Taxonomy & naming
Phallotorynus dispilos was described by Paulo H. F. Lucinda, Ricardo S. Rosa and Roberto E. Reis in 2005, in a Copeia paper that revised the whole genus Phallotorynus Henn, 1916 and named three new species (Copeia 2005(3):609–631). The genus belongs to the family Poeciliidae, subfamily Poeciliinae — the New World livebearers — within the order Cyprinodontiformes. Eschmeyer's Catalog of Fishes treats Phallotorynus dispilos Lucinda, Rosa & Reis, 2005 as a valid species.
The naming is a study in transparent description. The generic name Phallotorynus combines Greek phallos (penis) with elements referring to the ornate male intromittent organ that characterises the genus; the specific epithet dispilos is built from Greek dis (twice, two) and spilos (a spot or blotch), a noun in apposition that points directly at the species' diagnostic colour pattern — two dark blotches along the lower half of the flank. The English vernacular "Two spot toothcarp" used by FishBase is a literal translation of the same feature.
One taxonomic complication is worth recording plainly. FishBase carries a remark, following Meyer & Etzel (2006), flagging P. dispilos as a possible synonym of Phallotorynus victoriae and noting that the page "will be removed." That synonymy has not, however, been generally accepted: both the IUCN (which assessed the species in 2021) and the Catalog of Fishes continue to treat P. dispilos as valid. Lucinda et al. distinguished it from its congeners by gonopodial structure, colour pattern and meristics — notably 13–14 epipleural ribs versus 11–12 in P. jucundus and P. victoriae.
Morphology
Phallotorynus dispilos is a very small fish. The original description gives a maximum of about 0.5 in standard length for males and unsexed specimens and around 1 in SL for females, making it one of the smaller poeciliids and a fish that would be easy to overlook in the field. Counts reported via the original description are a dorsal fin of 7–10 soft rays, an anal fin of 9–10 soft rays, and 32 vertebrae.
The defining external mark — and the source of the name — is a pair of round to elliptical dark blotches running along the ventral half of each flank against an otherwise plain, pale body. Two further diagnostic features set the species apart from its relatives. The first is the male gonopodium, which in P. dispilos bears a distinctive wide, short, trowel-like appendix at the tip of ray 3; the elaborately modified gonopodium is a genus-wide hallmark of Phallotorynus and is the structure that gives the genus its name. The second is a striking sexual dimorphism in the teeth: males have pointed, acute teeth while females have truncate teeth.
Beyond these characters the species is, externally, an unremarkable small silvery toothcarp — which is partly why so little is recorded about its life in the wild. The combination of the two ventral blotches, the trowel-tipped third gonopodial ray, the sexually dimorphic dentition and the 13–14 epipleural ribs is what reliably separates it from the other members of the genus.
Habitat
Phallotorynus dispilos is known from the Rio Guyraugua, a tributary of the Monday (Monday/Acaray) River in eastern Paraguay, within the upper Paraná basin. FishBase gives the distribution more loosely as the "Rio Paraná drainage in Paraguay," but the IUCN assessment resolves this to the single known drainage — the Guyraugua — and it is on that small, localised range that the conservation status rests.
The habitat is freshwater inland wetland. The IUCN account, drawing on Meyer & Etzel (2006), describes the species as occurring in slow-flowing streams and in swampy areas alongside streams, among submerged vegetation. This is a fish of quiet, vegetated, sluggish water rather than open river, the kind of marginal swamp-and-stream habitat that is both productive for a small livebearer and acutely vulnerable to drainage and channelisation.
The extent of occurrence is small — about 1,569 mi², treated by the IUCN as a single location. FishBase reports the species as benthopelagic and tropical, consistent with a small fish living close to the bottom and vegetation of warm, shallow, slow water. Because there is no published in-situ measurement series for temperature, pH or hardness specific to this drainage, those values are best regarded as data sparse and inferred only broadly from the tropical lowland setting.
Feeding
There is no published diet study for Phallotorynus dispilos specifically, so its feeding ecology has to be read from its body, its habitat and its relatives rather than from direct observation. FishBase places the species at a trophic level of roughly 3.0, which is consistent with a small omnivore-to-micropredator rather than a strict herbivore or a piscivore.
Small poeciliids of slow, vegetated streams of this kind typically take a mixed diet of tiny invertebrates — micro-crustaceans, insect larvae and other small drifting or substrate-associated items — together with algae, diatoms and organic detritus gleaned from plants and the bottom. The species' small mouth and benthopelagic habit fit that generalist, opportunistic feeding pattern.
