Danio rerio, the zebrafish of home and laboratory

Danio (Brachydanio) rerio. Zebrafish. When I was a kid, no community tank was complete without a few zebrafish. I took them so much for granted in those days that I haven't kept them for years now, I've got to admit. I hope you can resist the clumsy long-finned mutants, though — and the golden forms, where their snappy stripes are bleached to ghostly traces.
 
There are about 26 valid species of Danio in the broader sense, including "Brachydanio," which was formerly separated out. The Brachydanio characteristics, like a shorter anal fin and a shorter lateral line, are after all only functions of Brachydanio 's smaller size, it now appears. The group are being called Danionin.
 
The Danio genus has a wide range, from the upper reaches of the Mekong River, in southern China, to the island of Sri Lanka, which has some rare Danios with very spotty distribution. Recently Fang Fang, working with Sven Kullander at the Swedish Museum of Natural History, has revised the Danios, including six new danios from barely-explored northern areas of Burma, Thailand and Laos. If you're interested, look at the brief outline of Fang's project, with some photos of rare newly discovered Danios you're unlikely to see at the LFS.
 
The striped danios that even you and I might identify as quite distinct from the vertically barred danios are in fact a species group that is separate from a less familiar third group of species, which belongs with Danio dangila, which we never see in the trade. For technical reasons, since Danio dangila was described first (in 1822), it looks as though the obscure stranger will keep the familiar name, while the familiar "danios" we love are to be cast off in their own genus, as Devario. Stay tuned.
 
There are also many casual hybrid populations of Danio species in the trade: for instance, it will be rare for you to find the true Giant Danio (D. aequipinnatus) not blurred with the Queen Danio (D. regina), or to discover the authentic Danio nigrofasciatus, which has been supplanted in the trade by the hybrid Leopard Danio, a fish that appeared in 1963. Max Gibbs illustrated some of these trade hybrids in Aquarium Fish, December 1997; see an archived text  version of his article, "Dashing and delightful: Danios and their close cousins". A gene for phosphorescence was transfered from a sea jelly and inserted into the genome of D. rerio (reported in the New Scientist, 1995)  to create the transgenic "Glo-Fish"  which was patented and appeared on the market in late 2003, the first genetically-modified animal to become available as a pet. Now it's available absolutely  everywhere, the aquarium equivalent of the pink plastic lawn flamingo.  
 
Because of its cast-iron dependability in the aquarium, the zebrafish became one of the standard organisms for laboratory study: The Zebrafish: Biology filled volume 59 of Methods in Cell Biology, 1999 Beginning in 2001 our familiar long-suffering zebrafish became the first fish to have its complete genome mapped: its genome is continually being refined. Because the Zebra is used as a model for studying vertebrate development, you can find on the web all sorts of arcane stuff about zebrafish DNA and other cutting-edge molecular biology that's over my head. But it never hurts just to have a look, to see what they're up to.  Check out the Zebrafish Information Network (ZFIN) centered at University of Oregon. "The Zebrafish Book" at the site has detailed suggestions for raising Zebrafish in the labs that might give you some pointers too . And don't miss, the page "Zebrafish for K-12!" Alan Alda hosts a PBS-TV series, "Scientific American Frontiers:" in the segment "Gene Hunters" he visits Nancy Hopkins in her M.I.T. lab with 150,000 zebrafish, to discuss this gene isolating project. You'll also find a BioScience article on zebrafish genetic research.
 
Links. Abbreviated Danio rerio species profile at Tropical Fish Hobbyist.  Danio rerio at FishBase. Zebrafish at Wikipedia.