Taxonomy & naming
Phalloceros mikrommatos was described by the Brazilian ichthyologist Paulo H. F. Lucinda in 2008, as part of a sweeping systematic and biogeographic revision of the genus Phalloceros published in Neotropical Ichthyology. That single paper recognised twenty-one new species, transforming what had long been treated as one variable, widespread fish — the familiar "caudo" or dusky millionsfish, Phalloceros caudimaculatus — into a genus of many narrowly distributed endemics tied to particular coastal drainages of south-eastern and southern Brazil and neighbouring countries. P. mikrommatos is one of these newly split forms, and Eschmeyer's Catalog of Fishes treats Phalloceros mikrommatos Lucinda, 2008 as the valid name.
The genus sits within the subfamily Poeciliinae of the family Poeciliidae, the New World livebearers, in the order Cyprinodontiformes. Within Phalloceros, P. mikrommatos belongs to a group of species sharing a roundish, well-defined ocellated lateral spot — a dark eye-like mark on the flank ringed by a paler halo — a feature it holds in common with P. leticiae and P. ocellatus. Lucinda separated the three on fine anatomical grounds, including the angle of the gonapophyses (the modified vertebral processes associated with the male's intromittent organ) relative to the vertebral column and, in females, the count of anal-fin rays: P. mikrommatos females typically carry eleven anal-fin soft rays against ten in its two closest relatives.
Morphology
Phalloceros mikrommatos is a very small fish even by the modest standards of its family. Males and unsexed specimens reach only about 1 in in standard length, while females grow somewhat larger, to roughly 1 in SL — placing it among the smallest of all livebearers. The body is the typical compressed, elongate shape of a Phalloceros, built for life among vegetation in shallow water. Meristic counts from the original description give eight to nine dorsal soft rays, nine to eleven anal soft rays, and thirty-one to thirty-two vertebrae.
The most conspicuous marking, and the source of the fish's diagnosis, is a roundish, well-defined ocellated lateral spot on the flank — a dark blotch with a defined margin that gives the impression of a small eye-spot. This ocellus is shared with P. leticiae and P. ocellatus, and the species epithet mikrommatos ("small-eyed") refers to it. As in all poeciliids the sexes differ in the anal fin: mature males carry the gonopodium, a slender rod formed from modified anal-fin rays, whereas females retain a normal fan-shaped anal fin. Beyond the gonopodium and the female's larger size, detailed external coloration notes for this species are sparse in the available literature.
Habitat
Phalloceros mikrommatos is a freshwater, benthopelagic fish of tropical South America, and it is a narrow endemic. It is known from the Rio João de Tiba drainage in the state of Bahia, in eastern Brazil — one of the many small Atlantic coastal river systems that drain directly to the sea along Brazil's eastern seaboard. The IUCN records its habitat simply as inland wetlands, consistent with the small, slow, vegetated coastal waters typical of the genus.
Like its congeners, P. mikrommatos is a fish of shallow, warm, often densely planted margins rather than open rivers — the kind of ditch, pool, swamp and stream-edge habitat where small livebearers can shelter among plants and feed at the surface. Because the species is confined to a single drainage, its conservation prospects are tied closely to the health of that one coastal system; restricted-range endemics of this kind are inherently more vulnerable to localised habitat change than the wide-ranging, weedy Phalloceros species that have spread far beyond their native ranges.
Feeding
No species-specific feeding study has been published for Phalloceros mikrommatos, but its ecology can be read from its body, its habitat, and its relatives. FishBase places it at a trophic level of about 3.0, the value expected of a small omnivore that takes a mixture of plant and animal matter. Phalloceros species in general are micro-grazers of the vegetated shallows, feeding on algae and biofilm, detritus, tiny aquatic invertebrates, and small insect larvae taken from plants, surfaces and the water column.
