the eggs. There are many ways to do this. I personally used a large homemade basket, constructed from plastic mesh, that I hung inside the breeding aquarium. The females were put in the basket a few days before the males. It is suggested to keep the breeding aquarium about two degrees warmer, than the fish are used to, compared to their normal holding tank. After the males are put into the basket, the fish will soon start to spawn, usually the next day. When the eggs are released, they will fall through the holes in the mesh and will be out of reach from the gluttonous adults. After the eggs are released and the spawning seems to be over, you can move the adults back to the holding tank, or wait a few more hours to do so, in case there is still enough time left, for more fish to spawn, before the first batch of eggs begin to hatch.

If you don't want to use mesh, you may use gravel, pebbles, or glass marbles, (on the aquarium bottom), that the eggs can fall in between.

If you keep the adults permanently in an aquarium with gravel, they may often spawn over the gravel. If you have a large shoal of adult zebra danio, there may be new eggs in the aquarium gravel every day, from different spawnings. Any small fry that hatch from an egg in the aquarium, with the adults, will usually get eaten, unless the fry have plenty of hiding places, and/or there are not that many adults in the aquarium. You may be able to harvest eggs simply by vacuuming the gravel using an aquarium syphon and let the dirty water drain into a bucket. There is often eggs hidden within the mulm and feces etc. Let the bucket with dirty water sit for a few days. You may start to see some newly hatched free swimming fry in the dirty water in the bucket, maybe within a few hours, or the next day. The fry may take up to about two days to hatch from their eggs in the bucket, depending on the water temperature and how many hours it was since each batch of eggs were scattered and fertilized, since they may, perhaps, originate from a few different spawning occasions.

An other way, is to use a lot of plants, or spawning mops, where the zebra danios also eagerly spawn.

The rearing of the fry is fairly easy. When the tiny fry hatch, they still have a small yolk sac. When they start to look for food, it is reccomended to feed them small live infusoria, or other very tiny types of food, for a week, before moving on to live baby brine shrimp (artemia nauplii) and other foods. The fry will soon turn into juveniles. Within a few months they will grow up and become adults. The ordinary lifespan of a zebra danio, in an aquarium, is about three years.

It is forbidden to buy, sell, or possess any type of GloFish within the EU. In Sweden, it is Havs- och vattenmyndigheten, HaV, that follows up that these rules are obeyed. However, outside the EU, in some (but not all) countries, and/or states, genetically modified GloFish variants of the zebra danio (combined with genes from jellyfish), may be commonly available in pet stores.

Related external links and references:

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