Taxonomy & naming
Krobia petitella was described in 2013 by Sarah E. Steele, Elford Liverpool and Hernán López-Fernández, in the journal Zootaxa (3693(2):152-162), from material collected in the Berbice River drainage of northeastern Guyana. The holotype (UG/CSBD 1662, formerly ROM 86851) was taken in the East Berbice River just upstream of Kwakwani. The genus Krobia is a small Guiana-Shield lineage erected by Sven O. Kullander and Han Nijssen in their 1989 monograph The Cichlids of Surinam to hold 'acara'-type cichlids that did not fit comfortably in the catch-all Aequidens — with Krobia guianensis (Regan, 1905) as the type species and Krobia itanyi (Puyo, 1943) the second original member. The genus is diagnosed by an unusual reach of the lateral band, which runs from behind the eye up toward the base of the dorsal fin rather than straight back along the flank, together with nearly scaleless vertical fins and other osteological details. Within the subfamily Cichlinae, Krobia sits in the tribe Cichlasomatini — the New-World acara assemblage that also includes Aequidens, Bujurquina and Andinoacara. Steele and colleagues' description brought the genus to four valid species: alongside petitella and the two originals, Kullander (2012) had added Krobia xinguensis from the rio Xingu in Brazil. The epithet petitella derives from the Latin petitus, 'reaching out for' or 'inclining towards,' chosen for the way the species' lateral-band spots lie adjacent to the upper lateral line anteriorly and intercept it posteriorly — a subtle feature, but a diagnostic one. The Catalog of Fishes lists the name as valid.
Morphology
Krobia petitella is a small, moderately deep-bodied, laterally compressed cichlid reaching about 4 in in standard length (FishBase) — a typical Krobia size, a little smaller than its better-known relative Krobia itanyi. In the genus's general build it has the rounded, gently rising head profile and compact body of an acara, with the colour pattern dominated by the lateral markings that give the genus and this species their diagnostic characters. The defining feature is the arrangement of the lateral band: in petitella the band spots lie close against the upper lateral line at the front of the body and then drop to intercept the lateral line further back, the 'reaching out and inclining towards' that the name petitus describes — the character that separates it from its congeners. As in other Krobia, the band overall trends up toward the dorsal-fin base rather than running level to the tail, the genus's hallmark, and the vertical (dorsal, anal and caudal) fins are nearly scaleless. Detailed sexual dimorphism is not separately documented for this newly described species; on the genus pattern, males of Krobia tend to grow somewhat larger with longer-pointed dorsal, anal and pelvic fins, while ripe females are rounder-bodied and shorter-finned, but for petitella specifically this should be read as the expected genus pattern rather than a published account.
Habitat
Krobia petitella is endemic to the Berbice River drainage of northeastern Guyana — as far as is known, it occurs nowhere else, making it the most geographically restricted member of its genus. The original description records its habitat in unusual detail, because the type series came with full water-chemistry data. This is a blackwater fish: the Berbice waters it lives in are soft and acidic, with conductivity measured at just 2.0 to 39.0 microsiemens, pH ranging from a strongly acidic 4.2 up to about 7.1, temperatures of roughly 77–91 °F, dissolved oxygen of 5.1 to 6.7 mg/L, gentle current velocities of 0–1 ft/s, and Secchi-disk visibility of only 2–3 ft in the tea-coloured water. FishBase classes it as a benthopelagic tropical freshwater fish. In short, it is a fish of warm, very soft, tannin-stained, slow-flowing shield water — the classic clear-to-blackwater Guiana stream environment, here on the acidic, low-mineral end of that range. The whole of its range lies within the Guiana-Shield freshwater province, a region of ancient rock and a fish fauna rich in local endemics.
Feeding
No diet study has been published for Krobia petitella specifically; FishBase estimates a trophic level of about 3.3 based on the species' size and the diets of close relatives, placing it in the omnivore-to-carnivore band typical of the genus. On the pattern of Krobia and the wider cichlasomatine acaras, it is best understood as a benthic micro-predator and opportunistic omnivore, gleaning small aquatic invertebrates — insect larvae and small crustaceans — from the substrate and leaf litter of its forest streams, and taking some plant and detrital material along the way. Acaras of this kind work the bottom, taking mouthfuls of sediment and sorting out edible items, but they are flexible generalists rather than specialised sifters. In its blackwater habitat petitella occupies the middle of the food web: a small invertebrate predator that is itself prey for the larger fish and other predators of the Berbice.
