H. eschwartzae is distinguished from other laterally-striped Hyphessobrycon species from the Madre de Dios watershed by the following characters: 4 simple anal-fin rays (vs. 3 in other species listed); 13-15 dentary teeth (vs. 5-11); 6 teeth in the internal premaxillary row (vs. 5); 3 teeth in the external premaxillary row (vs. 4 except in H. heterorhabdus and H. loretoensis, which may have three or four).
It further differs from H. loretoensis in possessing 7 (vs. 9-10) pored scales in the lateral line and 5 (vs. 3-4) scales between the lateral line and dorsal-fin, and from H. heterorhabdus by 30-31 (vs. 35-37) lateral scales, 7 (vs. 23-24) pored lateral line scales, 9 (vs. 10) predorsal scales and 4 (vs. 2) maxillary teeth.
It may be distinguished from two other similar species, H. amapaensis and H. montagi, by colour pattern; in the former the dark lateral body stripe is reduced or absent towards caudal peduncle (vs. continuous in H. eschwartzae), and the latter possesses two distinct humeral spots (vs. a single spot in H. eschwartzae).
H. eschwartzae is a member of the putative ‘H. heterorhabdus-group’ of closely-related species within the genus as proposed by Géry (1977). This originally comprised around 15 members characterised by a 'longitudinal pattern' consisting of a thin, usually dark, lateral body stripe, but has since been modified on a number of occasions.
Most recently, Lima et al. (2014) proposed a putatively monophyletic H. heterorhabdus-group containing only three species; Hyphessobrycon heterorhabdus, H. amapaensis, and H. eschwartzae. These 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 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, but is similar to that of Géry's H. agulha group. The latter contained species with a colour pattern comprising "lower half of the body dark, especially above anal fin; usually a horizontally elongate humeral spot, more or less united with the assymetrical, broad band", and included H. agulha, H. loretoensis, H. peruvianus, H. metae, and H. herbertaxelrodi.
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. agulha 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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