The green terror, is a colorful freshwater fish in the cichlidfamily. The fish originates from the Pacific side of South America from the Tumbes River in Peru to the Esmeraldas River in Ecuador. It is polymorphic and can have white or gold-orange edging to the tail and dorsal fins. It has historically been confused with two other species that always have narrow, clearly defined white edging, the more southernly distributed A. stalsbergi and the more northernly A. blombergi. Males of A. rivulatus grow larger than females and may reach lengths of up to. Females are sexually mature at around. As the name implies, late juvenile-phase and adult-phase specimens can be very aggressive. This is not guaranteed, however, and peaceful individuals can be observed. Juveniles of A. rivulatus are often sold in aquarium stores. This species is not to be confused with the blue acara which is similar in appearance; the blue acara is not as aggressive and does not grow as large as the gold saum. The fish is somewhat deep-bodied, possessing a prominent forehead. Adult males develop a pronounced forehead hump, composed offatty tissue. Juveniles are tan colored with silver-blue flecks and lack the bright iridescent blue, green, and orange coloration and long, flowing fins of adult specimens. In nature, A. rivulatus lives in a tropical climate and prefers water with a 6.5–8.0 pH, a water hardness of 25.0 dGH, and a temperature range of.
In the aquarium
A. rivulatus is a popular fish in the aquarium trade and is noted for its hardiness, as well as its aesthetic appeal. The fish may grow up to 30 cm in length, though 15 to 20 cm lengths are more typical, often depending on the size of the habitat. A. rivulatus is an inherently aggressive fish. With the exception of fully mature P. dovii and P. managuense, which rarely cohabit peacefully, A. rivulatus is cunning enough to survive—and even thrive—with larger cichlids. However, as a rule, a preying cichlid is capable of consuming any animal whose dimensions are inferior to that of the cichlid's fully expanded jaw. A. rivulatus of an appropriate size can often be assimilated with R. octofasciata, A. citrinellus, T. meeki, flowerhorns, smaller P. managuense and P. dovii, and other similarly aggressive cichlids.
Role in hormone research
A. rivulatus has also been used as a model for the effects of testosterone on lateralization.
