Taxonomy & naming
Lucania interioris was described in 1965 by Carl L. Hubbs and Robert Rush Miller, two of the twentieth century's most prolific chroniclers of the isolated desert-basin fish faunas of northern Mexico and the southwestern United States. The specific epithet interioris (Latin, "of the interior") reflects the species' confinement to an interior, endorheic desert drainage basin far from the coast — in contrast to the two better-known Lucania species, L. goodei and L. parva, both of which occur in Atlantic and Gulf coastal drainages of the United States. Eschmeyer's Catalog of Fishes lists Lucania interioris Hubbs & Miller, 1965 as a valid species within the family Fundulidae.
Lucania is a small genus of North American topminnows placed within Fundulidae, and L. interioris represents a striking biogeographic outlier: its presence in an isolated Chihuahuan Desert basin, disconnected from the rest of the genus's Gulf- and Atlantic-coastal range, points to an old and now fragmented distribution across what was likely a much better-connected Pliocene or Pleistocene drainage system.
Morphology
Lucania interioris is a small, slender topminnow consistent in general body plan with its congeners: a laterally compressed body, a small, slightly upturned mouth, and the fine scalation typical of Fundulidae. Detailed published measurements specific to this species were not confidently available from the sources consulted here, but a size in the range of 1–2 in, consistent with other small Lucania, is a reasonable expectation.
Sexual dimorphism in Lucania generally is modest: males typically show somewhat more intense fin colouration than the plainer females, without the strongly barred or highly ornamented patterns seen in some other North American killifish genera.
Habitat
Lucania interioris is restricted to spring-fed habitat within an isolated interior desert basin of northern Mexico, part of the same broad Chihuahuan Desert fish fauna that produced the region's famous and often critically endangered Cyprinodon pupfish radiation. These habitats are typically small, clear, spring-fed pools and their outflow channels, often with dense submerged and marginal vegetation providing cover.
As an isolated endemic of a single desert basin, the species is entirely dependent on continued spring discharge, which across the Chihuahuan Desert region has been widely reduced by groundwater extraction for irrigated agriculture — the same broad threat responsible for the extinction or critical decline of numerous co-occurring pupfish species.
Feeding
No species-specific dietary study of Lucania interioris was located. Lucania generally are omnivorous micropredators, taking small aquatic invertebrates — insect larvae, micro-crustaceans — along with some algae and detritus, a diet broadly consistent with the small surface- to mid-water-oriented mouth typical of the genus.
Mating
Courtship behaviour specific to Lucania interioris has not been documented in the sources consulted. Lucania generally are non-annual, egg-scattering spawners that deposit adhesive eggs among aquatic vegetation over an extended breeding season, without nest-building or notable parental defence — a pattern presumed, but not directly confirmed, for this species.
Breeding
Published captive-breeding information specific to this species is data sparse. Its better-known relative, the bluefin killifish (Lucania goodei), breeds readily in captivity given planted, well-vegetated tanks and warm-temperate to subtropical temperatures, with eggs deposited among fine plant roots and hatching within one to two weeks; whether comparable ease of breeding holds for L. interioris has not been confirmed.
In the aquarium
Lucania interioris is essentially unknown in the aquarium trade outside dedicated North American native-fish and conservation circles. General husbandry for small Lucania — a well-planted tank, moderate hardness, and peaceful tankmates — likely applies, but species-specific parameters for L. interioris are not confirmed, and given its restricted natural range any captive stock should be treated as conservation-oriented rather than casually sourced.
Conservation
Lucania interioris is listed by the CARES Preservation Program at the Endangered (CEN) priority level, consistent with its confinement to a single isolated Chihuahuan Desert basin. The overriding threat to the species, as for the wider desert-spring fish fauna of northern Mexico, is groundwater extraction for agricultural irrigation, which reduces or eliminates the spring discharge these fish depend on entirely. Because the species occupies such a narrow range, any further decline in aquifer levels or direct habitat modification at its native spring system would represent a serious threat to its continued survival.