One of the most popular and instantly recognisable fishes in the aquarium hobby, this species has been selectively-bred to produce a number of additional strains including albino, 'golden' (leucistic), short-bodied, long-finned and 'diamond' (in which the dorsal surface is covered in reflective scales).
Unfortunately years of commercial breeding have somewhat affected the genetic vigour of the ornamental strain meaning morphological defects and ill-health are widespread and care must be taken when selecting stock.
This species also susceptible to an ailment commonly referred to as 'neon tetra disease' (NTD) which is so named because it was first diagnosed in neons but can actually affect many other species.
It's therefore recommended to spend time observing the fish in the shop and if any are displaying signs of restlessness, white patches on the skin or setting themselves apart from the main school, all classic signs of the disease, it's best to avoid purchase entirely.
NTD is caused by a microsporidian parasite called Pleistophora hyphessobryconis and is virtually untreatable once it reaches an advanced stage.
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. Here they produce cysts which cause rapid degeneration of the muscle tissue and development of the characteristic white patches.
Once the disease becomes established the rate of mortality can be very high and the only way to treat the aquarium is by physically removing any fish exhibiting signs of infection.
Similar symptoms may be observed in a common illness caused by the pathogenic bacteria Flavobacterium columnare which is often referred to as 'columnaris' or 'false neon tetra disease' and can be successfully treated using antibacterial products, meaning it's always worth isolating and treating a fish showing symptoms of NTD as you may be able to cure it if the bacteria is to blame.
As in all members of the genus some wild specimens possess a variable number of golden metallic scales and are marketed as P. innesi '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. innesi can be told apart from its congeners P. axelrodi and P. simulans by the following combination of characters following Weitzman and Fink (1983): premaxilla possessing a single row (the anterior teeth may sometimes give the appearance of two rows) of 5-7, rarely 8, teeth having no more than 3 cusps each; maxilla with 2-3 teeth having no more than 3 cusps each; dentary teeth with no more than 3 cusps each; ventral-limb gill rakers 10-11, usually 11; pored lateral line scales 3-6, usually 4-5; red pigmentation on body of live specimens not extending anterior to the pectoral-fin base.
For aquarists perhaps more useful are the subtle differences in colour pattern with P. innesi differing from P. axelrodi in possessing relatively less red pigment on the body, a narrower blue lateral stripe which extends less posteriorly and a more distinct white line 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 a more distinct white marking on the anal-fin.
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 Paracheirodon species has apparently been discovered at a single locality within the rio Purus system, the main channel of which enters the Amazon upstream of the rio Negro but little additional information is currently available.
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)[1] 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.
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