Taxonomy & naming
Ilyodon cortesae was described by Jesús Paulo-Maya and Patricia Trujillo-Jiménez in 2000, in the Revista de Biología Tropical (48(2/3): 465–472), from a holotype (IPN-P 4380) of an adult male 2 in in standard length collected on 10 May 1986. The terra typica is the springs of Cutzarondiro, roughly 8 mi south-east of the Río San Juan Tacámbaro at about 1,1519 ft elevation in Michoacán, Mexico. The species belongs to the order Cyprinodontiformes, family Goodeidae (the splitfins), subfamily Goodeinae, and is placed in the genus Ilyodon. Eschmeyer's Catalog of Fishes governs the valid name and treats Ilyodon cortesae Paulo-Maya & Trujillo-Jiménez, 2000 as valid.
The specific epithet honours María Teresa Cortés, a Mexican ichthyologist. The genus name Ilyodon derives from the Greek ilyos (mud or cave) and odous (tooth), a reference to the distinctive dentition of these fishes. The English name most often applied is "Freckled splitfin" (a translation of the Mexican Mexcalpique pecoso), with "Cortes' splitfin" and the local Mexclapique del Tacámbaro also in use.
The systematics of Ilyodon are unresolved and the validity of I. cortesae is disputed. The Goodeid Working Group places the species within the I. whitei complex (its Clade IE, informally "Ilywh2"), and some authors do not consider it distinct from its nearest relative, I. whitei. Diagnostic features cited in the original description include jaws bearing two irregular series of strongly implanted bifid teeth with a comparatively deep gap between the cusps, wide preorbital lateral-line pores arranged in a straight line, and short prezygapophyses on the caudal-peduncle vertebrae. Because much of the genus is morphologically conservative and geographically structured, the boundaries between named Ilyodon forms remain a live question.
Morphology
Ilyodon cortesae is the smallest member of its genus, a dwarf splitfin that does not reach 3 in in total length. FishBase records a maximum of 2 in standard length, and the original describers report a largest known specimen of 2 in SL. The body is fusiform with an oval cross-section, the typical streamlined shape of a small, current-tolerant goodeid. Meristic counts include 15–18 dorsal soft rays, 11–13 anal soft rays, and 52–60 scales in the longitudinal series.
In life the back is light brown and the belly paler and yellowish, with a dark lateral line formed of a row of dots. The coloration is sexually dimorphic in pattern more than in form: males carry numerous marks across the belly and caudal fin and two to three longitudinal rows of black dots within the dorsal fin, while females are more sparsely marked, their pigment concentrated along the lateral line and their fins essentially unmarked. This freckled, dotted patterning is the source of the common name.
The original describers suggested that the species' dwarf size may be an ecological response to the food-poor conditions of clean spring habitats, and noted that small body size appeared to persist even under tank breeding — hinting at a heritable rather than purely environmental basis for its diminutive stature. Detailed morphometric data beyond the type series are sparse, in keeping with how little the species has been studied in the field.
Habitat
Ilyodon cortesae is endemic to Michoacán in central Mexico and is known only from the springs of Cutzarondiro and the upper Río San Juan Tacámbaro, within the Río Balsas basin. The type locality lies at roughly 19°10′42″ N, 101°20′45″ W, at about 1,1519 ft elevation. This is an extremely restricted range — a single spring system and its immediate headwaters — which makes the species a textbook narrow-range endemic.
The documented habitat is a small spring of clear water, about 4 by 6 metres in area and a metre deep, with a sand bottom interrupted by gravel zones and patches of Potamogeton and filamentous algae. Recorded water conditions are gentle and stable: dissolved oxygen around 7.2 mg/L, pH 7.5, and a water temperature near 68 °F. FishBase characterises the species as freshwater, pelagic, occurring at depths of 0–3 ft, and tropical, with the latitudinal band of 18–20° N.
The only fish reported co-occurring with I. cortesae at the type locality is the cyprinid Hybopsis boucardi, underscoring how species-poor and isolated this spring community is. Such small, clear, spring-fed waters in the Mexican highlands are precisely the habitats most vulnerable to extraction, pollution, and the introduction of exotic fishes — the pressures that have driven the regional decline of goodeids.
Feeding
Direct dietary observations for Ilyodon cortesae are sparse, but its trophic ecology can be inferred from its dentition and from FishBase's low estimated trophic level of about 2.0. The jaws bear strongly implanted bifid teeth, the functional toolkit of a fish that rasps and grazes as well as picks at small prey. The Goodeid Working Group notes that the gap between the tooth cusps is deeper than in congeners, and infers that the species is omnivorous but leans more towards carnivory than its relatives I. whitei or I. furcidens.
In practical terms this points to a mixed diet of algae, small aquatic invertebrates and the organic matter of its spring habitat, with a somewhat greater share of animal prey than in the more herbivorous members of the genus. The clean, food-poor character of its spring environment was itself suggested by the original describers as a possible driver of the species' small body size.
