Taxonomy & naming
Mimagoniates microlepis was originally described by Franz Steindachner in 1877 as Paragoniates microlepis, and the species passed through several generic reassignments over the following century before settling in Mimagoniates. FishBase still maintains the synonym Coelurichthys microlepis, under which the species occasionally appears in older aquarium literature. Eschmeyer's Catalog of Fishes treats Mimagoniates microlepis (Steindachner, 1877) as the valid name; the parenthetical authority reflects the move from the original genus.
Mimagoniates is a small genus within the subfamily Stevardiinae of the family Characidae. The stevardiin clade is defined in part by the remarkable internal fertilisation strategy shared by its members — sperm transfer via a modified anal fin in males — placing Mimagoniates in a biologically distinct group among the New World characids. The genus name alludes to resemblance to Goniates, a related form; the specific epithet microlepis is Latin for 'small-scaled', referring to the relatively small body scales that give the species a delicate appearance.
Morphology
The croaking tetra is a slender, fusiform fish reaching up to about 2.5 in standard length, though aquarium specimens are typically 2–3 in total length. The body is translucent to pale silver with a faint bluish or greenish iridescence along the flanks, and a narrow, often iridescent lateral stripe runs from behind the gill cover toward the base of the caudal fin. The fins are mostly clear or faintly yellowish.
Sexual dimorphism is present but subtle: males are more slender and somewhat smaller than females, and they possess the sexually modified anal fin characteristic of Stevardiinae — a structure that facilitates internal fertilisation. Males also possess pheromone-releasing caudal glands used in courtship display. Females can appear noticeably plumper when carrying developing eggs or retained sperm. Neither sex develops the intense colour of many popular aquarium tetras; the croaking tetra's appeal is behavioural rather than chromatic.
Habitat
Mimagoniates microlepis inhabits freshwater streams and small rivers along the Atlantic Forest coast of southern Brazil, ranging from coastal drainages in southern Bahia south through Espírito Santo, Rio de Janeiro, São Paulo, Paraná, Santa Catarina, and Rio Grande do Sul, with additional records from the upper Iguaçu drainage. These are typically clear, cool, well-oxygenated streams flowing through or near remnant Atlantic Forest.
The species is benthopelagic in habit, occupying the water column above stream substrates in shaded, often leaf-littered environments. The IUCN assessment notes that while the species is widespread and abundant in preserved areas of São Paulo, Rio de Janeiro, and Paraná, habitat degradation from agriculture and urbanisation has reduced its presence in Espírito Santo and Bahia. Water temperatures in these streams range from roughly 64–73 °F (64–73 °F), making this one of the cooler-water tetras available to hobbyists.
Feeding
Mimagoniates microlepis is an omnivore in the wild, feeding opportunistically on small invertebrates, insect larvae, and organic matter available in its stream environment. In captivity it is undemanding and readily accepts a wide range of standard aquarium foods, including quality flake, small pellets, and frozen or live foods such as Daphnia, Artemia nauplii, and bloodworm.
As a small, active fish it feeds most naturally in frequent small amounts rather than one large daily meal. Varied feeding that includes live or frozen invertebrate prey helps maintain condition and may stimulate the courtship displays that make this species so interesting to observe. Competitive feeding is rarely a concern in a conspecific group, though the fish may be outcompeted by larger or more boisterous tankmates.
Mating
The croaking tetra's courtship is among the most elaborate in the hobby. Males possess specialised caudal glands that release pheromones, and during courtship they perform an energetic fin-spreading display in which the male orients broadside to the female, quivers, and releases these chemical signals — behaviour described in the aquarium literature as a 'mating dance'. The croaking sound that gives the species its common name is produced by males during or adjacent to these displays, apparently via rapid movement of the swim bladder.
What sets Mimagoniates microlepis apart biologically from the vast majority of characids is internal fertilisation. The modified anal fin of the male acts as an intromittent organ for sperm transfer; once a female has been inseminated she can store viable sperm for weeks or even months, spawning through the dry season and releasing fertilised eggs into the rainy season without ongoing access to a male. This strategy — rare among characins but characteristic of Stevardiinae — means females can arrive in an aquarium already carrying sperm from a previous encounter.
Breeding
Breeding Mimagoniates microlepis in captivity is possible but requires patience and the right conditions. Following internal fertilisation, females scatter adhesive eggs among fine-leaved plants or spawning mops; the eggs attach to the substrate or plant material and are left without parental care. Both parents will consume eggs given the opportunity, so breeders typically remove adults or harvest the spawning medium to a separate rearing container.
The species' internal fertilisation means that females can spawn multiple times from a single insemination event, and gravid females acquired from a shop may produce fertilised eggs even in the absence of a male. Cool, clear, well-oxygenated water replicating the species' stream habitat appears to be important for breeding success; temperatures in the 64–72 °F range (64–72 °F) are often recommended. Fry are small and require fine first foods such as infusoria or commercial liquid fry food before graduating to newly hatched brine shrimp.
In the aquarium
The croaking tetra is a lively, schooling fish that does best in a group of six or more in a well-planted aquarium with some open swimming space. A tank of 23.5–31.5 in or longer suits a small group well. Because the species originates from cool, shaded Atlantic Forest streams, it can be kept at lower temperatures than most tropical tetras — 64–75 °F (64–75 °F) is appropriate, with Aquarium Glaser recommending 64–68 °F as optimal. It tolerates typical aquarium pH and hardness ranges without difficulty.
The croaking tetra is peaceful and suitable with other small, similarly sized fishes that share its temperature preference; subtropical companions such as certain livebearers, small danionins, and other cool-water characids are natural partners. It is best avoided in tanks heated above 77 °F for extended periods. The species is particularly rewarding when kept in a group large enough to trigger courtship behaviour — the croaking sounds and pheromone displays of courting males are a genuine novelty that few aquarium fishes can match. Water quality should be maintained carefully, as the species is sensitive to poor conditions in the same way as other small, clear-water stream fishes.
Conservation
Mimagoniates microlepis is assessed as Least Concern (LC) on the IUCN Red List, with the most recent assessment completed in November 2018. The species is widespread along the Atlantic coast of southern Brazil and remains abundant in preserved stream habitats across São Paulo, Rio de Janeiro, and Paraná. Its broad distribution across multiple drainages buffers it against localised losses.
Nevertheless, the IUCN assessment acknowledges habitat reduction in Espírito Santo and Bahia due to agricultural conversion and urbanisation, and there is no active conservation programme targeting the species in Brazil. The broader Atlantic Forest biome — one of the world's most biodiverse and most threatened ecosystems — continues to lose extent, and the cool, clear-water stream habitats that Mimagoniates microlepis depends on are vulnerable to temperature increases, sedimentation, and riparian clearance. Aquarium populations are almost entirely captive-bred and pose no collecting pressure on wild fish.