This species is also known as 'Río Meta tetra' in the aquarium trade, although it is often misidentified and its name has frequently been applied to other species. In particular, the congener H. peruvianus has repeatedly appeared under the name H. metae in aquarium literature.
Some variability in colour pattern has been observed in populations collected at different localities, particularly in the colour and intensity of the pale lateral body stripe. This is unsurprising given the extent of its natural range, and some of these variants have been traded with names such as H. cf. metae 'gold line', 'teal line', etc.
H. metae can be identified via the following combination of characters as per Géry (1977): dorsal-fin insertion posterior to mid-body; body depth fits less than 3.6 times in SL; head length fits 3.5-4.0 times in SL; caudal peduncle length usually fits 1.5-1.75 times in SL; snout relatively short, maxilla not reaching pupil; usually 9-10 predorsal scales; 30-31 longitudinal scales; 18-19 branched anal-fin rays.
In addition, it can be distinguished from all congeners inhabiting the Río Orinoco basin in Colombia and Venezuela by presence of a dark marking on the caudal peduncle, between the pale lateral body stripe and anal-fin, but is often confused with the similar-looking Peruvian congeners H. loretoensis and H. metae.
For aquarists, the easiest way to identify these species correctly is to examine the colour pattern; in both H. loretoensis and H. peruvianus there is a broad dark band in the lower portion of the body, between the opercle and caudal peduncle, whereas in H. metae the dark band is prominent only on the caudal peduncle itself (as per the original description by Eigenmann and Henn). In addition, the dark band extends to the tips of the median caudal-fin rays and there is a bright red marking at the base of both caudal-fin lobes in H. peruvianus, but in H. loretoensis the dark band extends only slightly onto the caudal-fin, which is a more-or-less solid brick red colour. In H. metae the dark caudal peduncle marking continues onto the base of the median caudal-fin rays, and there is a yellow-to-orange marking at the base of each caudal-fin lobe.
All three species are included in the putative ‘H. agulha-group’ of closely-related species within the genus as proposed by Géry (1977), of which all members possess a dark lower half of the body, especially above the anal-fin, and usually a horizontally elongate humeral spot, more or less united with the asymmetrical, broad band. Other species in this group include H. agulha, H. loretoensis, H. peruvianus, and H. herbertaxelrodi, although some authors have included one or more of these, including H. metae, in the H. heterorhabdus group.
Lima et al. 2014 noted similarities between the H. aghula group and their revised H. heterorhabdus group which comprises Hyphessobrycon heterorhabdus, H. amapaensis, and H. eschwartzae. This trio all possess: a well-defined, elongate humeral blotch which is continuous with a dark, well-defined midlateral stripe that becomes blurred towards the caudal peduncle; a longitudinal red stripe extending along the body above the midlateral line; upper half of the eye red.
This pattern is different to that presented by several members of Géry’s original H. heterorhabdus group, such as H. vilmae, H. cachimbensis, and H. stegemanni, which possess a continuous, solid dark lateral stripe on the body and no obvious humeral blotch.
Hyphessobrycon was raised by Durbin in Eigenmann (1908) as a subgenus of Hemigrammus, differing from the latter by the absence of scales on the caudal-fin.
The grouping was revised by Eigenmann (1918, 1921) while Géry (1977) created artificial groups of species based on colour pattern, and these definitions are still widely used today, e.g., the H. flammeus group, the H. heterorhabdus group, etc. These cannot be considered to represent monophyletic assemblages, however, and their concepts continue to be redefined.
Weitzman & Palmer (1997) hypothesised the existence of a monophyletic assemblage within the genus based on colour pattern and male fin morphology that they termed the ‘rosy tetra clade’, with one of the characters supporting its monophyly being presence of a prominent dark marking on the dorsal-fin. This assemblage, plus other morphologically similar species, is considered to represent Hyphessobrycon sensu stricto by some authors, with the remaining species included in a much-expanded H. heterorhabdus group.
Others have proposed conflicting, typically more restricted, views of both the genus and/or its constituent species groups, and significant confusion remains. What is clear is that, as currently recognised, Hyphessobrycon is a polyphyletic lineage containing several genera.
The process of splitting it up has already started, and Malabarba et al. (2012) revalidated the genus Ectrepopterus Fowler, previously considered a synonym of Hyphessobrycon. They also analysed its relationships within the Characidae in the context of Mirande's (2010) previous work, but included the type species, H. compressus, for the first time in such a study. The results demonstrated that H. compressus is more closely-related to 'rosy tetra' representatives such as H. eques, H. pulchripinnis, and H. socolofi than other species then placed in the genus, including Psalidodon anisitsi, H. bifasciatus, H. elachys, H. herbertaxelrodi, and H. luetkeni.
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