Taxonomy & naming
Nannostomus grandis was described by H. Zarski in 2011, the original account appearing in Vertebrate Zoology 61(3): 283–298. The type material was based on specimens originating from the ornamental-fish trade out of Belém, Pará state, Brazil, which serves as the type locality. Eschmeyer's Catalog of Fishes is the authority for the valid name Nannostomus grandis Zarski, 2011.
The genus Nannostomus belongs to the family Lebiasinidae and encompasses the pencilfish — slender, pencil-shaped characoid fishes distributed across tropical South America. N. grandis sits within a cluster of closely related species centred on N. beckfordi, from which it differs primarily in its larger size and in the absence of pale fin-tip colouration on the ventrals. The species name grandis is Latin for 'large', an unambiguous reference to the fish's status as the longest-bodied member of the genus.
Morphology
Nannostomus grandis is the largest known species in its genus, reaching a maximum recorded standard length of 2 in. In body form it conforms to the classic pencilfish plan: elongate, laterally compressed, with a pointed snout, small terminal mouth, and a streamlined profile that accounts for the common name. The caudal fin is forked, the dorsal fin placed well back on the body above or just behind the pelvic insertion, and the adipose fin is present.
Like most Nannostomus, the species displays a bold horizontal body stripe running from the snout to the caudal peduncle, often accompanied by a secondary stripe below it and reddish pigment on the fin bases. The most reliable diagnostic character separating it from the closely related N. beckfordi is the lack of pale white or bluish colouration on the tips of the ventral fins — a useful field mark once you know to look for it.
Habitat
The type locality for Nannostomus grandis is given as Belém, in Pará state at the mouth of the Amazon basin in northeastern Brazil, with the original material traced to the ornamental-fish trade operating out of that city. Precise field collection data for wild-caught individuals are limited, but the species is broadly associated with lowland Amazonian waters.
As with other pencilfish from the Pará region, it is likely found in slow-moving or still blackwater and clearwater habitats — flooded forest margins, stream edges, and the shallows of Amazonian tributaries — where it moves through the upper water column among aquatic vegetation and submerged leaf litter. Water conditions reported in the aquarium literature for successful husbandry range from 72–82 °F, with pH from 5.0 to 7.0, consistent with the soft, often acidic waters of the lowland Amazon basin.
Feeding
Nannostomus grandis is a micropredator in the wild, foraging on small invertebrates in the upper water column and at the surface. Natural prey items include tiny crustaceans, insect larvae, small worms, and other minute aquatic organisms of a scale that the small mouth can handle.
In the aquarium the species responds well to a varied diet of small live and frozen foods — newly hatched brine shrimp, Daphnia, micro-worms, and finely crushed or graded flake. Good-quality small-particle dried foods form a useful staple, but regular provision of live or frozen items improves condition and can stimulate spawning behaviour. Feeding in small amounts several times per day suits the species' naturally opportunistic foraging style.
Mating
Nannostomus grandis is an egg-scattering open spawner, as is typical for the genus, with no parental care provided after spawning. Males display to females using posture and colour intensity, spreading fins and orienting broadside to showcase their body markings. Courtship typically takes place in the upper water column or among fine-leaved plants.
The species is most active in its social and courtship behaviours when kept in a group rather than as a pair, and the presence of multiple males encourages natural display. Spawning generally takes place among fine-leaved vegetation or over a substrate of soft plant matter, with pairs briefly coming together to release and fertilise small batches of eggs.
Breeding
Breeding Nannostomus grandis follows the standard pencilfish approach: condition a group of adults on varied live and frozen foods, provide fine-leaved plants such as Java moss or synthetic spawning mops, and allow the fish to spawn over several sessions. The adhesive eggs are small and are distributed through the planting material; adults show no inclination to protect them and will eat eggs if given the opportunity.
Eggs and spawning medium should be removed to a separate, shallow rearing vessel with soft, slightly acidic, warm water to prevent predation. At temperatures around 79–82 °F, hatching occurs within a few days. The tiny fry are initially very small and require infusoria or commercial liquid fry food as first foods before graduating to newly hatched brine shrimp nauplii. Growth is gradual, and juveniles take several months to reach a size at which they can be introduced to a community setting.
In the aquarium
Despite being the largest Nannostomus species, N. grandis is still a small fish by any general measure, and it suits a well-planted aquarium of modest size. A tank of 23.5 in or more provides sufficient swimming space for a group of eight to ten individuals, which is the minimum worth keeping to allow natural social behaviour. The species is peaceful and does best with quiet, similarly sized companions that will not outcompete it at feeding — small tetras, dwarf corydoras, and other Nannostomus species are all suitable.
Water chemistry should err toward the soft and slightly acidic: pH 5.0–7.0 and temperatures of 72–82 °F. The upper range suits breeding; the lower end suits general long-term maintenance. A dark substrate and tannin-stained water from driftwood or dried leaves will encourage the fish to show their colours and reduce shyness. The species appreciates surface cover in the form of floating plants, as it spends much of its time in the upper third of the water column. Like all pencilfish, it is a skilled jumper and the tank must be covered securely.
Conservation
Nannostomus grandis has not been formally assessed by the IUCN Red List, and its conservation status is listed as Not Evaluated (NE). This is a common situation for species described relatively recently from trade material, where the scientific baseline required for a population assessment — range maps, abundance estimates, threat analysis — has not yet been assembled.
The species' origin from the Belém fish-trade pipeline means that wild populations are harvested for the ornamental trade, though the scale and sustainability of that collection is unquantified. Given the broad and partially intact nature of lowland Amazonian habitats in Pará, there is no immediate reason to consider the species threatened, but the absence of an assessment means no formal safeguard is in place. Captive-bred stock is preferable where available, both to reduce wild-collection pressure and to maintain a supply independent of import variation.