Taxonomy & naming
The biajaca was described by the French ichthyologist Achille Valenciennes in 1831 as Centrarchus tetracanthus, on material from Havana, Cuba — he placed it, mistakenly, in a North American sunfish genus. The epithet reflects that error: tetracanthus means 'four-spined', because Centrarchus sunfishes carry seven or eight anal-fin spines while this fish has only four. In 1862 Theodore Gill erected the genus Nandopsis for it, the name combining the leaffish genus Nandus with the Greek opsis ('appearance'); Gill apparently saw a superficial resemblance between the Cuban fish and the Asian leaffishes (family Nandidae), then thought to be cichlid relatives — a likeness later authors have struggled to see. For much of the twentieth century the Antillean species were shuffled through Cichlasoma and 'Cichlasoma' as that vast wastebasket genus was dismantled, and Nandopsis tetracanthus is still encountered in the older literature under those names. Nandopsis is now restricted, in its strict sense, to the three living Antillean endemics — tetracanthus and ramsdeni of Cuba and haitiensis of Hispaniola — plus the fossil Nandopsis woodringi from the late Oligocene of Haiti. Chakrabarty's 2006 molecular study (Molecular Phylogenetics and Evolution) confirmed Nandopsis as a phylogenetically and geographically coherent group, the only cichlid lineage endemic to the Greater Antilles, and the Catalog of Fishes treats Nandopsis tetracanthus (Valenciennes 1831) as valid with type locality Havana.
Morphology
Nandopsis tetracanthus is a robust, deep-bodied heroine cichlid. Reported maximum size is about 8 in SL in FishBase, with aquarium and field reports of roughly 10–12 in total length for large males; females stay somewhat smaller. The ground colour is a clean white to pale beige or pearl-grey, overlaid with a striking pattern of rounded black blotches scattered across the flanks and converging into broken bars — a bold, almost dalmatian-like livery that makes the species unmistakable among Caribbean fishes. A dark mid-lateral blotch and a spot on the caudal peduncle are usually present. Fins are long and pointed in mature fish, and dominant males develop the heavy forehead typical of breeding heroine cichlids. Sexual dimorphism is modest: males grow larger, longer-finned and more nuchally humped, while ripe females tend to show somewhat heavier markings and faint vertical barring and darken markedly when in spawning condition. The four anal-fin spines that gave the species its name remain a reliable meristic feature.
Habitat
Nandopsis tetracanthus is endemic to Cuba and is the only cichlid distributed across essentially the whole island — a remarkable thing for a strictly freshwater fish on an oceanic island. It is a habitat generalist, recorded in fast-flowing rivers, slower lowland streams, natural lakes and man-made reservoirs, and the brackish mouths of rivers and coastal lagoons; FishBase classes it as a freshwater and brackish, benthopelagic species that favours stretches with aquatic vegetation. Reported water temperatures span roughly 75–86 °F, and the species tolerates a wide chemical envelope from soft, near-neutral inland water (pH around 6) to harder, alkaline and salt-influenced conditions near the coast (pH to about 8). This breadth of tolerance — including genuine euryhalinity — is itself a clue to the genus's biogeographic history: an ability to enter brackish and even near-marine water is exactly the trait that would let a 'freshwater' cichlid disperse between, or persist on, islands. Within Cuba the biajaca occurs from western drainages near Havana across to the east, making it the default native cichlid of the island's inland waters.
Feeding
The biajaca is a generalist carnivore and opportunistic predator. Wild fish take worms, aquatic insects and their larvae, shrimps and other crustaceans, and small fish, hunting among vegetation and over the bottom; larger individuals are capable predators of anything they can swallow. FishBase places it low-to-mid on the food web as a benthopelagic feeder of invertebrates and small vertebrates. In the relatively species-poor freshwater fauna of Cuba — where cichlids did not radiate into the dozens of specialist niches seen on the African Rift lakes — Nandopsis fills the role of a mid-sized predatory cichlid, a generalist that exploits whatever invertebrate and small-fish prey its varied habitats provide rather than specialising on any single resource.
