T. stellatus is superficially similar to its only congener T. securis, but can immediately be identified by the presence (vs. absence) of a prominent dark spot in the dorsal-fin. It is sometimes traded as 'platinum hatchetfish'.
The genus Thoracocharax was originally erected by Fowler in 1906 as a subgenus of Gasteropelecus, but was elevated to generic status by Weitzman in 1960 for the subject species and T. securis.
The family Gasteropelecidae is separated from other Characiformes by the following combination of characters: frontal bone longitudinally corrugated with a strong longitudinal ridge; posttemporal and supracleithrum fused into a single bone; pelvic fins and associated bones minute; presence of an enlarged, strongly convex muscular pectoral girdle region, consisting of greatly expanded coracoids fused to a single fan-shaped and corrugated median bone; lateral line extending ventro-posteriorly to approach the anal-fin insertion; 0-2 or 3 scales behind head and one or very few scales on caudal-fin base; 10-17 dorsal-fin rays; 22-44 anal-fin rays; adipose-fin present in larger species (those in Gasteropelecus and Thoracocharax), absent in smaller species (those in Carnegiella).
Gasteropelecids are commonly-referred to as ‘freshwater hatchetfishes’ due to their heavily-keeled body shape which has evolved in such a way due to possessing an enlarged, heavily-muscled pectoral girdle, and which resembles the shape of a hatchet head.
They are sometimes said to be capable of propelled flight above the water surface by beating their pectoral fins but in fact this is not the case and was disproven by Wiest (1995) who used high speed video photography to demonstrate that the pectoral fins are used in leaving the water but not while the fish is in the air. The action is more accurately described as a powerful jump and is actually a modified threat response which only appears to be used in extreme circumstances much like in many other fishes.
The researcher also found that the fish only seem to be able to jump once or twice before requiring a rest period due to the large amount of energy required to work the massive pectoral girdle muscles, and when fatigued their threat response is comparable to that of other small species in that they dive towards the substrate.
In addition anecdotal evidence from fish collectors and icthyologists working in the field would suggest that gasteropelecids rarely jump and they also tend not to do so in aquaria, even when being chased with a net (pers. obv.). However, leaving the tank cover open during darkness is likely to result in one or more hatchetfish on the floor by morning, so clearly they can and will jump.
The family Gasteropelecidae contains three genera. Thoracocharax, the most primitive and distinguished by its impressive keel, contains two species, T. securis and T. stellatus. Gasteropelecus contains three species, G. sternicla, G. levis and G. maculatus. These two genera contain the largest in size of the hatchetfish, and all are silver in colouration; G. sternicla is the more frequently seen of these five species, though any of them may be offered as "Silver Hatchetfish." The third genus, Carnegiella, contains four described species that are the most derived or specialized of the hatchetfishes, and all are smaller and lack an adipose fin.
The genus Thoracocharax is the most widespread of the hatchetfishes and the two species have distinct chromosomal and morphological characteristics in different geographic regions. Thoracocharax stellatus presents a considerable morphological polymorphism among populations, mainly due to the geographic isolation (Silva et al, 2012)[1]. This type of diversity may sometimes lead to subspecies or distinct species within the complex. Phylogenetic analysis by Kelly T. Abe et al. (2013) determined that this species is in a monophyletic lineage, but there were four well-supported clades found among the samples, "separated by relatively large genetic distances and following a well defined geographic orientation," suggesting that "T. stellatus may represent a species complex instead of a single species."
Characiformes is among the most diverse orders of freshwater fishes currently including close to 2000 valid species distributed among 19 families. This tremendous taxonomical and morphological diversity has historically impaired the ability of researchers to resolve their genetic relationships with many genera remaining incertae sedis. A further limiting factor has been that in many cases exhaustive study of these on an individual basis is the only way to resolve such problems. Modern molecular phylogenetic techniques have allowed some headway, though, and research papers by Javonillo et al. and Oliveira et al., published in 2010 and 2011, respectively, revealed some interesting hypotheses.
The results of the former suggest that Gasteropelecidae forms a monophyletic clade nested within the family Characidae, while the latter expanded on this and concluded that within the family Carnegiella is the sister group of Gasteropelecus with Thoracocharax sister to that clade, and that Gasteropelecidae appears most closely-related to the authors’ expanded Bryconidae which contains the genera Brycon, Henochilus and Salminus.
Others such as Oliveira et al. (2011) have concluded that the family Erythrinidae is also closely-related to this grouping with Hepsetidae and Alestidae more distant.
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