Taxonomy & naming
Cichlasoma zarskei was described in 2011 by the Brazilian ichthyologist Felipe P. Ottoni, in the journal Vertebrate Zoology (61(3):335-342), from material collected in the rio Maranhão basin of northern Brazil. The holotype (MNRJ 37576, 3 in SL) was taken at lago Malhada Grande in Maranhão state and is held at the Museu Nacional in Rio de Janeiro. It was the first species of Cichlasoma described since Sven Kullander's foundational 1983 revision, which had cut the genus down from a sprawling assemblage of around a hundred nominal species to a small core of South American acaras built around the type species Cichlasoma bimaculatum; Ottoni's species took that core from twelve valid species to thirteen. The story behind it is one of a population hiding under another name: when Kullander described the eastern-Brazilian Cichlasoma orientale in 1983, he was uncertain how far west its range ran, and material from the rio Itapecuru of Maranhão — a closely allied but distinct form — was eventually recognised by Ottoni as a separate species and named zarskei. The two remain very close: Ottoni's paper provides a revised diagnosis of orientale specifically to separate the two without overlapping characters. Cichlasoma zarskei sits in Cichlasoma sensu stricto, tribe Cichlasomatini of subfamily Cichlinae, and the Catalog of Fishes lists it as valid. The epithet honours the German ichthyologist Axel Zarske, of the Senckenberg Natural History Collections in Dresden.
Morphology
Cichlasoma zarskei is a small acara reaching about 4 in in standard length (the type series ran from roughly 2–4 in SL), with a moderately deep, laterally compressed body — body depth around 45-49% of standard length — and the generalised build typical of the genus. It is closely similar to Cichlasoma orientale in shape, colour pattern and general counts, and Ottoni separated the two on a careful suite of characters. The species and its sister orientale are diagnosed from the rest of the genus by a conspicuous posterior band of the longitudinal (lateral) stripe combined with faintly dotted dorsal, anal and caudal fins; zarskei is then distinguished from orientale itself by fewer pleural ribs (8-9 versus 11), more scales in the E1 series (25-26 versus 23-24), shorter last dorsal- and anal-fin spines, a longer snout and a wider head with a shorter lower jaw, along with osteological details of the entopterygoid and premaxilla. In life and in alcohol the flank is light brown with eight darker trunk bars, a dark or black interrupted longitudinal stripe between the lateral lines, a conspicuous spot at the caudal-fin base and a second at the junction of the stripe and the sixth bar, plus a dark cheek spot — the genus's characteristic markings. The species carries a trisserial (three-row) predorsal scale pattern. Sexual dimorphism is not described in detail in the original account.
Habitat
Cichlasoma zarskei is endemic to the rio Maranhão basin of northern Brazil, in Maranhão state — a drainage of the Brazilian northeast, distinct from the better-known rio Maranhão headwater of the Amazon in Peru. The type series came from floodplain settings: the holotype from lago Malhada Grande, with paratypes from the igarapé (forest stream) Arari in the Arari municipality, indicating a fish of the lakes, streams and connected backwaters of this lowland river system. FishBase classes it as a benthopelagic tropical freshwater fish. These are warm, often seasonal waters of the northeastern Brazilian lowlands, and as a small generalised acara zarskei is the kind of cichlid that thrives in the slower, structured habitats — floodplain lakes such as its type locality, forest streams and quiet river margins with cover — rather than fast open channels. Its restricted, single-basin range makes it one of the more geographically confined members of the genus.
Feeding
No dedicated diet study has been published for Cichlasoma zarskei; FishBase estimates a trophic level of about 3.0 from the species' size and the diets of close relatives, placing it in the omnivore-to-carnivore band typical of the genus, and the carnivore-leaning jaw — premaxilla and dentary bearing rows of conical, caniniform teeth, as Ottoni described — fits a small invertebrate predator. On the pattern of Cichlasoma and the broader cichlasomatine acaras, it is best understood as a generalist benthic feeder, taking small aquatic invertebrates — insect larvae and small crustaceans — together with detritus and some plant material from the substrate and the structured margins and floors of its lakes and streams. Acaras of this kind work the bottom and the leaf litter, sorting edible items from sediment, but are flexible opportunists rather than specialists. In its floodplain community zarskei sits in the middle of the food web, a small invertebrate-leaning omnivore that is itself prey for larger predatory fishes.
