Taxonomy & naming
Heros sp. "Casiquiare" is not a formally described species. It has no scientific name, no describing author, no year of publication, and no entry in Eschmeyer's Catalog of Fishes, because no taxonomist has published a description of it. It is an informal taxon of the aquarium and field literature, named in the usual convention for undescribed cichlids — the genus Heros, the abbreviation "sp.", and a collection name in quotation marks taken from the Casiquiare, the natural canal (Brazo Casiquiare) that links the upper Orinoco to the Rio Negro. The fish sits squarely within Heros, the round, deep-bodied severum genus of the heroine radiation, and within the genus it belongs to the severus/efasciatus complex of green, banded blackwater severums — the same assemblage that includes the true Heros severus of the Rio Negro and Orinoco. Hobby references treat the Casiquiare fish as a distinct regional population ("Heros sp. Casiquiare") within that complex, valued because the Casiquiare is a genuine biological bridge between the Orinoco and Amazon faunas and its severums are of particular interest. Whether it warrants species rank or is best read as a geographic colour form of Heros severus is unresolved; no authority or date should be attached to it. (Its affinity to described severums is inferred from morphology and provenance, not from any published act of nomenclature.)
Morphology
The Casiquiare severum has the classic severum build: a tall, strongly compressed, disc-like body with high dorsal and anal fins and the rounded outline of the genus. By analogy with its congeners in the severus/efasciatus group, the maximum size is on the order of 8–10 in total length, with most aquarium fish somewhat smaller. The ground colour is the green-to-silvery-gold typical of the complex, overlain by the dark vertical barring that can flush in and out with mood, and a dark blotch toward the caudal peduncle. Regional severums from the upper Negro/Orinoco blackwaters often carry warm gold, green, and reddish tones on the head and flanks, and head vermiculation — dark spots and worm-lines on the cheeks and forehead — is generally heavier in males. Sexual dimorphism follows the genus pattern: males grow larger, show stronger facial vermiculation and somewhat more extended unpaired-fin tips, while ripe females are rounder-bodied and plainer-headed. Because the form is undescribed, no formal diagnostic counts distinguish it from neighbouring severum populations; it is recognised in the hobby chiefly by provenance and overall colour.
Habitat
This form is tied to the Casiquiare and the surrounding upper-Orinoco / upper-Rio-Negro blackwater province of Amazonas State, Venezuela. The Casiquiare is unique in being a large natural canal that diverts part of the upper Orinoco's flow southwest into the Rio Negro, so its waters mix Orinoco and Amazon influences — a setting of great biogeographic interest. The fish inhabits the soft, acidic, tannin-stained blackwaters characteristic of the region: low-conductivity water draining nutrient-poor white-sand catchments, with submerged wood, leaf litter, and root tangles along the margins providing cover. Like other severums it is a quiet-water, structure-associated cichlid of the river edges, flooded forest, and slow side-channels rather than fast or open water. Its known range is essentially this corridor and its connected blackwaters, which makes it a more geographically circumscribed form than the widely distributed severum complex as a whole.
Feeding
As a member of the severum group, Heros sp. "Casiquiare" is an omnivore with a pronounced herbivorous and frugivorous bias. Wild severums of this complex feed largely on plant matter, fallen fruit and seeds, algae, and detritus, taken among the root tangles and leaf litter of blackwater margins, supplemented by aquatic invertebrates and small benthic prey. The genus sits at a comparatively low trophic level for a cichlid, in keeping with that vegetable-leaning diet. There is nothing predatory about its feeding mode — it is a deep-bodied browser and picker rather than a hunter of fishes. In the aquarium this translates into a need for a vegetable-forward menu, as set out below.
Mating
Heros sp. "Casiquiare," like all Heros, is a monogamous, biparental, pair-bonding cichlid. Pairs form as fish mature within a group, and a bonded male and female together select and clean a spawning surface and defend a territory around it. Courtship is the standard severum routine of lateral displays, fin-spreading, jaw-locking contests, and joint substrate-cleaning, the male's heightened coloration and facial vermiculation serving as the chief nuptial signal. Outside breeding the fish is comparatively peaceable for its size, in line with the generally mild temperament of the severum group, though pairs become territorial around the nest at spawning time. As usual for the genus, allowing a pair to self-select from a group of young fish gives the best results.
Breeding
The Casiquiare severum is an open, biparental substrate spawner in the standard Heros pattern. Adhesive eggs are laid in rows on a cleaned vertical or sloping hard surface — a flat stone, broad leaf, driftwood, or the glass — and tended by both parents, the female fanning and directly caring for the eggs while the male concentrates on perimeter defence, with the roles overlapping. After hatching the wrigglers are moved to pits and guarded; once free-swimming, the fry are herded and defended as a school by both parents, and severum fry are known to glean mucus from the parents' flanks in the first days, as in other heroine cichlids. Clutch sizes in the severus/efasciatus group run to several hundred eggs in well-grown pairs. Parental care is extended across eggs, larvae, and free-swimming fry, and conditioned pairs re-spawn at intervals; soft, warm, acidic water of the kind the fish comes from favours fertilisation and egg development. (No clutch figures are separately published for this particular form; the numbers here are genus-typical and flagged as such.)
In the aquarium
The Casiquiare severum reaches the hobby only as occasional wild-caught imports from southern Venezuela, so it is far less commonly seen than the staple Red Shoulder and gold severum lines. When available it is kept exactly as one would keep any soft-water, blackwater severum.
Provide a tall tank with a generous footprint — on the order of 47 in length and a good volume for an adult or pair — furnished as a blackwater margin: soft sand substrate, abundant driftwood and root structure, botanical leaf litter, and tannin-stained, soft, warm, acidic water. Temperatures in the mid-to-high 20s Celsius (roughly 79–86 °F) and a soft, acidic pH (around 4.5–6.8) match its Casiquiare origins; wild fish in particular need soft, low-conductivity water and respond poorly to hard, alkaline conditions. Filtration should be effective but gentle in flow, suiting a quiet-water fish.
Feed a vegetable-forward omnivore diet: a quality cichlid or herbivore pellet as the staple, supplemented with blanched greens, spirulina, and plant matter, plus periodic frozen or live invertebrates for protein. Expect it to nibble soft plants. For tankmates, choose peaceful to moderate companions of similar size — calm large characins, peaceful catfish, mild cichlids — and avoid aggressive cichlids that would bully this gentle fish or anything small enough to be eaten. The usual mistakes are keeping wild fish in hard alkaline water, keeping the tank too cool, and housing this mild severum with belligerent tankmates.
Conservation
Heros sp. "Casiquiare" has not been evaluated by the IUCN, and because it is an undescribed form without a formal scientific name it has no Red List assessment of any kind — no status, no category, no published criteria. As a member of the broader severum complex of the upper Orinoco and Rio Negro, it is not a narrow single-locality endemic in the strict sense, but its known range is comparatively circumscribed to the Casiquiare corridor and connected blackwaters, which warrants a degree of caution. The general pressures on the region — deforestation, gold mining and associated mercury contamination in the upper Orinoco/Negro, and the ornamental fishery — form the backdrop for any future assessment, though none is documented as a population-level threat to this particular form. No conservation status should ever be fabricated for it; the honest statement is that it is unassessed and that its restricted, biogeographically notable range merits attention rather than alarm.