Taxonomy & naming
The fish was described by the American ichthyologist Carl L. Hubbs in 1936 as Cichlasoma amarum, in his monograph on the fishes of the Yucatán Peninsula collected during the Carnegie Institution expeditions. The type material came from a salt or brackish lagoon on Isla Mujeres (Mujeres Island), off the eastern coast of Yucatán near the tip of the peninsula in present-day Quintana Roo, Mexico; the holotype (USNM 50504) and a small paratype series are held at the U.S. National Museum. For most of the twentieth century the species sat in the sprawling catch-all genus Cichlasoma, listed by Kullander (2003) as an incertae sedis species awaiting revision. The genus Mayaheros was erected by Říčan, Piálek, Dragová & Novák in 2016 to hold the Mayan-cichlid group, and the Catalog of Fishes (Fricke, Eschmeyer & Van der Laan) accordingly lists it as Mayaheros amarus (Hubbs 1936), the name adjusted to the masculine to agree with the genus. Its validity is the open question. Catalog of Fishes retains it because it has never been directly re-examined against urophthalmus, while the geometric-morphometric and molecular survey of Yucatán Mayan cichlids by Mejía and colleagues (Copeia 2018) found the named island and cenote forms to be phenotypic variants of a single variable lineage rather than separate species. Several sister names from the same 1936 monograph — alborum, cienagae, conchitae, mayorum, zebra — have since been formally synonymized with urophthalmus, which makes amarus a nominally valid but unverified species, a name awaiting a formal revision.
Morphology
Mayaheros amarus is a small-to-modest heroine cichlid, maturing well under the robust 12 in-plus reached by Mayaheros urophthalmus; FishBase and the type series point to a fish on the order of 4–4.5 in. The body is the typical oval, laterally compressed heroine shape, with a brassy-tan to olive ground colour overlaid by the lateral barring and dark flank markings characteristic of the Mayan-cichlid group. The diagnostic feature of the whole group is the conspicuous ocellus — a black, pale-ringed eyespot — high on the caudal peduncle, the 'tail-eye' (Greek oura + ophthalmos) that gives the parent species its name urophthalmus. Several dark vertical bars cross the flanks and can be flashed or suppressed with mood. Sexual dimorphism is slight throughout the genus: males average a little larger and develop somewhat longer dorsal- and anal-fin tips, while ripe females are deeper-bellied, with no dramatic colour difference. Because amarus has scarcely been studied beyond its original description, fine meristic and colour detail beyond the group-typical pattern is not well documented.
Habitat
The species is known only from Isla Mujeres, the small limestone island off the northeastern corner of the Yucatán Peninsula, where Hubbs collected it from a salt or brackish coastal lagoon — the 'bitter water' of its name. This is the salient point of its biology: while most of the Mayan-cichlid group lives in fresh cenotes, aguadas and rivers, amarus was taken from a saline island pool, underscoring the group's remarkable euryhalinity. Mayan cichlids as a whole tolerate the full sweep from fresh through brackish to nearly marine conditions, and amarus sits at the salty end of that range. The water of such a lagoon is warm (commonly mid-to-upper 20s °C), hard, highly alkaline and mineral-rich, subject to evaporative concentration and seasonal swings in level and salinity. The species' apparent micro-endemism to a single island is at least partly an artifact of limited collecting; whether the Isla Mujeres population is genuinely isolated and distinct, or simply a salt-tolerant outpost of the wider urophthalmus lineage, is exactly the question the molecular work leaves open.
Feeding
No dedicated diet study exists for Mayaheros amarus, but its membership in the Mayan-cichlid group places it among opportunistic omnivores with a carnivorous lean. Mayaheros urophthalmus, the best-studied relative, is a generalist predator-omnivore taking small fish, crustaceans, aquatic and terrestrial insects, snails, detritus and some plant material, shifting its diet readily with whatever each habitat offers — a flexibility that underlies the group's success across such varied waters. In a coastal salt lagoon, amarus would be expected to work the bottom and margins for small crustaceans, insect larvae, snails and organic debris, taking small fish when available. The robust heroine jaw and pharyngeal mill suit a mixed diet that includes hard-shelled prey, useful in a brackish habitat rich in molluscs and crustaceans. Trophically the group sits in the mid-range of the food web as a flexible secondary consumer.
