Taxonomy & naming
Guianacara oelemariensis was described in 1989 by the Swedish ichthyologist Sven O. Kullander and the Dutch ichthyologist Han Nijssen in their monograph 'The Cichlids of Surinam (Teleostei: Labroidei)', published by E. J. Brill of Leiden — the work that erected the genus Guianacara itself. The description appears on page 126; the holotype is held in the Royal Belgian Institute of Natural Sciences in Brussels under register ISNB 750, collected in a tributary of the upper Oelemari River in the Marowijne District of Suriname. Kullander and Nijssen built the new genus to hold three Surinamese 'bandit acaras' that had long been shuffled among the catch-all genus Aequidens but were clearly something else; they argued the group was not related to Aequidens at all, but to Acarichthys and other Geophagus-like eartheaters. Within Guianacara they recognised that oelemariensis stood apart from its congeners — it has a single supraneural bone rather than two, its anterior dorsal-fin spines lack the produced lappets seen in the others, and it shows no midlateral bar, only a midlateral spot — and on those characters they placed it alone in the subgenus Oelemaria, leaving the remaining species in the nominal subgenus Guianacara. That arrangement still stands: in their 2006 revision of the genus, López-Fernández, Taphorn and Kullander retained oelemariensis as the sole species of subgenus Oelemaria. The genus sits in the Neotropical cichlid subfamily Geophaginae; Kullander's 1998 morphological phylogeny placed Guianacara as the sister of Acarichthys in a tribe Acarichthyini, a relationship since broadly supported by molecular and combined analyses (Farías et al.; López-Fernández et al., 2005).
Morphology
This is a small, deep-bodied cichlid: FishBase records a maximum of about 3 in standard length in males and roughly 3 in in females, making it one of the more diminutive Guianacara. The body is laterally compressed with a steeply rising forehead, the typical eartheater silhouette in miniature. Ground colour is a yellowish to greyish beige that can flush with iridescent bluish flecking on the flanks, brightening considerably in dominant or breeding fish. Two dark markings define the genus and frame this species in particular. The first is the 'bandit' mask — a black supraorbital and infraorbital stripe running through and below the eye across the cheek — that gives Guianacara their common name. The second is the flank marking, and here oelemariensis breaks from its relatives: instead of the broad vertical midlateral bar or 'saddle' blotch carried by Guianacara owroewefi, Guianacara sphenozona, Guianacara geayi and the Venezuelan species, it shows only a discrete, roughly rectangular black spot centred on the flank, with no continuous bar developing even in adults. This single difference — together with the reduced one supraneural (rather than two) and the absence of produced lappets on the front dorsal spines — is the diagnostic signature of the subgenus Oelemaria. The dorsal fin is dusky with rows of small pale spots on its soft portion. Sexual dimorphism is modest and follows the genus pattern: females stay somewhat smaller and look noticeably daintier, while males grow larger and heavier-bodied, with more prolonged, pointed dorsal and anal fins that carry a greater number of light spots than the female's, and with the largest males appearing notably beefier overall. The sexes are otherwise close to isomorphic and colour differences are slight and mood-dependent.
Habitat
Guianacara oelemariensis appears to be endemic to the Oelemari River, a headwater tributary of the Marowijne (Maroni) River drainage in the Marowijne District of southeastern Suriname, on the eastern flank of the Guiana Shield. The Oelemari forms part of the border region with French Guiana, and aquarium fish photographed under the name have been taken on the French Guianese side of this same upper-Marowijne system. The species is one of two Guianacara found together in the Oelemari: Kullander and Nijssen recorded it as sympatric there with Guianacara owroewefi, an unusual overlap in a genus whose species are otherwise almost entirely allopatric, each confined to its own river. Specific in-situ water chemistry for oelemariensis has not been published, and FishBase simply notes that it 'inhabits rivers' and is likely to occur in the same biotopes as its congeners. Those biotopes are well characterised across the genus: Guianacara are fishes of clear, soft, slightly acidic water flowing over a substrate of large rocks intermixed with sandy patches, typically in smaller rivers, creeks and the faster-flowing reaches and rapids of larger ones. Field workers describe the genus as ecologically conservative — strikingly uniform in habitat preference from one species to the next — so the rocky, sand-floored clearwater rapids of the Oelemari are almost certainly where this fish lives. As a small shield-stream cichlid it is a shallow-water animal, occurring in the well-lit, current-swept riffle and run zones rather than in deep pools.
Feeding
No dedicated dietary study of Guianacara oelemariensis has been published, and FishBase carries an estimated trophic level of about 3.3 — the value typical of a small omnivore that takes a meaningful share of animal prey. Across the genus, Guianacara are benthic foragers of the eartheater lineage: they work the sand and fine gravel between the rocks of their stream beds, taking small invertebrates, insect larvae, and the organic particles and micro-organisms bound up in the substrate, and supplementing this with bottom-growing algae and plant matter. In aquaria the genus is described as readily omnivorous, accepting live, frozen and prepared foods and occasionally browsing vegetable matter such as spirulina or blanched greens — behaviour consistent with a generalist that grazes the productive rock-and-sand interface of a clearwater shield stream. Within its community the species occupies the role of a small benthic invertebrate-and-detritus feeder of the riffle zone, the same niche filled by Guianacara throughout the eastern Guianas.