Mating
Reproductive behaviour has not been observed for Cichlasoma zarskei, but the genus and the wider cichlasomatine acaras are highly uniform in their breeding biology, and the species can be expected to follow that template. These are pair-bonding, biparental substrate spawners. A male and female form a pair, select and clean a spawning surface — typically a flat stone or a cleared patch of firm substrate — and defend a territory around it together. Outside of breeding small Cichlasoma are relatively peaceable, loosely social fish, but a pair coming into condition becomes territorial around its chosen site. Courtship follows 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 choose their own partners from a group rather than being forced together.
Breeding
On the genus-typical pattern, Cichlasoma zarskei is an open, biparental substrate spawner. The female lays adhesive eggs in rows on a cleaned hard surface, and both parents tend the brood through the standard cichlasomatine division of labour — the female staying close to mouth and fan the eggs, the male patrolling the perimeter and repelling intruders. After hatching the wrigglers are moved to one or more pits dug in the substrate, and once free-swimming the fry are herded and guarded as a school by both parents. Specific clutch counts for zarskei are not published; small Cichlasoma of this size lay on the order of a couple of hundred eggs per spawn. This biparental open-substrate strategy — eggs on a surface, larvae in a pit, a guarded free-swimming shoal — is the reliable acara pattern; the species' breeding has not been documented in detail, and there is no published account of any departure from the genus norm.
In the aquarium
Cichlasoma zarskei is a recently described, range-restricted Brazilian species and is essentially absent from the aquarium hobby; the keeping notes that follow are extrapolated from the genus, several members of which (notably Cichlasoma portalegrense and Cichlasoma dimerus) are familiar, easygoing aquarium acaras. At around 4 in it is a small, manageable cichlid — a pair would be comfortable in a tank of around 25–30 US gal or more, with a longer footprint helping a pair hold a territory at breeding time and letting a group sort out a pairing naturally.
Water should track its warm northeastern-Brazilian floodplain lakes and streams: warm temperatures in the mid-to-upper twenties Celsius and broadly neutral, soft-to-moderate water suit a generalised lowland acara, with stable, clean water and regular partial changes mattering more than precise chemistry. Aquascaping should echo the floodplain-lake and forest-stream habitat — a soft substrate the fish can dig in, driftwood and leaf litter for cover and to soften the water, rockwork for sightline breaks, and at least one flat stone as a potential spawning slate. Feeding is straightforward for an invertebrate-leaning omnivore: a varied diet of small prepared foods alongside live and frozen invertebrates brings out the best condition. On the genus's temperament, expect a comparatively peaceable fish away from spawning that becomes territorial around a nest; robust, similarly sized South American community fish make sensible companions, and raising a group together to let a pair form is the surest route to breeding. For now this remains far more a fish of the rio Maranhão than of the hobby.
Conservation
The IUCN Red List assesses Cichlasoma zarskei as Data Deficient (assessed 7 November 2018) — a reflection of how little is known about it rather than evidence of either security or decline. Described only in 2011 and known from a single drainage, the species has not been studied enough to gauge its population size, trend or the threats it faces, and the Red List accordingly declines to assign it a threat category. It carries no CITES listing. As a narrow, single-basin endemic of the northeastern Brazilian lowlands, it is inherently more exposed to localised habitat change — deforestation, sedimentation, water abstraction and pollution affecting its floodplain lakes and forest streams — than a widespread species would be, which is precisely the kind of situation a Data Deficient listing flags for further study. Until its distribution and population are better understood, its conservation status is best treated as genuinely uncertain, and its outlook tied to the condition of the rio Maranhão basin.