Mating
Reproductive behaviour has not been documented specifically for Mayaheros amarus, but the Mayan-cichlid group is uniformly biparental and strongly territorial when breeding. In Mayaheros urophthalmus, pairs form a monogamous bond, clear and defend a substrate territory, and become markedly aggressive toward intruders during courtship and brood care; spawning is timed to the warm season when temperatures are reliably above about 75 °F and food is abundant. A pair courts with lateral displays, fin-spreading and substrate-cleaning over a chosen surface. There is every reason to expect amarus follows the same template — a monogamous pair holding and defending a territory — though spawning in a fluctuating, salinity-variable lagoon would be tied to the periods when conditions are most stable and food most plentiful.
Breeding
As with mating, breeding is undocumented for Mayaheros amarus itself and is inferred from the group. Mayan cichlids are open substrate spawners: the pair lays adhesive eggs in rows on a cleaned hard surface — a flat stone, root, or firm patch of substrate — and both parents guard the clutch, fanning the eggs and later shepherding the free-swimming fry. Clutch sizes in Mayaheros urophthalmus run from a few hundred to over a thousand eggs depending on female size; eggs hatch in a few days at warm temperatures, and the wrigglers are moved to pits before becoming free-swimming. Parental care is extended — the pair guards the school of fry for several weeks, and the group is documented producing multiple broods through a long warm season. A small-bodied form such as amarus would lay a correspondingly smaller clutch, but the biparental, multiple-brood substrate-spawning pattern of the group is almost certainly retained, and its salt tolerance would let fry develop in brackish water that excludes many competitors.
In the aquarium
Mayaheros amarus is essentially absent from the aquarium hobby as a named fish — it is not collected or traded under its own name, and what reaches keepers as 'Mayan cichlid' is overwhelmingly Mayaheros urophthalmus. Any husbandry guidance is therefore by extension from its better-known relatives, which are hardy, undemanding, and well suited to keepers who can manage their aggression. A pair would want a tank on the order of 55 US gal or more with an open sandy bottom, scattered rock and wood for cover and territory boundaries, and the hard, alkaline water of its limestone home (pH comfortably above 7, moderate-to-high hardness, temperature around 77–82 °F). Given the salt lagoon it came from, amarus would take readily to slightly brackish water, and the group as a whole copes with a wide chemical envelope. The standing caution for the whole Mayan-cichlid group applies: they are robustly territorial and become genuinely combative when spawning, so tankmates must be sturdy and the tank large enough to defuse aggression. Feeding is trivial — they accept pellets, frozen and live foods readily. None of this is specific to amarus, and a keeper should treat the name as a poorly-defined, salt-tolerant member of the urophthalmus complex rather than a distinct aquarium fish.
Conservation
Mayaheros amarus has not been assessed in its own right by the IUCN Red List; the conservation evaluations in this group treat the Mayan cichlid as the single widespread species Mayaheros urophthalmus (assessed Least Concern), reflecting the same taxonomic uncertainty that surrounds amarus's validity. As a nominally single-island form, however, it would warrant a cautious read if it were ever confirmed distinct: a saline lagoon on a small, heavily developed tourist island like Isla Mujeres is precisely the kind of habitat most exposed to coastal development, water extraction, pollution and the spread of introduced fishes across the Yucatán. The broader Mayan-cichlid lineage is in no danger — it is abundant, adaptable and even invasive where introduced (notably in Florida) — but any genuinely range-restricted island form would be far more vulnerable than its wide-ranging relatives. For now the honest statement is that the taxon is unassessed and its very distinctness unresolved.