Mating
Like other heroine cichlids, Nandopsis tetracanthus forms monogamous pairs that jointly hold and defend a breeding territory, and pairing is preceded by the colour change, jaw-locking and lateral displays typical of the group. Both sexes intensify in pattern as they come into condition, the female often darkening dramatically. Pair formation can be fraught: these are large, powerfully aggressive fish, and an unready or mismatched partner may be attacked. In the wild, breeding pairs establish themselves over firm substrate or near cover during the warmer, wetter part of the year, the male defending an outer territory while the female focuses on the immediate nest site — the standard division of labour for a substrate-spawning cichlid.
Breeding
Nandopsis tetracanthus is a biparental substrate spawner. Eggs are laid on a cleaned hard surface — a rock, root or, in aquaria, a flat stone, slate or the inner wall of a flowerpot — and the species will readily use a cave or burrow as a spawning site. Clutches are large: aquarium accounts report roughly 300–400 eggs from an established female, with up to about 600 from very large, experienced fish (FishBase gives a maximum near 600). At around 81–82 °F the eggs hatch in roughly two days, and the female tends and fans them while the male patrols and defends the wider territory; the pair then guard the mobile fry through the wriggler and free-swimming stages. As in its Hispaniolan congener, parental aggression is intense, and the female does not hesitate to drive off — or attack — her own mate if he intrudes on the nest. The fry grow steadily on small live and prepared foods once free-swimming.
In the aquarium
The Cuban cichlid has a long, if specialist, history in the hobby and is the most frequently kept Nandopsis, but it is a big, boisterous, predatory fish for experienced keepers rather than a community species. A single adult or a compatible pair needs a large tank — on the order of 105 US gal or more, with a footprint long enough that a defending pair and any tankmates can keep their distance; bigger is always better for managing the species' territoriality.
Water chemistry is undemanding by cichlid standards, reflecting the fish's broad wild tolerance: temperature 75–86 °F, pH anywhere from about 6.5 to 8.0, and soft to moderately hard water are all acceptable, with stability mattering more than precise values. Good filtration and regular water changes are important because these are large, messy carnivores that produce a heavy waste load. A substrate of sand and a layout of robust rockwork, flat stones and a cave or flowerpot give the fish places to claim and a surface to spawn on; delicate plants will be uprooted, so any greenery should be tough and well anchored.
Diet is straightforward: a varied carnivore menu of frozen and live foods — mysis, krill, shrimp, mussel, earthworms, larger insect larvae — alongside good-quality sinking cichlid pellets, with some plant-based food (spirulina) to round it out. Avoid feeder fish, which add little nutritionally and risk disease.
The real challenge is temperament. Nandopsis tetracanthus is territorial and predatory; it will eat tankmates small enough to fit in its mouth and harass those that intrude on its space, especially when breeding. It is best kept as a single specimen, as a bonded pair, or with other large, robust cichlids and tough dither fish in a very spacious tank — never with small or timid companions. Many keepers maintain it in a species set-up, which also suits its conservation value as a regional endemic worth keeping pure.
Conservation
The IUCN Red List assesses Nandopsis tetracanthus as Least Concern (assessed 7 April 2020 by T.J. Lyons and S. Rodríguez-Machado, published 2021), on the strength of its wide distribution across Cuba, its broad habitat tolerance and its apparently stable, locally abundant populations. It is one of the few Antillean freshwater fishes not currently of conservation concern — a sharp contrast with its Cuban congener Nandopsis ramsdeni, a narrow eastern endemic listed as Endangered. The biajaca is harvested locally for food and enters the aquarium trade, but neither is flagged as a population-level threat. Its long-term security is nonetheless tied to that of Cuba's freshwater ecosystems, which face the familiar island pressures of pollution, water extraction, habitat alteration and introduced species. As the most widespread member of a genus found nowhere else on Earth, Nandopsis tetracanthus is a flagship for the singular, irreplaceable freshwater fauna of the Greater Antilles.