Beyond this inference the record is genuinely sparse: no gut-content analysis, no prey list and no feeding observations have been published for the species. The responsible summary is that P. dispilos is most likely a small omnivorous micropredator of its swampy-stream habitat, with the specifics unknown.
Mating
Like all members of the family Poeciliidae, Phallotorynus dispilos reproduces by internal fertilisation. The male transfers sperm to the female using the gonopodium, an intromittent organ formed from modified rays of the anal fin; in this genus the gonopodium is especially elaborate, and in P. dispilos it carries the diagnostic wide, short, trowel-like appendix at the tip of the third ray. The very name Phallotorynus — "penis" plus an ornamentation element — registers how central this male structure is to the group.
The sexually dimorphic dentition (pointed teeth in males, truncate in females) hints that the sexes differ in more than just the reproductive organ, though the behavioural meaning of that difference has not been studied in this species. As in poeciliids generally, fertilisation is internal and the male's role ends with insemination; there is no nest, no spawning site and no paternal care.
No courtship behaviour, mating system, or sperm-storage data have been published for P. dispilos itself. Many poeciliid females can store sperm and produce successive broods from a single insemination, but whether and to what degree this occurs in P. dispilos is undocumented, and is recorded here as data sparse rather than assumed from better-known relatives such as the guppy.
Breeding
Phallotorynus dispilos is a livebearer: as a poeciliine, it is viviparous, the female carrying internally fertilised embryos and giving birth to free-swimming young rather than laying eggs. Most poeciliines, including this lineage, are understood to be lecithotrophic — the developing young are nourished chiefly by yolk laid down before fertilisation, with little post-fertilisation maternal provisioning — rather than placental, matrotrophic breeders.
Beyond that family-level mode, the specifics are unknown. No published figures exist for gestation length, brood size, size at birth, or reproductive frequency in P. dispilos, and there is no evidence one way or the other regarding superfetation (the simultaneous carrying of broods at different developmental stages) in this species. These blanks reflect a genuinely understudied, non-commercial fish rather than an oversight, and they are left as data sparse here.
Given the species' small adult size — males under 1 in, females around 1 in SL — any broods are necessarily small, but no actual brood counts have been recorded. FishBase classifies the species as having high resilience, with a minimum population doubling time of under fifteen months, which implies a reasonably productive reproductive output typical of small livebearers even though the underlying brood data are absent.
In the aquarium
Phallotorynus dispilos is effectively absent from the aquarium hobby. It is a tiny, range-restricted Paraguayan endemic with no commercial collection, no established trade and, as far as can be found, no published species-specific aquarium or breeding account. A targeted search for husbandry information returned only generic livebearer guides with no bearing on this species.
Nothing reliable can therefore be said about its captive requirements from direct experience. Any keeper who encountered it would have to reason from its wild habitat — warm, slow-moving, well-vegetated, swampy stream water in the tropical lowlands of the upper Paraná — and from general small-poeciliid husbandry, while treating temperature, pH and hardness targets as inferred rather than documented for this fish.
On balance this is a species of scientific and conservation interest rather than an aquarium subject. Its small range, its endangered status and the absence of any trade mean the honest entry here is that captive care is undocumented; presenting guppy- or platy-style care figures as if they applied to P. dispilos would be fabrication, and is deliberately avoided.
Conservation
Phallotorynus dispilos is assessed as Endangered (EN) on the IUCN Red List under criteria B1ab(ii,iii), in an assessment dated 3 February 2021 (e.T176510310A176510489). The listing rests on its very small geographic range: an extent of occurrence of about 1,569 mi², treated as a single location, with a projected continuing decline in area of occupancy and in habitat quality. The population trend is recorded as unknown.
The threats are habitat-based and concrete. The species' swampy streams sit in a region important for agriculture and cattle, and the IUCN account lists land-use change, urbanisation, road construction and the channelisation of swampy areas for irrigation as the principal pressures; the known range also lies close to a main road. For a fish confined to a single small drainage of slow, vegetated water, the draining and reshaping of that water is an existential threat rather than a marginal one.
For an aquarist or atlas reader the conservation message is straightforward. This is a narrowly endemic, Endangered livebearer whose persistence depends on a single Paraguayan stream system remaining wet, vegetated and unchannelised. It is not a fish to seek out for the tank; it is a fish whose habitat is the thing that needs protecting.