Mating
The reproductive biology of Guianacara oelemariensis specifically has not been documented in the wild or in print, but the genus is consistent enough that its pattern can be described with confidence. Guianacara form monogamous pairs and are biparental substrate spawners. Outside breeding they are loosely social and, despite a reputation for fairly high intraspecific aggression and territoriality, are basically peaceable fish whose disputes rarely end in injury. As a pair forms, the two fish select and clean a spawning site — a cleft beneath a rock or a cave-like recess — and a division of labour sets in that holds through the whole cycle: the female concentrates on the eggs and brood while the male takes the lead in defending the territory against intruders. Courtship is comparatively brief, with intensive cleaning of the chosen surface preceding the spawning itself. This female-tends, male-guards arrangement, common to the genus, is the template that almost certainly governs pairing and spawning in the Oelemari population.
Breeding
Guianacara oelemariensis has not been confirmed bred and reported under its own name, but there is one tantalising near-miss in the record. In 1991 the aquarist Stalsberg wrote up a spawning of Guianacara imported and kept under the name oelemariensis, though Weidner is careful to flag that the identification was never positively confirmed — the accompanying photographs, taken at the Río Caciporé in Amapá, Brazil (a river far from the Oelemari and outside any confirmed Guianacara oelemariensis record), in fact appear to show the more common Guianacara geayi rather than this species. Treated with that caveat, Stalsberg's account is nonetheless the most detailed breeding record associated with the name: his pair produced roughly 200-300 light grey, opaque eggs, spawned at a total hardness of 2 °dGH, a pH of 5.2, and a temperature of 79 °F — about as soft and acidic as aquarium water gets. The larvae hatched after 24 hours and were moved to a pre-dug pit; a further six days on, the fry became free-swimming and were immediately able to take Bosmina and Cyclops nauplii without difficulty. Stalsberg noted the young were not particularly fast-growing. If this pair really was oelemariensis, its breeding biology tracks the rest of the genus closely: Guianacara are biparental cave or cleft spawners, and congeners such as Guianacara sphenozona typically lay on the order of one to a few hundred eggs, hatching in roughly six days and reaching free-swimming after a further five or so, with the pair then leading and guarding a mobile school of fry large enough at first feeding to take newly hatched brine shrimp — a sequence Stalsberg's fish essentially match. Until a specimen of confirmed identity is bred and documented, though, this remains the best available account rather than a settled one, and the triggers that bring the genus into breeding condition generally are those of its clear, soft, slightly acidic stream habitat — warm temperatures, soft water and a rich diet of small live and frozen foods.
In the aquarium
Guianacara oelemariensis is essentially never seen in the aquarium trade under a confirmed identification — its tiny native range in the remote upper Marowijne keeps it out of commercial collecting, and fish sold as Guianacara almost always belong to the more widespread and more colourful congeners. A handful of trade fish have circulated under the oelemariensis label (Stalsberg's 1991 stock among them), but as noted under Breeding, the identification behind these imports has never been positively confirmed and may in fact be Guianacara geayi. Nonetheless, the genus as a whole is well established in the hobby and undemanding, and a keeper who did obtain genuine oelemariensis could maintain it on the standard Guianacara regime. A single pair needs a footprint of at least around 47 in in length and on the order of 40 US gal or more; for keeping several together, or for reducing the intraspecific friction that runs fairly high in this genus, a larger tank of 80 US gal or so is better. The aquascape should reproduce the rocky-sandy clearwater stream the fish comes from: a sand or fine-gravel substrate (the fish dig, so plants are best potted or robust), with larger rocks, roots and driftwood arranged to create caves and broken sightlines that double as territories and spawning sites. Water should be warm, soft and close to neutral or slightly acidic — temperatures of about 25-30 degrees C, pH in the range 6.0-7.0, and modest hardness (roughly 1-10 dGH) — matching the soft, slightly acidic shield-stream water the genus inhabits; the temperature should not be allowed to drop much below 25 degrees C. Feeding is easy: a varied diet of live and frozen foods (bloodworm, brine shrimp, small crustaceans), good-quality dried foods, and occasional vegetable matter keeps the fish in colour and condition. For tankmates, peaceful mid- and upper-water species from the same soft, warm-water community — tetras, hatchetfish, pencilfish, peaceful catfish and the like — suit it well; the fish do not harm plants beyond uprooting them while digging. The main husbandry watch-point is intraspecific: pairs and rivals can be territorial, especially around spawning, so adequate space and a broken-up layout that lets subordinate fish retreat are the keys to keeping a group settled.
Conservation
The IUCN Red List assesses Guianacara oelemariensis as Least Concern. The assessment (Red List ID 167698) was carried out on 1 March 2007 by S. O. Kullander and H. Nijssen — the same specialists who described the species — and remains the current listing. The Least Concern category reflects an absence of identified major threats rather than abundance data: the fish is naturally rare in collections simply because its range is so small and remote. That narrow distribution is itself the species' chief vulnerability. As a probable single-river endemic of the upper Oelemari, it has no geographic insurance — a localized disturbance to that one clearwater system would affect the entire known population. The principal pressures on Guiana-Shield headwater streams are habitat-scale: small-scale and artisanal gold mining (and the associated mercury contamination and sediment loading), deforestation in the catchment, and changes to water clarity and flow, all of which bear directly on the clear, soft, fast-flowing rock-and-sand biotope this cichlid depends on. Much of the upper Marowijne basin remains forested and sparsely populated, which has so far worked in the species' favour, but for a fish this geographically confined the long-term outlook is tied closely to the health of that single river.