Mating
Reproductive behaviour has not been observed directly in Krobia petitella, but the genus and the broader cichlasomatine assemblage are remarkably uniform in their breeding biology, and there is every reason to expect petitella to follow the template set by its close relative Krobia itanyi. These are pair-bonding, biparental substrate spawners. A male and female form a pair, select and clean a spawning surface — usually a flat, exposed stone — and defend a territory around it together. Away from breeding the fish are relatively peaceable and loosely social, but as a pair comes into condition they become conspicuously territorial around the chosen site. Courtship runs through the familiar acara repertoire of mutual lateral displays, fin-spreading and quivering and trial passes over the cleaned surface before spawning. As in related acaras, pairs form most reliably when fish are allowed to choose their own partners from a group rather than being forced together.
Breeding
On the genus-typical pattern — established in detail for Krobia itanyi and consistent across the cichlasomatine acaras — Krobia petitella can be expected to be an open, biparental substrate spawner. The female lays adhesive eggs in rows across a cleaned flat surface, and both parents tend the brood through the standard division of labour: the female stays close to the eggs, mouthing and fanning them, while the male patrols the perimeter and drives off intruders. After hatching the wrigglers are moved to one or more pits dug in the substrate, and once the fry are free-swimming the pair leads and guards the school, herding strays back and standing over them. Clutch size has not been published for petitella; congeners lay on the order of a couple of hundred to several hundred eggs. This biparental open-substrate strategy — eggs on a surface, larvae in a pit, a guarded free-swimming shoal — is the reliable acara pattern, and direct breeding accounts of petitella specifically are not yet on record, reflecting how new and how localised the species is.
In the aquarium
Krobia petitella is a recently described, range-restricted Guiana-Shield endemic and is not an established aquarium fish; it appears rarely if at all in the trade, and the keeping notes that follow are extrapolated from its close, well-known relative Krobia itanyi and from the detailed wild water chemistry recorded in the original description. Like other Krobia it is a small, manageable cichlid — at around 4 in a pair would be comfortable in a tank of roughly 30 US gal or more, with a longer footprint helping a pair hold a territory at breeding time.
The one unusually specific thing the description tells us is the water this fish actually lives in, and it is demanding: very soft and often strongly acidic blackwater, with conductivity as low as a few microsiemens and pH dipping to around 4.2. A keeper aiming to do right by petitella would target soft, low-mineral, tannin-stained water on the acidic side — the warm, dark, lightly mineralised conditions of a Guiana blackwater stream — rather than hard tap water, with temperatures in the mid-to-upper twenties Celsius. Aquascaping should echo the home creek: a soft substrate the fish can dig in, leaf litter and driftwood to stain and shade the water, smooth rocks and at least one flat stone as a potential spawning slate, and gentle flow. Feeding is straightforward for an invertebrate-leaning omnivore — small meaty live and frozen foods alongside prepared fare. On the genus's temperament, expect a comparatively peaceable fish away from spawning that becomes territorial and defensive around a nest; robust, similarly sized soft-water community fish make the best companions, and a group raised together to let a pair form naturally is the surest route to breeding. For now, though, this is far more a fish of the Berbice than of the hobby.
Conservation
The IUCN Red List assesses Krobia petitella as Least Concern (assessed 1 November 2023). It is a single-drainage endemic, restricted to the Berbice of northeastern Guyana, which is a comparatively narrow range; but within that drainage it is not flagged as facing a population-level threat, and the assessment rests on the species being reasonably represented across its blackwater habitat. It carries no CITES listing. The general pressures on Guiana-Shield freshwaters form the backdrop against which its status should be read — deforestation and sedimentation, and in particular alluvial gold-mining with its associated mercury contamination and turbidity, which is a real concern in Guyana's interior and would bear especially on a soft-water, clear-to-blackwater specialist. None of these is documented as an active danger to petitella at present, but as a narrow endemic of a single river it is inherently more exposed to localised habitat change than a widespread species, and its conservation outlook is effectively tied to the condition of the Berbice.