Because the species is so little kept and studied, no detailed account of seasonal or ontogenetic diet shifts exists. Data here are genuinely sparse, and any aquarium feeding regime is best extrapolated from the well-documented omnivory of better-known Ilyodon and other goodeids rather than from species-specific records.
Mating
Like all Goodeidae, Ilyodon cortesae is a viviparous fish with internal fertilisation, but its reproductive anatomy differs fundamentally from that of the New World poeciliid livebearers such as guppies and mollies. Goodeids do not possess a true gonopodium. Instead, the male's anal fin is modified into a short crowded structure called the andropodium — formed by a notch separating a small anterior group of fin rays from the rest of the fin — which functions in transferring sperm to the female. There is no elongated rod equivalent to the poeciliid gonopodium.
Reproductive specifics for I. cortesae itself were not detailed in its description, and the account here is drawn from genus- and family-level knowledge. Goodeid courtship generally involves the male positioning alongside the female and bending the andropodial region toward her vent to achieve internal insemination. The pronounced colour difference between the sexes in this species — heavily marked males versus sparsely marked females — is consistent with the visually mediated mate assessment typical of the family.
Unlike many poeciliids, goodeids do not store sperm for long periods and do not exhibit superfetation (the simultaneous carrying of broods at different developmental stages). Each female therefore carries a single cohort of embryos at a time, fertilised afresh for each reproductive cycle rather than from a stored sperm reserve.
Breeding
The hallmark of goodeid reproduction is matrotrophy: the developing embryos are nourished internally by the mother through trophotaeniae, ribbon- or rosette-like extensions of the gut that protrude from the embryo's vent and absorb nutrients directly from maternal tissue. This is a far more intimate form of maternal provisioning than the lecithotrophic (yolk-fed) development of poeciliid livebearers, and the young are born comparatively large and well developed, sometimes still trailing remnants of the trophotaeniae at birth.
Species-specific breeding data for Ilyodon cortesae are not published; the species has been bred in aquaria within the goodeid-keeping community, but quantitative records of brood size and gestation are sparse. At the genus and family level, gestation typically lasts on the order of six to eight weeks, broods are modest in number compared with the prolific poeciliids, and only a single brood is carried at a time (no superfetation). The dwarf body size of I. cortesae implies correspondingly small broods.
The original describers' remark that the species' small size appeared to persist under tank breeding is one of the few reproductive observations on record, and it is notable as a hint that the dwarfism is at least partly heritable rather than a plastic response to the wild spring's poor food supply. For a fish of such restricted range, maintained captive populations in dedicated goodeid programmes represent a meaningful insurance against loss in the wild.
In the aquarium
Ilyodon cortesae is essentially a specialist's fish rather than a general-hobby species. It is maintained almost entirely within dedicated goodeid-conservation circles such as the Goodeid Working Group, where keeping it is as much an act of ex-situ preservation as it is a hobby. It is not a fish encountered in the general aquarium trade, and reliable husbandry detail specific to the species is limited.
What husbandry guidance exists is extrapolated from the species' habitat and from the keeping of other Ilyodon and highland goodeids. Its native spring is clear, well-oxygenated, neutral (pH ~7.5) and cool to temperate at around 68 °F, which suggests it does best in clean, well-filtered water that is not kept too warm — sustained high temperatures suit the highland goodeids poorly. As a small, active, schooling splitfin it is appropriately kept in a group, and like most goodeids it can be nippy, so tankmates should be chosen with that in mind.
Given its conservation significance, the most valuable role an aquarist can play with this species is to maintain a genetically managed captive line within a coordinated programme rather than to keep it casually. Detailed, species-specific care parameters remain data-sparse, and keepers should treat published goodeid husbandry as the working baseline.
Conservation
Ilyodon cortesae has not been formally evaluated by the IUCN Red List. FishBase (Version 2025-2) reports its Red List status as Not Evaluated, and no species-specific assessment or numeric Red List page exists for it. Part of the reason is taxonomic: because the species is frequently subsumed under Ilyodon whitei, it has not been assessed as a distinct entity even as the broader plight of Mexican goodeids has drawn growing attention.
That broader context is grim. Reviews of Mexican goodeids — including the work of Domínguez-Domínguez and colleagues — flag nearly all species in the group as threatened, the consequence of habitat destruction, water extraction, pollution, and the spread of introduced fishes across the central Mexican plateau. A species known from a single small spring and a short stretch of headwater river, as I. cortesae is, fits the highest-risk profile within that already imperilled fauna: its entire global range could be compromised by a single local event.
The absence of a Red List category should therefore be read as a data gap rather than reassurance. The species' extreme range restriction, its dependence on a clean spring habitat, and the regional decline of its relatives all argue for precaution. For a narrow endemic like this one, coordinated captive maintenance through goodeid-conservation networks is a substantive safeguard against extinction while its taxonomic status and wild populations remain inadequately documented.