D. rerio is one of the most popular and ubiquitous fishes in the aquarium hobby, and its popularity is well deserved since it is peaceful, hardy and inexpensive.
Unfortunately it has been mass-produced in huge numbers for many years, thus inbreeding has occurred on a large scale and quality stock can be hard to find.
Alongside morphological deformities common ailments include 'neon tetra disease' (NTD), caused by the microsporidian parasite Pleistophora hyphessobryconis and basically untreatable once it reaches an advanced stage.
Symptoms include restlessness, increased respiration, white patches on the skin, and an individual separating itself from the group. The spores (larval stage) of the parasite enter the fish through the mouth and burrow through the walls of the gut before settling in the muscles. There they produce cysts which cause rapid degeneration of the muscle tissue and development of the characteristic white patches.
Once the disease takes hold mortality rates can rapidly become very high. No medication is available, and the only way to 'treat' the tank is to physically remove any fish displaying symptoms.
Similar symptoms are caused by a severe form of the pathogenic bacterium Flavobacterium columnare, and this is sometimes mistaken for NTD (Michel, Messiaen, and Bernardet, 2002)[12].
Another common problem is microsporidiosis, caused by the parasite Pseudoloma neurofilia (Matthews et al. 2001), with external symptoms including chronic emaciation and curvature of the spine having led to the vernacular term 'skinny disease'. Its life-cycle is similar to that of F. columnare in that it is ingested orally, but the main site of infection is the central nervous system, and it can be tricky to diagnose since infestations have been recorded in seemingly healthy individuals.
The 'leopard danio' is a spotted morph of D. rerio which was described as Brachydanio frankei (Meinken, 1963)[16], but is now generally referred to as D. rerio var. frankei. Its origin was considered something of a mystery for a number of decades, with the most accepted theory being that it was produced via selective breeding, but it is now known to be a spontaneous mutation of the striped form which also occurs in the wild (Spence et al., 2008)[17]. There do exist several ornamental strains, however, including a xanthistic 'golden' form and a long-finned variety.
More recently, transgenically-modified 'Glofish' which flouresce constantly have appeared on the market.
These were originally produced by the National University of Singapore in an attempt to mutate a fish so that it would literally glow in the presence of certain environmental toxins and are available in a number of colours, including yellow, green and red.
The substances causing this effect are proteins extracted from a particular jellyfish (green and yellow forms) or coral species (red form). An American company quickly purchased the rights to market Glofish in the United States where they've achieved a fair level of popularity.
As things stand they're illegal to buy, sell or own within the European Union although several instances of smuggling have been recorded, the punishment for which is a hefty fine.
D. rerio is a commonly-used model organism in scientific research, particularly the fields of vertebrate ontogeny and gene function, and is one of the few fishes to have been launched into space!
In recent years it's become commonplace to refer to the stripes on the body and fins of danionins as follows:
- P stripe: or "pigment stripe" is the central, dark, lateral stripe on the body which extends into the caudal-fin in some species. Stripes above it are numbered P+1, P+2, etc. and those beneath P-1, P-2, P-3.
- A stripe: the central stripe on the anal-fin; the proximal stripe (above it) is A+1 and the distal stripe (beneath) A-1.
- D stripe: The submarginal dorsal-fin stripe.
Following Fang (2003) Danio spp. are characterised by the presence of an A stripe on the anal-fin and two or more P stripes on the caudal, plus some internal characteristics such as enlarged nasal lamellae.
The genus has undergone some significant taxonomic reshuffling in recent years following the publication of a series of phylogenetic studies.
Older, molecular, phylogenies tended to agree that it represented a monophyletic group consisting of two major clades; the 'Danio devario' group containing the larger, deeper-bodied species and the 'D. rerio' clade comprising the smaller, slimmer fish.
However in 2003 Fang conducted a more detailed study based on morphological characters which included members of other related genera, and the results suggested for the first time that the genus Danio as previously considered represents a polyphyletic grouping, i.e., not all members derived from a single common ancestor.
The genus name Devario was suggested for the larger species with Danio being applied only to the smaller fish (with the exception of the type species, D. dangila which can grow to around 89 mm3.5 in SL). Recent molecular studies by Mayden et al. (2007) and Fang et al. (2009) resulted in further changes, with the latter study considering the genus Danio to be composed of three subclades. These were subsequently split into distinct genera by Kottelat (2013), as follows:
The former species D. erythromicron, D. margaritatus, D. choprae and D. flagrans were grouped together in the revalidated genus Celestichthys Roberts, 2007. These exhibit unique body patterning consisting of vertical bars (D. erythromicron, D. choprae, D. flagrans) or light spots (D. margaritatus) and possess either very short barbels or none at all.
Kottelat (2013) left only the type species, D. dangila, in Danio, separated on the basis of its larger size and the shape of the caudal-fin, which in adults is only slightly emarginate or even truncate in shape, a feature it shares only with Tinca tinca (the common tench) among other cyprinids.
The remaining species, of which D. rerio is thought to be the most ancient, were placed in the revalidated genus Brachydanio Weber & de Beaufort, 1916. Kullander (2015) did not follow this split, because phylogenetic analyses strongly support a single Danio in the sense of Fang (2003), with Devario a separate genus. The Catalog of Fishes and FishBase both keep all of these species in Danio.
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