Taxonomy & naming
Albert Günther named this fish in 1860 as Chromis rivulata, in his "Second list of cold-blooded vertebrata collected by Mr. Fraser in the Andes of western Ecuador" (Proceedings of the Zoological Society of London). The type material came from the Pacific slope of western Ecuador. Over the next century and a half it moved through several genera — Acara, then for decades Aequidens — until Zuzana Musilová, Oldřich Říčan and Jan Novák erected the genus Andinoacara in 2009 for a clade of trans- and cis-Andean cichlids. The name fuses andino- (the Andes, across whose slopes and valleys the genus is spread) with acará, the Tupí-Guaraní word for cichlids. The valid combination today is therefore Andinoacara rivulatus (Günther, 1860), in the subfamily Cichlinae (the Cichlasomatini of older usage).
The synonymy is unusually tangled and was only settled in the modern revision by Nicklas Wijkmark, Sven O. Kullander and Ramiro E. Barriga Salazar (2012), who redescribed Andinoacara rivulatus on specimens from the Guayas, Tumbes and Zarumilla drainages. They showed that Aequidens azurifer Fowler, 1911 is a junior synonym, and that the syntypes of Acara aequinoctialis Regan, 1905 are the same specimens as those of rivulatus — designating a common lectotype made aequinoctialis a junior objective synonym. Chromis rivulata Günther, 1860 is the original combination, and the catalogues (FishBase, Eschmeyer's Catalog of Fishes, IUCN) list Acara aequinoctialis, Aequidens azurifer, Aequidens rivulatus and Andinoacara rivulata as synonyms.
At the heart of the confusion is the hobby name "green terror," applied to at least three fish. As recounted by Stawikowski & Werner and on the andinoacara.com database, the story runs like this. In the early 1970s a white-edged ("Silbersaum") cichlid reached Germany from Peru and was called Aequidens rivulatus; soon a gold/orange-edged ("Goldsaum") fish turned up and was thought to be undescribed. When Werner and Stawikowski collected at Günther's Ecuadorian type locality in 1985, they found only the Goldsaum form — proof that the gold-edged Ecuadorian fish is the true Andinoacara rivulatus and that the white-edged Peruvian "Silbersaum" was something else. That something else was named in 2009 by Musilová, Schindler & Staeck as Andinoacara stalsbergi (for the Norwegian aquarist Alf Stalsberg, 1948–2022, who collected the holotype), restricted to Pacific coastal rivers of Peru south of the Tumbes. The same revision split off a third, closely allied fish from the río Esmeraldas basin of northern Ecuador in 2012 as Andinoacara blombergi (Wijkmark, Kullander & Barriga). The practical upshot: rivulatus in the strict sense is the gold/orange-saum fish of the Guayas and adjacent Ecuadorian and extreme-northern-Peruvian drainages, while the popular white-saum "green terror" of Peru is Andinoacara stalsbergi. To muddle things further, several hobby sources — andinoacara.com among them — argue that "green terror" is best reserved for stalsbergi and prefer "gold saum" or "orange saum" for rivulatus, yet FishBase, the IUCN and most of the trade still pin "green terror" on rivulatus. We keep "Green terror" and "Gold saum" as the established aliases here while flagging that the name is shared with stalsbergi; a fish sold as a "green terror" should always be checked against its drainage and saum colour before it is given a name.
Morphology
This is a robust, deep-bodied, laterally compressed cichlid with a steep forehead, a large head, and the conical teeth and interrupted (two-part) lateral line typical of the cichlasomatines. Reported sizes vary widely with how the fish is measured and whether wild or aquarium animals are meant. The lectotype designated by Wijkmark et al. (2012) is only about 3 in standard length (around 4 in total length); the largest preserved specimen those authors examined was 5 in SL, while Eigenmann (1922) recorded a museum fish of about 8 in total length. Field and aquaculture workers in Ecuador cite roughly 12 in as a maximum, and FishBase gives 8 in TL as its catalogued maximum, with aquarium males well known to reach the upper part of that range. In short, most wild adults fall in the 4.5–10 in range, and exceptional — usually tank-grown, well-fed — males approach 12 in.