In practice a fish of this size and habit would subsist on whatever small items its shallow coastal waters provide — diatoms and filamentous algae scraped from surfaces, micro-crustaceans, and the larvae of mosquitoes and other insects that breed in still, warm water. The genus's appetite for mosquito larvae is part of why its better-known members have been moved around the world. For P. mikrommatos specifically the dietary detail is sparse, and the safest statement is the general one: a small, opportunistic omnivore of vegetated wetlands.
Mating
As a member of the Poeciliidae, Phalloceros mikrommatos reproduces by internal fertilisation, the defining trait of the New World livebearers. The male carries a gonopodium — a mobile, intromittent organ built from the modified anterior rays of the anal fin — which he uses to deliver sperm to the female. The genus name itself records this anatomy: Phalloceros comes from the Greek phallos (penis) and keras (horn), a reference to the male's gonopodium. The skeletal apparatus that supports and aims this organ, the gonapophyses, provided Lucinda with some of the finest characters for telling the new species apart.
Courtship in Phalloceros, as in poeciliids generally, involves the male pursuing and positioning himself beside females to achieve insemination, without nest-building or territory defence. Detailed behavioural observations specific to P. mikrommatos have not been published — it is a recently described, hobby-absent endemic rather than a studied model species — so the account here is necessarily drawn from the shared reproductive biology of the genus and family rather than from species-level data, which remain sparse.
Breeding
Phalloceros mikrommatos is, like all members of its genus, viviparous: females retain the fertilised eggs internally and give birth to free-swimming young rather than laying eggs. Reproduction in Phalloceros is lecithotrophic — the developing embryos are nourished by yolk laid down in the egg, with no true placental connection to the mother. Unlike some poeciliids such as Poeciliopsis and Heterandria, superfetation (carrying several broods at different stages of development simultaneously) is not characteristic of Phalloceros, so a female matures and delivers one brood at a time.
No gestation period or brood size has been published specifically for P. mikrommatos, and that data gap should be stated plainly rather than filled by guesswork. By analogy with better-studied congeners such as P. caudimaculatus, the expectation is small broods of a modest number of well-developed live young, with the fry born fully formed and able to swim and feed at once. FishBase rates the species' resilience as high, with an estimated population doubling time of under fifteen months — the rapid turnover typical of small, early-maturing livebearers — but the underlying species-specific reproductive figures simply have not been recorded.
In the aquarium
Phalloceros mikrommatos is essentially unknown in the aquarium hobby. It is a recently described, single-drainage endemic from coastal Bahia, not a fish of the ornamental trade, and it has none of the colour variety or commercial history that made its relative the "caudo" (P. caudimaculatus) a familiar pond and aquarium livebearer. There is no established body of husbandry experience to draw on, and any keeping notes would be inference from the genus rather than tested practice.
Were the species ever maintained — for example in a conservation or research context — its requirements would be expected to mirror those of small, warm-water, vegetated-margin Phalloceros: a densely planted tank with gentle flow, stable tropical temperatures, and small foods suited to a fish barely two centimetres long, with floating plants and cover to give birth-ready females and newborn fry refuge from being eaten. None of that is documented specifically for P. mikrommatos, and an honest profile records it as a wild endemic rather than an aquarium subject.
Conservation
Phalloceros mikrommatos is assessed as Least Concern on the IUCN Red List, in an assessment dated 7 November 2018. The listing reflects that the species, though narrowly distributed, is not currently known to face a major threat across its range in the Rio João de Tiba drainage of Bahia, where it occupies inland wetland habitat.
The Least Concern status should nonetheless be read with the caution that attaches to any single-drainage endemic. A fish confined to one small Atlantic coastal system has no buffer of geographic spread: localised pressures on that drainage — habitat alteration, pollution, water abstraction, or the introduction of non-native fishes — would affect the whole global population at once. The contrast with the genus's weedy, widely introduced members is instructive: where some Phalloceros have become invasive far outside their native ranges, P. mikrommatos is the opposite kind of fish, a quiet, range-restricted endemic whose long-term security depends on the continued health of one coastal Brazilian waterway.