Taxonomy & naming
Psectrogaster rhomboides was described by Carl H. Eigenmann and Rosa Smith Eigenmann in 1889. It sits in the genus Psectrogaster within the family Curimatidae, part of the characiform superfamily that also contains the true curimatas (Curimata, Curimatella, Cyphocharax) — small to medium, toothless, detritivorous characins found throughout the Neotropics. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the standing authority for the valid combination Psectrogaster rhomboides, and nothing in the cached sources for this species indicates a synonymy or generic reshuffle, unlike the harder splits seen elsewhere in Serrasalmidae (silver dollars, pacus).
The genus name Psectrogaster is built from Greek roots referring to the fish's body and stomach/gut anatomy, a nod to the coiled, elongated intestine typical of detritivorous curimatids; the species epithet rhomboides describes the rhomboid (diamond-like) profile of the deep, laterally compressed body. Reliable common names for this species are not established in the cached sources, so none are listed here rather than inventing a trade name.
Morphology
Psectrogaster rhomboides is a small, laterally compressed, deep-bodied fish with the characteristic rhomboid or diamond-shaped silhouette its name describes — a body form shared with several other curimatids and adapted for maneuvering in slow to moderate current over soft substrate. FishBase gives a maximum recorded length of 7 in standard length, a modest size consistent with its place among the "minor characins" rather than the large food-fish or predatory lineages of the order.
Like other curimatids, it is presumed to lack functional jaw teeth, relying instead on a specialised pharyngeal and gut apparatus to process fine detritus and microvegetation; this is standard for the family rather than something documented specifically for this species in the cached sources. Colouration is not detailed in the available material, and general curimatid patterning — silvery flanks, sometimes with a faint humeral spot or dusky fin edges — should not be assumed without confirmation.
Habitat
The species is restricted to the Parnaíba and Jaguaribe river basins of northeastern Brazil, two of the region's major coastal drainages that run through a markedly seasonal, semi-arid landscape distinct from the classic Amazon or Paraguay-Paraná systems more commonly associated with Neotropical characins. FishBase classifies it as a benthopelagic, tropical freshwater fish, meaning it forages near the bottom while still moving through open water rather than staying strictly substrate-bound.
Specific water chemistry and temperature data for this species were not captured in the cached sources, and this article does not invent figures for them. Given its northeastern Brazilian range and benthopelagic, detritus-feeding habits, warm tropical freshwater conditions can be inferred in general terms, but any precise temperature or pH range should be treated as data sparse until a primary source documents it.
Feeding
Psectrogaster rhomboides is classified as benthopelagic on FishBase, and diet data for a closely related congener, Psectrogaster essequibensis, sampled in the Bolivian Amazon, describes a detritivore feeding on flocculated organic matter, microdetritus, microvegetation, filamentous algae and microcrustaceans, with the balance of the diet shifting toward more algae in higher-sediment waters. This is genus-level evidence rather than a direct study of P. rhomboides, but it is consistent with the toothless, gut-adapted body plan typical of Curimatidae and is the best available guide to its feeding ecology.
No aquarium feeding data exists for this species in the cached sources, since it is not established in the hobby. Any captive diet would reasonably need to emphasise fine, vegetable-and-detritus-based foods rather than meaty or predatory fare, by analogy with its wild feeding ecology and with other small curimatids that are occasionally kept.
Mating
No courtship, pairing or sexual dimorphism data for Psectrogaster rhomboides was found in the cached sources. This is not unusual for a small, geographically restricted curimatid with no aquarium-trade or aquaculture profile driving targeted study — reproductive biology for many minor South American characins simply has not been documented in accessible literature. Rather than infer behaviour from distantly related characins, this account leaves the topic open.
Breeding
There is no documented breeding record for Psectrogaster rhomboides in the cached sources, in the aquarium hobby or in aquaculture. Many Curimatidae are presumed to be open-water or scatter-spawners typical of small riverine characins, but this has not been confirmed for this species specifically, and the sources available here do not support asserting a spawning mode with confidence. This species is essentially unknown in captive breeding terms, and that gap is stated plainly rather than papered over with a guessed mode.
In the aquarium
Psectrogaster rhomboides is not established in the aquarium trade, and no aquarium-keeping literature for the species was found in the cached sources. Its modest adult size (to roughly 7 in SL), benthopelagic habits and detritivorous diet suggest it would, in principle, behave like other small curimatids — an active, likely shoaling grazer needing open swimming space, soft substrate to forage over, and a diet weighted toward vegetable matter and fine detritus rather than protein-heavy foods — but none of that is confirmed husbandry, and this account does not present it as such.
Given the total absence of hobby data, anyone encountering this species would be working from first principles and general curimatid care rather than from established guidance. It is included in this atlas primarily as a documented member of the Hemiodus & minor characins group native to a distinctive, less-studied corner of Brazil, not as a species with an established aquarium track record.
Conservation
The IUCN Red List assessed Psectrogaster rhomboides as Least Concern on 7 November 2018, and the cached assessment notes no documented threats or conservation issues for the species. This status likely reflects its presence across two river basins rather than a single highly localised population, though the Parnaíba and Jaguaribe systems in semi-arid northeastern Brazil face regional pressures common to the area, including damming and water extraction, that are not called out specifically for this species in the available assessment.
There is no evidence in the cached sources of commercial fishing, aquarium collection or trade pressure on this species; its conservation profile is essentially that of a data-adequate but low-profile freshwater fish with a currently secure status.