The body is metallic green to blue-green, its scales catching electric-blue and turquoise highlights, and the head and cheeks carry the "oblique, waving, bluish, shining silvery streaks" that Günther singled out in the name. The dorsal and caudal fins have a coloured margin — the "saum" — which in true rivulatus is a broad band, typically orange, gold or red (sometimes white in younger fish, often warming to orange with maturity) and conspicuously wider than the thin white edge of Andinoacara stalsbergi. A useful field character separates the two: in rivulatus each scale has a lighter blue-green edge around a darker centre, whereas in stalsbergi the scale has a dark centre with a dark edge. Diagnostic counts from western-Ecuadorian material include roughly 14 dorsal-fin spines and about 11 soft rays in a long, continuous dorsal fin, and 24 scales in the E1 lateral scale row (versus 25 in the río Esmeraldas congener Andinoacara blombergi). Sexual dimorphism is pronounced: males grow larger and longer-finned and develop a fleshy cephalic hump — a nuchal bump of subcutaneous adipose tissue — on the forehead, the classic mark of a dominant breeding male, while females stay smaller and more rounded in the fins.
Habitat
Andinoacara rivulatus lives in the Pacific (trans-Andean) drainages of western Ecuador and the extreme northwest of Peru. FishBase summarises the range as the Pacific slope "from the Esmeraldas River in Ecuador to the Tumbes River in Peru," but the modern revisions tighten it: the Esmeraldas-basin fish is now Andinoacara blombergi and the drainages south of the Tumbes belong to Andinoacara stalsbergi, so rivulatus in the strict sense runs through the Guayas, Santa Rosa, Zarumilla and Tumbes river systems, with the great Guayas basin as its heartland. The IUCN assessment gives an altitudinal range of roughly 5 to 1,656 ft above sea level, and the fish is common in both coastal lowland rivers and premontane reaches. Beyond its native range, ornamental releases have established populations in Mexico, in Java in Indonesia, and even on the Amazonian side of the Ecuadorian Andes in the Zamora drainage — the last traced by molecular work to a recent west-Ecuadorian introduction.
Its biotope is small and large coastal streams and the main channels and pools of premontane rivers: flowing, structured water rather than still ponds. Recorded water chemistry is near-neutral to alkaline, FishBase giving a pH range of about 6.5–8.0, a tropical climate and a quoted 68–75 °F band, while the hobby keeps it warm at roughly 72–77 °F. Hardness data are thin — aquarium guidance favours soft to moderately hard water, a few degrees dGH, consistent with FishBase's open-ended dH range up to about 25. The Guayas basin it dominates is a heavily worked landscape of banana, rice and sugar agriculture, cattle and major urban centres, and rivulatus is a tolerant generalist that persists across most of it.
Feeding
The green terror is a benthopelagic omnivore that sits fairly high in its food web — FishBase puts its trophic level near 3.3 — reflecting a diet that mixes animal and plant matter but leans carnivorous. Ecuadorian workers, who know it as the "vieja azul," describe an ecologically important species that takes small fishes and small invertebrates, picking and foraging over the river bottom and through submerged structure. Its catalogued foods run to aquatic insects and their larvae, crustaceans, worms, small fishes and plant material — the broad opportunism that lets it thrive in disturbed and variable rivers. In aquaculture and the aquarium that same generalist appetite makes it easy to feed on prepared pellets supplemented with live and frozen invertebrates and some vegetable matter; keepers are warned off heavy red-meat diets, which the fish never meets in the wild. As a comparatively large, abundant predator-omnivore in coastal Ecuadorian rivers, it is both a food fish for rural communities and a meaningful consumer of smaller fishes and benthic fauna.
Mating
Andinoacara rivulatus is a monogamous, pair-bonding cichlid, with one male and one female holding a lasting bond through a breeding cycle. As in its convict-cichlid relatives, the larger, more colourful male holds and defends a territory, and the bond firms up around a chosen spawning site on a hard surface. A male in breeding condition deepens in colour, flares his gold-edged fins and develops the pronounced nuchal hump that signals dominance; as spawning approaches, pairs turn markedly territorial and aggressive toward their own kind and other fishes alike — the behaviour behind the "terror" in the common name. Detailed wild courtship has not been documented to the depth available for some Central American cichlids, but FishBase, the Cichlid Room Companion and aquarium breeders agree on the picture: a classic substrate-spawning pair that divides labour, the female concentrating on the clutch while the male patrols and defends the surrounding ground. Pairs are best given room, and the older the fish, the harder they hold it.
Breeding
This is a biparental, open/substrate spawner. The pair cleans a hard surface — a flat rock, a slab, a root, or in aquaria a chosen stone or pot — and the female lays her eggs on it, where the male fertilises them externally. FishBase records clutches of up to about 600 eggs on hard substrate, the female tending eggs and larvae while the male defends the territory. Early development has been described in detail for cultured fish by Moshayedi and colleagues (2017): exogenous feeding — the fry beginning to take food from outside — starts at about three days post-hatch, and the young come to resemble small juveniles by roughly 28 days post-hatch. Parents then shepherd the free-swimming fry as a guarded school, both adults driving off intruders, the cooperative and fiercely defensive brood care that makes the species both rewarding and demanding. Spawning is not tied to a narrow season in this aseasonal tropical climate, and in good conditions pairs will breed again and again.
