This species is immensely popular in the aquarium hobby and is now commercially-bred on a large scale with selectively-produced ornamental strains including 'gold' and albino.
It was initially going to be described as Hyphessobrycon cardinalis by Myers and Weitzman (1956) but just before this was due to be published Schultz presented an alternative description in the hobbyist magazine 'Tropical Fish Hobbyist' and named the fish after the publication's founder, Dr. Herbert R. Axelrod.
A significant proportion of the fish available in the aquarium trade are still wild caught or derive from community-led breeding projects such as Project Piaba in the middle Negro region, where this species is the most important ornamental fish and in the municipality of Barcelos accounts for as much as 60% of annual income.
Weitzman and Fink (1983) observed differences in the number of branched pectoral-fin rays, branched anal-fin rays, predorsal scales, and ventral-limb gill rakers between rio Negro and Río Orinoco populations whilst also noting that no conclusions could be drawn due to the need for more extensive collections and sample sizes.
There are also differences in colour pattern. The Rio Negro form possesses a somewhat straighter and longer blue lateral stripe which terminates below the adipose fin base, and usually has the red pigment extending slightly further onto the belly. The Orinoco form has less red pigmentation in the ventral portion of the body, and the blue lateral stripe ends before the adipose fin and is frequently slightly curved in a slight arc shape. These characteristics give the Orinoco form a chunkier appearance than the Negro form.
As in all members of the genus some wild specimens possess a variable number of golden metallic scales and are marketed as P. axelrodi 'platinum' or 'gold'. Although naturally-occurring the condition is said to be caused by a parasite which stimulates increased guanine deposition in the scales.
P. axelrodi can be told apart from its congeners P. innesi and P. simulans by the following combination of characters following Weitzman and Fink (1983): premaxilla with a single row of 5 teeth each having 5 cusps (3 cusps in specimens measuring less than 10 mm0.4 in SL); maxilla with 1 tooth having 5 cusps; dentary with anterior teeth often having 5, up to 7 cusps; ventral-limb gill rakers 12-14, usually 12-13; pored lateral-line scales 5-8, usually 6-7; red pigmentation on the body of live specimens extending well anterior to the pelvic-fin base.
For aquarists perhaps more useful are the subtle differences in colour pattern with P. axelrodi differing from P. innesi in possessing relatively more red pigment on the body, a broader blue lateral stripe which extends more posteriorly and a less distinct white marking on the anal-fin.
It differs from P. simulans in possessing a bluer (vs. greener) lateral stripe which extends only as far as the adipose-fin (vs. extending to caudal-fin base), a greater amount of red pigment on the flanks and less blue pigment on the eye.
The taxonomic history of all three species is complicated with earlier authors such as Géry (1960, 1977) tending to include them in separate genera as Hypessobrycon simulans, Cheirodon axelrodi and Paracheirodon innesi, respectively, with this classification mostly based on dental morphology following the system proposed by Eigenmann (1915). Others such as Madsen (1975) placed both P. axelrodi and P. simulans in the genus Cheirodon while van Ramshorst (1981) included P. innesi in Cheirodon as well.
Weitzman and Fink (1983) provided the first evidence for the monophyly of Paracheirodon, containing all three species, via a number of osteological characters plus their distinctive colour pattern consisting of an intense blue or blue-green lateral stripe, dense red pigment on the body restricted to the portion ventral to the lateral stripe, and a stripe of dark brown or black chromatophores underlying and extending dorsally to the lateral stripe.
A fourth "neon" species was discovered by Heiko Bleher in 2006 at a single locality, a nameless igarape within the rio Purus system, the main channel of which enters the Amazon upstream of the rio Negro. The fish has to date not been described, but DNA sequencing by Axel Meyer on the few specimens recovered apparently indicates that the newly discovered species is genetically closest to P. inessi (Bleher, 2008)[8].
The closer relationships of Paracheirodon within the family Characidae have not yet been fully-confirmed with the grouping considered incertae sedis by the majority of recent authors.
Results obtained in the molecular phylogenetic analysis of Oliveira et al. (2011) recovered P. axelrodi to be most-closely related to certain members of Hyphessobrycon species (specifically H. eques and H. megalopterus) among those species included in the study.
The function of the intense blue lateral stripe in Paracheirodon spp. may have evolved as a predator evasion strategy according to the hypothesis of Ikeda and Kohshima (2009)[2] which was published following direct field study of the fish in blackwater streams near Requena in the Peruvian Amazon plus observation of wild specimens in aquaria under different conditions.
The reserachers found that the bright colouration of the lateral stripe in P. innesi is less conspicuous in blackwater except when viewed from a limited angle of around 30° above the horizon, and that this could aid in confusing predators via bright mirror images of the stripe being projected onto the underside of the water surface.
Whereas most characids displaying a laterally-striped colour pattern possess a dark-coloured lateral stripe with a lighter stripe dorsally, this pattern is reversed in Paracheirodon and may be an adaptation for forming such mirror images.
The fish are also able to change the intensity of their overall colour pattern to an extent, becoming lighter in bright conditions with pale substrate or background and clear water, and duller with a violet lateral stripe at night, and this may also be a response designed to make them less visible.
Editing needs JavaScript. This is the text as it was last saved.