Taxonomy & naming
Priapella olmecae was described by Meyer and Espinosa Pérez in 1990 from the Río Papaloapan drainage of southern Veracruz and adjacent Oaxaca, Mexico. Eschmeyer's Catalog of Fishes treats Priapella olmecae Meyer & Espinosa Pérez, 1990 as the valid name, placing the species in the small Middle American genus Priapella within the family Poeciliidae, the New World livebearers, and the subfamily Poeciliinae. FishBase carries it under species code 47309.
The genus Priapella is a compact group of surface-dwelling poeciliids restricted to the Atlantic-slope drainages of Mexico and northern Central America, all of them characterised by upturned mouths, a habit of swimming in loose swarms against the current, and — in several species — striking blue eyes. The species epithet honours the Olmec, the ancient Mesoamerican people of the Gulf-coast lowlands the fish inhabits; the genus name derives from Priapus, the classical god of procreation, a diminutive reference to the male's gonopodium, the modified anal fin that defines the live-bearing toothcarps. P. olmecae is most readily distinguished within the genus by its restricted Papaloapan range and its vivid eye colour.
Morphology
Priapella olmecae is a small, slender, surface-oriented poeciliid. Males reach about 2 in in total length, while females grow somewhat larger, to roughly 3 in. The body is elongate with a flattened dorsal profile and a terminal-to-upturned mouth, the typical build of a fish that feeds at and just below the water's surface, and the back-and-flank colouration is a restrained silvery to olive that lets the fish blend with sun-dappled flowing water.
The feature that names the fish in the hobby is its eye: a brilliant, iridescent blue iris, comparable to the blue-eyed lampeyes (Aplocheilichthys), that flashes as the fish turns at the surface. As in all poeciliids the sexes differ in the anal fin — males carry the gonopodium, a slender intromittent organ formed from modified anal-fin rays and used to transfer sperm internally, whereas females retain a normal fan-shaped anal fin and are deeper-bodied, especially when gravid. Beyond size, eye colour and the gonopodium, the species lacks the elaborate finnage or saturated pigment of domesticated livebearers; its appeal is the cleaner, wild elegance of a current-adapted fish.
Habitat
The species is endemic to the Río Papaloapan basin of the Gulf slope of Mexico, in the states of Veracruz and Oaxaca — the Papaloapan freshwater ecoregion. It is a freshwater, surface-associated fish of shallow water (recorded from depths of roughly 0–3 ft) in a tropical climate, with FishBase giving a temperature range of about 70–81 °F.
Within these waters P. olmecae favours slow- to fast-moving stretches, where it hangs near the surface and orients head-into-current in loose swarms — the behaviour the genus is known for. It shares its habitat with other Middle American poeciliids and swordtails: FishBase records it co-occurring with the green swordtail (Xiphophorus hellerii) and the shortfin molly (Poecilia mexicana), both characteristic of the same lowland Atlantic-slope streams. Its dependence on flowing, well-oxygenated surface water within a single small basin is exactly what makes it vulnerable to habitat degradation across that limited range.
Feeding
Priapella olmecae is mainly carnivorous, taking insects and insect larvae, small crustaceans, and planktonic invertebrates from the surface and upper water column; FishBase places it at a trophic level of about 3.0. Its upturned mouth and surface-swarming habit are adaptations for snapping small insects drifting on or falling onto the water, and specialist accounts note that the genus will even leap clear of the surface to seize prey, with flying and surface insects forming a major part of the natural diet.
In the aquarium this translates into a fish that feeds at the surface and in midwater rather than from the bottom. It accepts the usual small prepared foods, but does best with a strongly insect- and invertebrate-weighted diet — small live and frozen foods such as Daphnia, brine shrimp, mosquito larvae and similar items suit its natural feeding mode and help maintain condition. Because it does not forage from the substrate, food should be offered where the fish actually feed, at and near the surface.
Mating
Like all poeciliids, Priapella olmecae practises internal fertilisation. The male transfers sperm to the female using the gonopodium, the intromittent organ built from modified rays of the anal fin; there is no egg-laying and no external spawning. Males court and pursue females in the open water near the surface, and as in related livebearers females can store sperm internally, allowing successive broods to follow a single mating.
Reproduction in Priapella is viviparous and lecithotrophic — the developing young are nourished by yolk rather than through a placental connection — and no placental matrotrophy has been documented for the genus. Likewise, superfetation (carrying overlapping broods at different stages of development), which occurs in some other poeciliids, has not been reported for Priapella. The behavioural side of mating is dominated by the genus's surface, current-facing lifestyle, with courtship and copulation taking place in the same flowing, well-lit water in which the swarm feeds.
Breeding
Priapella olmecae is a livebearer: after internal fertilisation the female carries the developing embryos and gives birth to free-swimming young rather than laying eggs. Precise reproductive figures published specifically for P. olmecae are sparse, and they should not be invented; genus-level husbandry notes for Priapella give an indicative gestation of roughly 35 to 42 days and broods on the order of 8 to 25 young, but these are best treated as approximate guidance for the genus rather than verified counts for this species.
In captivity, success with the genus is reported to be helped by a diet rich in insects, reflecting the fish's natural prey. The young are born fully formed and able to swim and feed at once, and — as with most livebearers — providing cover such as dense or floating plants helps fry survive among the adults. Because the documented breeding data for this particular species are limited, an aquarist should rely on close observation of the fish in hand rather than on precise published brood and gestation numbers.
In the aquarium
Priapella olmecae is a peaceful, sociable, current-loving fish best kept as a group in a well-lit, well-filtered aquarium that offers some water movement to suit its head-into-current swimming. It is a surface and midwater swimmer that does not feed from the bottom, and it lives unobtrusively alongside plants, which it leaves undamaged. Specialist importers describe it as very peaceful and recommend keeping it in swarms, where its blue-eyed, current-facing behaviour is best displayed; tank-bred stock is sometimes available in the trade at around 1.5–2 in.
Water chemistry follows the genus's preference for clean, harder, alkaline conditions: a temperature around 70–79 °F, medium-hard to hard water, and a pH above about 7.5 are recommended by livebearer specialists, consistent with FishBase's broader 70–81 °F range for the species. Good filtration, oxygenation and gentle flow matter more than fancy decor for a fish adapted to flowing surface water, and an insect-rich feeding regime keeps it in condition. It is a fish for the dedicated livebearer keeper rather than a beginner's community centrepiece — undemanding in temperament but specific in its needs for current, surface feeding and stable, hard water.
Conservation
Priapella olmecae is assessed as Endangered (EN) on the IUCN Red List, under criteria B1ab(iii)+2ab(iii), in an assessment dated 13 March 2018 (species ID 191771). The listing reflects a very restricted range — a small extent and area of occupancy confined to the Río Papaloapan basin — combined with a continuing decline in the quality of its habitat, the combination that drives a B-criterion Endangered category. At the national level the species is also recognised as threatened (Amenazada) under Mexico's NOM-059.
For a single-basin surface fish dependent on clean, flowing water, the principal pressures are those affecting that limited habitat: degradation of water quality, alteration of stream flow, and the wider human impacts on the Gulf-slope lowlands of Veracruz and Oaxaca. Because the species occupies so small a geographic range, local habitat loss can translate quickly into range-wide decline, which is precisely what its restricted-range Endangered listing signals. Maintaining aquarium populations of tank-bred stock, and avoiding any pressure on wild populations, is the responsible position for hobbyists working with this fish.