In the aquarium
The green terror has been a fixture in the cichlid hobby since the early 1970s — widely bred, reliably available, and popular for its metallic colour and conspicuous personality — but keepers who underestimate its adult size and aggression routinely regret it.
Tank. An adult male in typical home conditions grows to 8–10 in and can push toward 11–12 in; females are smaller but still reach 6–7 in. A single specimen or a confirmed pair needs a minimum of around 105 US gal, both to satisfy its territoriality and to dilute the load a heavy-bodied, strong-feeding cichlid produces. Longer beats taller — aim for a footprint of at least 59 in. Use a deep layer of fine to medium gravel or sand, because the fish digs: pre-spawning pairs excavate real craters and rearrange anything not anchored to the glass. Large, stable rockwork and heavy driftwood divide the territory, and flat-topped rocks or slate slabs make the preferred egg-laying surface. Tough, fast-growing plants (Vallisneria, potted Amazon swords) can work, but loosely rooted ones get uprooted.
Water. The Pacific-slope rivers it comes from are neutral to slightly alkaline and moderately hard, matching the documented pH 6.5–8.0 and 68–77 °F. In practice it does well at pH 7.0–7.8 and 5–20 dGH, so most neutral tap water suits it. It is less at home in soft, acidic water; kept below pH 6.5 long-term it can fade in colour and condition. It tolerates brief warmth toward 82 °F but not sustained, and sustained cold below 68 °F stresses it. Strong biological and mechanical filtration plus generous 30–40% weekly water changes handle the heavy bioload of a fish that eats hard and reaches 10 in.
Tankmates. Even outside breeding it will intimidate, chase and injure fish it reads as rivals, and intraspecific aggression is the worst of it: two males without space and visual barriers fight severely, and an unbonded male will persecute an unpaired female to exhaustion. Only robust fish of comparable or greater size in different zones — large South or Central American cichlids, big catfish, large plecostomus — are realistic, and even these need watching; small or timid fish are simply prey. Many experienced keepers run it as a single specimen, a bonded pair, or a species-only tank.
Buying. A shop "green terror" can be true Andinoacara rivulatus (broad orange or gold saum), Andinoacara stalsbergi (thin white saum), or a juvenile of either whose margin has not yet coloured up. Check the fin margin under good light, but note that juveniles at 1–2.5 in are ambiguous since the saum develops with age — provenance from a breeder who tracks origin is the surest guide. Andinoacara stalsbergi tends to come from cooler, softer Peruvian rivers and may be marginally more aggressive; keep the two apart, as they hybridise.
Verdict. In species-appropriate conditions this is a straightforward fish — hardy, disease-tolerant, unfussy about food and easy to breed. The difficulty is social and spatial, not delicate care: the tank must be genuinely large, the aggression is genuinely high, and pairing is genuinely demanding. For a keeper ready for those realities it is a rewarding long-term fish and a reliable spawner with fascinating biparental care — but it is not for beginners or mixed community aquaria.
Conservation
The IUCN Red List assesses Andinoacara rivulatus as Least Concern (assessed 14 September 2020; assessment by T. J. Lyons for the ABQ BioPark, published 2021). Under its revised concept the species ranges across the Guayas, Santa Rosa, Zarumilla and Tumbes drainages of Pacific Ecuador and northern Peru, with an extent of occurrence of about 49,299 mi²; the population trend is listed as unknown, but there is no evidence of a major range-wide decline and the fish remains common in coastal and premontane rivers. It carries no CITES listing.
The pressures on it are nonetheless real and pervasive. The Guayas basin — its core — supports roughly 5.5 million people and much of Ecuador's industrial agriculture, including a large share of the country's bananas and rice, which brings heavy pesticide and nutrient pollution, deforestation, dams and reservoirs, river-bank modification, intensive cattle grazing and expanding urban effluent. In the Puyango-Tumbes basin, artisanal gold mining around Portovelo discharges mercury-laden wastewater straight into the watershed, and the Tumbes delta has been extensively converted to shrimp farming. Introduced African tilapias compete with native fishes across the Pacific slope. The assessors judged that these stresses are driving continuing declines in habitat quality, but that their specific impact on rivulatus has not been quantified and the species shows no clear population crash — hence Least Concern rather than a threatened category. It is taken in subsistence and artisanal fisheries and is heavily cultured for food and the ornamental trade, and Ecuador has introduced seasonal fishing closures and a minimum harvest size on parts of the Guayas to protect food fishes, this one among them.