Taxonomy & naming
Phalloceros heptaktinos was described by the Brazilian ichthyologist Paulo H. F. Lucinda in 2008, in a monographic revision of the genus published in Neotropical Ichthyology (6(2): 113–158). That paper transformed the understanding of Phalloceros: where the genus had long been treated as essentially one widespread, variable species — the familiar Phalloceros caudimaculatus — Lucinda recognised a swarm of regionally restricted forms and erected twenty-one new species, of which heptaktinos is one. Eschmeyer's Catalog of Fishes treats Phalloceros heptaktinos Lucinda, 2008 as the valid name, placing it in the genus Phalloceros within the subfamily Poeciliinae of the family Poeciliidae, the New World livebearers, in the order Cyprinodontiformes.
The species is diagnosed on fine anatomical grounds rather than colour. Lucinda separated it from its congeners by the absence of a basal process on the first gonactinost — one of the small bones supporting the male copulatory fin — together with a distinctive sexual dimorphism in the number of dorsal-fin rays. The specific epithet records that last feature directly: heptaktinos is built from the Greek hepta, "seven," and aktinos, "ray," for the seven dorsal rays typical of females. Because the genus was only recently dismantled into its component species, much of the older literature on "Phalloceros caudimaculatus" from southern Brazil may in fact refer to heptaktinos or its neighbours, and the species has accumulated little independent study of its own.
Morphology
Phalloceros heptaktinos is a very small fish even by livebearer standards. Lucinda's original description gives a maximum of about 1 in standard length for males and unsexed specimens and roughly 1 in standard length for females, with females the larger and deeper-bodied sex — the usual poeciliid pattern in which females outgrow the smaller, slimmer males. Meristic counts run to seven or eight dorsal soft rays, nine to eleven anal soft rays, and thirty-one vertebrae.
The most useful external character is also the one behind the species name: the sexes differ in dorsal-fin ray count. Males typically carry eight dorsal rays (very exceptionally seven), while females typically carry seven (very exceptionally eight). This is a subtle, countable distinction rather than a flashy one, and it underlines the fact that Phalloceros species are told apart chiefly by anatomy and by the structure of the male's gonopodial complex rather than by bold differences in pattern. Like other members of the genus, heptaktinos is a modestly built, dusky little fish; detailed published colour notes for this particular species are sparse.
Habitat
The known range of Phalloceros heptaktinos is extraordinarily narrow. It has been recorded only from tributaries of the arroio dos Ratos, part of the Jacuí basin, in Rio Grande do Sul — the southernmost state of Brazil. On present evidence it is a single-drainage endemic, and the type locality essentially defines the limits of its documented distribution.
FishBase classes the species as a freshwater, benthopelagic fish of tropical waters, which is consistent with the small streams and slow margins favoured by Phalloceros generally — shallow, vegetated, often sluggish habitats where these tiny livebearers shelter among plants and feed near the surface and bottom alike. Beyond the basin identity and these broad habitat traits, specific information on the water chemistry, depth and seasonality of the arroio dos Ratos tributaries as they pertain to this species has not been published in detail, so its precise ecological envelope is best described as data sparse.
Feeding
No diet study specific to Phalloceros heptaktinos has been published, and its trophic level is given by FishBase only as a modelled estimate of around 3.0 — a value that places it among small omnivores and micro-predators rather than dedicated herbivores or piscivores.
By the well-documented habits of the genus, Phalloceros are opportunistic feeders of small streams, grazing algae, biofilm and detritus and taking small aquatic invertebrates, insect larvae and other tiny drifting items from the water column and surface film. There is no reason to expect heptaktinos to depart from this generalist pattern. In the absence of published gut-content data, any more specific account of what this particular species eats would be speculation, and the honest summary is that its feeding ecology is inferred from close relatives rather than directly recorded.
Mating
Like all poeciliids, Phalloceros heptaktinos reproduces by internal fertilisation. The male carries a gonopodium — a slender intromittent organ formed from modified rays of the anal fin — which he uses to transfer sperm directly to the female. The fine structure of this organ and its supporting bones is in fact central to how the species is identified: Lucinda diagnosed heptaktinos partly by the absence of a basal process on the first gonactinost, one of the skeletal elements underpinning the gonopodial complex. In the small toothcarps of this genus, courtship is a matter of persistent male pursuit rather than territory-holding or nest-building.
Females of internally fertilising poeciliids commonly store sperm, allowing successive broods to develop from a single mating, and there is no reason to think heptaktinos differs in this basic respect. No species-specific observations of its courtship behaviour, mate choice or mating frequency have been published, however. Within Phalloceros there is no report of superfetation — the carrying of more than one brood at different developmental stages simultaneously, a trait seen in some other poeciliid lineages — so the species is best understood as a conventional single-brood-at-a-time livebearer rather than a superfetating one.
Breeding
Phalloceros heptaktinos is viviparous: after internal fertilisation the female carries the developing embryos and gives birth to fully formed, free-swimming young rather than laying eggs. Reproduction in the genus is lecithotrophic — the embryos are nourished by yolk laid down in the egg rather than by a placental connection to the mother — so there is no matrotrophy and, as noted, no superfetation reported in Phalloceros. Broods develop to term and are released as miniature versions of the adults.
No published gestation period or brood-size figure exists for heptaktinos specifically; as a tiny fish in a genus of small, fast-maturing livebearers, it can be expected to produce correspondingly small broods, but exact numbers are simply not on record. FishBase scores the species as highly resilient, with an estimated minimum population doubling time of under fifteen months — a value that, like the trophic level, is a model output rather than a field measurement, and reflects the general tendency of small livebearers to reproduce quickly. For this particular species the practical breeding details remain undocumented.
In the aquarium
Phalloceros heptaktinos is essentially absent from the aquarium hobby. It is an obscure, narrow-endemic fish known from a single drainage, and no species-specific care article or organised breeding record — including in the brood-record programmes of livebearer specialist societies — could be located for it. Aquarists who keep Phalloceros almost always keep the widespread Phalloceros caudimaculatus or its more available relatives rather than heptaktinos.
Any husbandry guidance for this species is therefore extrapolated from the genus, not established for the fish itself. The small southern Phalloceros are generally undemanding in captivity — tolerant, peaceful, plant-loving livebearers suited to a quiet, well-vegetated tank — but pinning specific temperature, pH or feeding recommendations on heptaktinos would overstate what is actually known. The responsible position is to treat it as a fish of scientific rather than hobby interest, and to be candid that its captive requirements are inferred rather than tested.
Conservation
The IUCN Red List assesses Phalloceros heptaktinos as Data Deficient, in an assessment dated 7 November 2018 (species ID 187252). The Data Deficient category reflects exactly the situation described above: the species is known only from a single small drainage in Rio Grande do Sul, and there is too little information on its population size, trend and the threats acting on it to place it in a meaningful threat category.
A narrow, single-basin range is itself a source of concern, because a fish confined to one small drainage is intrinsically vulnerable to localised habitat change — pollution, water abstraction, agriculture and urban expansion in the Jacuí basin around the arroio dos Ratos. Without targeted survey work, however, it is impossible to say whether heptaktinos is genuinely rare or merely under-recorded. CITES and CMS have not evaluated the species. The clearest conservation need is simple fieldwork: the kind of focused sampling that would replace a Data Deficient listing with an evidence-based assessment of how this tiny, recently described endemic is actually faring.