The genus Fluviphylax Whitley, 1965 was comprized of five valid species (F. pygmaeus Myers et Carvalho, 1955, F. zonatus, F. simplex, F. obscurus Costa, 1996, and F. palikur Costa et Le Bail, 1999), which are endemic to the Amazon region. And finally two new species were described in 2020 by Bragança with the species F. gouldingi and F. wallacei. These fishes are the smallest known South American vertebrates and among the smallest know vertebrates on Earth. Many species but the type F. pygmaeus have been described in after the 1990's, and much remains unknown about the biology, taxonomy and systematics of this group of fishes. The aims of the present study were to establish the diploid and haploid number of F. zonatus and F. simplex, and to find species-specific markers for the discrimination of taxa. The diploid number for both species was 48 chromosomes, with no sex chromosome heteromorphism. Fluviphylax zonatus exhibited the karyotypic formula 4m+8sm+22st+14a and FN=82, and F. simplex exhibited 4m+16sm+18st+10a and FN=86. The determination of the total mean length of the chromosomes and their grouping into five size classes demonstrated different chromosome composition of the two species. This difference was further supported by the distribution of constitutive heterochromatin. The meiotic analysis revealed 24 bivalents in both species, but F. zonatus exhibited chromosomes with late pairing of the telomeric portions in the pachytene. These data reveal that cytogenetic characterization is useful and important for the discrimination of these species. Our study further indicates that this method could be employed in the analysis of other species of small fishes that are difficult to distinguish using traditional morphological traits or are morphologically cryptic.

Taken, corrected to latest (2023) situation) from KilliFlash 2011, Killi Data Online, E.R. Souza et al. 2012.

Fluviphylax gouldingi spec. nov. from the upper Rio Negro drainage and F. wallacei spec. nov. from the middle Rio Negro drainage, Rio Amazonas basin, northern Brazil, are described. Fluviphylax gouldingi is distinguished from all other species of Fluviphylax in having a sharp ventral process in the opercle, a narrow subopercle and interopercle, a filamentous second pelvic-fin ray, absence of mesethmoid and the presence of an orange bright blotch on the preorbital region of both males and females. Fluviphylax wallacei differs from all congeners in having a sexual dimorphic pattern of the cephalic lateral line system and by the presence of a pointed dorsal fin in males. Fluviphylax gouldingi and F. wallacei differ from all congeners by the presence of a slender retroarticular, a triangular opercle, an orange colouration on the posterior region of the pectoral fin in males and the head free neuromasts placed at the same level of head surface. The genus Fluviphylax is diagnosed by the presence of eye extremely large, absence of vomer, absence of interarcual cartilage, absence of interhyal, derived shape of maxilla, basihyal cartilage enlarged, reduction of caudalfin rays and cephalic sensory system, and a unique colour pattern, consisting of melanophores concentrated on dorsal and ventral midlines of body.

These extremely small fish live in the mouths of smaller steams where they drain into larger rivers. There they form schools of a few thentals that feed at the water's surface on small insects that land on the water to deposit eggs and other floating food. I experienced firsthand how beautifully this species behaves in the wild when I went into the side creeks to look for Rivulus. They swim quietly together but at the slightest danger they shoot in all directions and therefore it is difficult to catch a group. They are very delicate fish that die quickly after being caught if one does not take the proper measures. To transport this species over a longer period, one absolutely must bring clean water as a kind of stock if one wants to keep them alive during the rest of the trip. Packing should be in breathable bags and preferably one fish per bag. For this I used small sealed bags that I have already made upfront for this kind of small delicate fish.

Breeding is not very easy either. The young are extremely small and are best fed with paramecium or other extreme small live food. Artemia will be good for the adults but cannot be eaten by the young. The water should be soft and slightly acidic with a pH of 6.5.

Remarks:

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Dorsal 00.0,
Anal 0.0,
D/A 0.0,
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Costa, W.J.E.M. 1996. Relationships, monophyly and three new species of the neotropical miniature poeciliid genus Fluviphylax (Cyprinodontiformes: Cyprinodontoidei). Ichthyol. Explor. Freshwat. 7(2):111-130.

Bragança, P.H.N. 2018. Fluviphylax gouldingi and F. wallacei, two new miniature killifishes from the middle and upper Rio Negro drainage, Brazilian Amazon (Teleostei, Cyprinodontiformes, Cyprinodontoidei). Spixiana, 41 (1): 135, figs. 1-2.