Taxonomy & naming
Hoplias intermedius was described by the English ichthyologist Albert Günther in 1864. It belongs to the family Erythrinidae — the trahiras and wolf fishes — within the order Characiformes, a small family of predatory South American characins that also includes the genera Erythrinus and Hoplerythrinus. The genus Hoplias, to which this species belongs, comprises a group of large-bodied, cylindrical ambush predators long known to anglers and aquarists collectively as "trahira" or "wolf fish"; species-level identification within the genus has historically been difficult, and the taxonomy of several nominal Hoplias species has been revised as more material and molecular data become available.
The common names "wolf fish" and "trahira" are applied loosely across the genus and are not unique to this species; "intermedius" (Latin, "in-between, intermediate") most likely alludes to body proportions or meristic counts that Günther judged intermediate between other trahira forms known at the time. As with other Hoplias, positive identification of aquarium specimens sold under this name should not be assumed without expert confirmation, since several similar-looking trahira species circulate in the trade under overlapping common names.
Morphology
Hoplias intermedius has the unmistakable trahira build: a thick, rounded, torpedo-shaped body, a broad and somewhat flattened head, and a large mouth armed with heavy, conical, interlocking teeth built for gripping and crushing prey rather than slicing it. FishBase lists 42–46 scales along the lateral line and a high trophic level (4.2 ± 0.7), consistent with an apex predator taking mostly fish and other vertebrates. The species reaches a maximum recorded length of 23.5 in standard length, though aquarium-trade material is often offered at a smaller, still-substantial size — one aquarium supplier lists typical adult length around 21.5–22 in.
Colouration in the genus is generally cryptic — mottled olive, brown or grey-green tones that break up the outline against submerged wood and leaf litter — well suited to an ambush hunter that relies on stillness and camouflage rather than pursuit. Hoplias, like other erythrinids, also possess an accessory air-breathing organ, allowing them to tolerate oxygen-poor water and take gulps of air at the surface, a trait shared across the family.
Habitat
Hoplias intermedius is native to South America, with FishBase recording its distribution in Brazil; as an erythrinid trahira it is associated with the sluggish, often vegetated or structurally complex freshwater habitats typical of the genus — river margins, backwaters, flooded forest and standing pools — where its air-breathing capacity lets it persist through low-oxygen conditions that would exclude many other predatory fish. FishBase classifies it as a benthopelagic, tropical freshwater species.
The cache does not provide a specific published temperature or pH range for this species. Aquarium-trade sources for the closely related, similarly sized Hoplias in the trade cite mild, tropical conditions — roughly 75–82 °F and a pH close to neutral (around 6.8–7.5) — which is broadly consistent with lowland South American freshwater habitats, but keepers should treat this as a general guide from trade experience rather than a confirmed wild reading for H. intermedius specifically.
Feeding
Hoplias intermedius is an obligate piscivore and generalist predator, an ambush hunter that lies motionless amid cover before striking at passing prey. FishBase's trophic-level estimate (4.2 ± 0.7) places it firmly among carnivorous predators rather than omnivores. As with other Hoplias, the diet in the wild is centred on fish, and larger individuals will take any vertebrate or invertebrate prey they can engulf.
In the aquarium, keepers report that trahiras of this genus accept meaty frozen and fresh foods readily — whole fish such as smelt and sprats, prawn, mussel and worms — and that any tankmate small enough to be swallowed will eventually be eaten. This is not a fish for a mixed community: its diet in captivity should be built entirely around meaty, appropriately sized foods, and live feeder fish should be used sparingly if at all given the nutritional and disease risks of a live-fish-only diet.
Mating
No courtship behaviour specific to Hoplias intermedius is documented in the cache. Within the genus Hoplias, several species are known to be substrate nest-spawners in which a male prepares and defends a pit or a nest site among plants or debris ahead of spawning; whether this applies to H. intermedius specifically has not been confirmed in the available sources, and no sexual dimorphism is documented for this species — data sparse.
Breeding
Breeding data for Hoplias intermedius specifically is not available in the cache. More broadly, several Hoplias and related erythrinids (e.g. Erythrinus) are reported as substrate nest-spawners in which the male guards a pit or plant-associated nest and the resulting fry, but captive breeding of large Hoplias is rarely documented in the aquarium hobby — these are large, solitary, aggressive predators poorly suited to the paired conditioning and close observation that breeding usually requires. None of the cached sources report captive spawning of this species, and no breeding mode should be assumed confirmed for H. intermedius; treat it as not established in the aquarium hobby.
In the aquarium
Hoplias intermedius must be sized for what it actually becomes: FishBase's maximum of 23.5 in SL, and trade listings citing an adult length around 21.5–22 in, put this fish well outside what most home aquaria can comfortably house long-term. One aquarium-trade source recommends a minimum of around 120 gallons (roughly 120 US gal) for a single adult, with a securely fitted, heavy lid — Hoplias, like other large predatory characins, are strong enough to knock loose or leap through an inadequate cover, and their air-breathing habit brings them regularly to the surface.
This is a solitary predator, not a shoaling or community fish: keepers should expect it to attack and kill any tankmate it can catch and fit in its mouth, and even conspecifics are not safely housed together outside a deliberate breeding attempt. Water should be kept clean with strong filtration given the fish's size and meaty diet; trade sources cite a pH around 6.8–7.5 and a temperature of roughly 75–82 °F as workable, mild tropical conditions. Handling requires real caution — the heavy conical dentition of a fish this size can inflict a serious bite, and specimens should be moved by container rather than by hand or a standard net.
Conservation
Hoplias intermedius is assessed by the IUCN Red List as Least Concern. The cached IUCN source lists the assessment without a stated restriction on distribution, consistent with FishBase's listing of a Brazilian range for the species; no major threats specific to H. intermedius are documented in the available sources. As with many freshwater fish across Brazil's river systems, the general long-term pressures on Neotropical freshwater habitats — damming, deforestation-linked sedimentation and water-quality change — are relevant background context even though this species is not currently considered threatened.