Taxonomy & naming
Characidium grajahuensis was described by the Brazilian ichthyologist Lauro Travassos in 1944, with a type locality at Grajaú, in the state of Rio de Janeiro, Brazil — the source of the specific epithet. It belongs to the family Crenuchidae (the South American darters) within the order Characiformes, and specifically to the subfamily Characidiinae, the streambed-darting characins that also include the genus Melanocharacidium. Some taxonomic databases, including the South American Characiformes Inventory (SACI), list the current accepted spelling as "Characidium grajahuense", a minor orthographic variant of the original description; this article follows the spelling used in the cited source material, Characidium grajahuensis.
Characidium is a large and taxonomically active genus of small benthic characins found across much of South America, many of them narrowly distributed stream endemics that are difficult to tell apart without close examination of fin-ray counts and body markings. C. grajahuensis is one of several Characidium species restricted to the short, steep coastal drainages of southeastern Brazil's Atlantic Forest — a region with many single-basin endemics because the coastal mountain range isolates adjacent stream systems from one another.
Morphology
Characidium grajahuensis has the elongated, subcylindrical body typical of the genus — built for a life spent perched on or near the substrate rather than cruising open water. FishBase records a maximum standard length of 3.5 in, making it a modestly sized fish and, at the smaller end of that range, suitable for a nano or small streambed setup. As in other Characidium, the pelvic and pectoral fins are enlarged relative to typical characins and are used to prop the fish against the current on rocks, gravel or leaf litter between short, rapid darts — the behaviour that gives the whole subfamily its common name of "darters".
Detailed colouration and pattern data specific to this species were not found in the sources consulted for this article; data sparse. Across the genus generally, Characidium species tend toward cryptic, camouflaged patterning — dusky bars, blotches or a dark lateral stripe over a pale to olive body — consistent with a benthic fish that relies on stillness and substrate colour-matching as much as on speed to avoid predators.
Habitat
Characidium grajahuensis is endemic to a narrow stretch of coastal Atlantic Forest streams in Rio de Janeiro state, Brazil, with a range confined to the drainages between Guanabara Bay and Mangaratiba. It is placed within the Serra do Mar freshwater ecoregion, the steep, forested coastal mountain belt that runs along southeastern Brazil and that supports a high proportion of narrowly endemic fish because its short, fast-flowing streams are hydrologically isolated from one another.
As a benthic darter, it favours flowing sections of small to moderate streams with a mix of rock, gravel, sand and leaf litter substrate, where it partitions habitat by water speed and sediment type rather than ranging widely through the water column. Specific published temperature and pH data for this species were not found in the cache; the wider genus is generally associated with cool-to-moderately-warm, soft, slightly acidic to near-neutral running water, and Atlantic Forest coastal streams of this kind typically run cooler and better oxygenated than lowland Amazonian systems.
Feeding
Characidium grajahuensis is a micro-predator, not a grazer or scavenger. Genus-wide accounts of Characidium describe a diet of tiny aquatic and terrestrial insects and their larvae, along with small crustaceans, picked from the streambed and drifting current during short foraging darts. Darter characins characteristically ignore flake food and prefer small live or frozen items.
In the aquarium, keepers should offer bloodworm, white and black mosquito larvae, vitamin-enriched brine shrimp, daphnia and cyclops rather than relying on dry foods; a fish that appears to be refusing food may simply be waiting for something closer to its natural micro-prey. Regular small feedings of appropriately sized live or frozen foods better match its foraging habits than one or two large flake feedings a day.
Mating
Species-specific courtship behaviour for Characidium grajahuensis was not found in the sources consulted; data sparse. Hobby accounts of the genus describe spawning as being triggered by a shift toward softer, more acidic water and increased water movement, conditions often replicated with a cooler partial water change. No reliable external sexual dimorphism is documented for this species in the cache; genus-wide, sexing is generally difficult from colouration alone, and breeders typically rely on relative body fullness in ripe females.
Breeding
Characidium species, including the wider darter-characin group this species belongs to, are understood as substrate egg-scatterers: eggs are released over fine-leaved plants or similar cover rather than deposited in a guarded nest, and there is no reported parental care after spawning. Hobby breeding accounts for the genus describe conditioning pairs in soft, acidic water with fine-leaved plants such as Java moss, good water movement and a protected filter inlet to prevent fry loss, with spawning often following a cool partial water change and typically occurring overnight or the following morning.
No species-specific breeding reports for Characidium grajahuensis were found in the cache, so the account above should be read as representative of the genus rather than confirmed for this species specifically; parental care is presumed absent, consistent with the scattering mode documented for its relatives.
In the aquarium
At a documented maximum of 3.5 in, Characidium grajahuensis is a small, unobtrusive fish well suited to a modest streambed or biotope aquarium rather than a large showpiece tank. It is a bottom-oriented micro-predator that spends most of its time propped on the substrate or darting between cover, so a tank furnished with smooth gravel, rounded stones and leaf litter, plus a current-generating filter outflow, suits its natural behaviour far better than a still, heavily planted community layout. Because it often ignores flake and prefers small live or frozen foods, keepers should be prepared to offer bloodworm, daphnia, cyclops or brine shrimp regularly rather than assuming it will adapt to dry food.
As with many Characidium, it is best kept in a species or quiet-community setting with similarly sized, non-aggressive tankmates that will not out-compete it for the small live foods it needs. Given its tight endemism to a small set of Atlantic Forest coastal streams, hobbyists should also be mindful of sourcing — wild collection pressure on such narrowly distributed species is a real conservation consideration even where the species itself has not been formally assessed.
Conservation
Characidium grajahuensis has not been formally assessed by the IUCN Red List and its status is recorded here as Not Evaluated (NE). It is a narrow-range endemic, confined to coastal Atlantic Forest streams between Guanabara Bay and Mangaratiba in Rio de Janeiro state, Brazil — a heavily populated and developed part of the Brazilian coast where habitat loss, water pollution and stream modification from urban and coastal development are the general pressures affecting Atlantic Forest freshwater fish.
No species-specific population trend or threat data were found in the sources consulted for this article; data sparse. Given the very restricted range typical of coastal Serra do Mar endemics, a formal assessment would be valuable, and any aquarium sourcing of wild specimens from this region should be treated with the same caution warranted for other narrowly distributed Atlantic Forest stream fish.