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Dario dario

Dario dario
Scarlet Badis
This species is one of the more well-known Badids in the hobby and has started to gain a greater following in recent years as makes a good choice for 'nano' planted aquaria. It can be easily told apart from congeners as it's the only Dario in which males possess a series of seven iridescent blue vertical bars on the flanks. You may see it referred to as Badis badis bengalensis/B. bengalensis by some resources; a result of an inadequately-researched 'description' published by a German magazine in 1999. 
Prior to 2002 the family Badidae included just five species of which only B. badis and, to a lesser extent, 'Badis' dario were popular in the aquarium hobby. However an extensive revision paper by Kullander and Britz published that year resulted in the erection of no less than ten new species along with the genus Dario into which 'B.' dario was moved and designated the type species. Dario currently contains three members which are most easily distinguished from Badis by their small adult size (usually less than 1"/2.5cm), predominantly red colouration, more extended first few dorsal rays/pectoral fins in males, straight-edged (vs. rounded) caudal fin, lack of visible lateral line and less involved parental behaviour. All were previously considered to represent 'Badis' dario. There are also a handful of undescribed species from India, Bangladesh and Myanmar so the genus is set to grow in size over time. 
Badids have historically been considered members of the families Nandidae or Pristolepididae and it wasn't until 1968 that Barlow proposed a separate grouping for them. They share some characteristics with anabantoids, nandids and channids; most notably the typical spawning embrace in which the male wraps his body around that of the female. Recent studies have concluded that this procedure is an ancient trait inherited from a common ancestor to all these families. In the 2002 revision all Badis, Dario and Nandus species were found to share a uniquely bifurcated (split) hemal spine on the penultimate vertebra and the authors hypothesised that this may represent evidence of the monophyly of this group. They further proposed that the family Nandidae should be restricted to include only Nandus species with the other genera, Polycentrus, Monocirrhus, Afronandus and Polycentropsis, grouped together in Polycentridae. Following this system the Nandidae and Badidae are only separated by differences in morphology and egg structure although the phylogenetic relationships between them are yet to be fully-studied.