Taxonomy & naming
George S. Myers described Rivulus beniensis in 1927 in the Bulletin of the Museum of Comparative Zoology (v. 68, no. 3, p. 133), from specimens collected by N. E. Pearson during the Mulford Expedition's survey of the Beni basin in 1921–1922 — fish Pearson had first reported under the name Rivulus strigatus. The type locality is Ivon, on the Río Beni in lowland northern Bolivia. Myers divided his material into two subspecies: the nominal form from Ivon and Rivulus beniensis lacustris from lagoons along the shores of Lake Rogagua. Fels and Huber, who redescribed the species in 1985 and designated lectotypes for both names, considered lacustris doubtfully distinct — the two type localities sit on the same Amazonian plain with no geographic barrier between them — and Eschmeyer's Catalog of Fishes now treats it as a synonym. A third name, Rivulus bolivianus Seegers 1988, described from Laguna Suárez about 2 mi southeast of Trinidad on the Río Mamoré, was synonymized with beniensis by Costa (in Reis et al. 2003, and again in 2006) and is likewise carried as a synonym in the Catalog.
Costa's 2006 revision placed the species in its own subgenus, Benirivulus, named for the Beni; his 2011 dismantling of the old catch-all genus Rivulus then transferred it to Anablepsoides, the genus holding the bulk of the Amazonian and Guianan rivulines. The Catalog of Fishes records the current status as valid — Anablepsoides beniensis (Myers 1927), family Rivulidae, subfamily Rivulinae — with the parentheses marking the move out of the original genus, and lists the combination as accepted in subsequent work from Costa et al. (2013) through the Bolivian faunal literature (Sarmiento et al. 2014; Careaga et al. 2020) to Drawert (2023). One further tangle deserves mention: for decades the name was misapplied to a Peruvian fish from Tingo María that was only separated in 1985 as Rivulus derhami (today Anablepsoides derhami), so much of the older literature under 'beniensis' concerns that species instead.
Morphology
Anablepsoides beniensis is a small, elongate rivuline. FishBase gives a maximum of 1.6 in total length, and Fels and Huber's 1985 redescription puts normal adult size at about 1.57 in; the male lectotype measures 1.04 in standard length (1.35 in total). The body is slender and the caudal fin long and lanceolate — one of the characters that separates the species from the superficially similar Anablepsoides strigatus, whose tail is shorter and spade-shaped. Counts from the redescription: 7–8 dorsal-fin rays, 12–13 anal-fin rays, 33–35 scales in the longitudinal series, and a body depth of 18–20% of standard length. Fels and Huber grouped the species morphologically with Anablepsoides ornatus of the middle Amazon and Anablepsoides speciosus of Peru.
Because the redescription rests on preserved museum material, the colours in life remain essentially undocumented. In preservative, males show fine dark-brown longitudinal lines along the flanks — the lines between them presumed red in life — with dusky fins, a dark line along the anal-fin base, and no caudal ocellus. Females carry darker marbling that resolves into a chequered pattern on the lower rear body, spotted fins with a dark anal-fin edge, and a large black ocellus on the caudal peduncle; in the Lake Rogagua types this ocellus sometimes covers up to half the peduncle. Both sexes have a dark margin to the lower lip.
Habitat
This is a lowland Amazonian fish of the Madeira drainage. The Catalog of Fishes gives the distribution as the Madeira River basin in Bolivia and Brazil; FishBase words it as the Mamoré-Guaporé river system. Confirmed collections span the lower Río Beni at Ivon, lagoons fringing Lake Rogagua, Laguna Suárez near Trinidad on the Mamoré (type locality of the synonym Rivulus bolivianus), and the Río Iténez (Guaporé) near its confluence with the Río Baures opposite Costa Marques, where Fels and Huber also examined fish from a grassy pool on the nearby plain. These are floodplain waters — lakes, lagoons and pools of the seasonally flooded Beni savannas.
FishBase characterises the habitat simply as shallow water between plants, and records the species as a facultative air-breather — a capability widespread among rivulines that lets them use warm, oxygen-poor margins — in very soft (hardness 0–1 dH), slightly acidic to neutral water (pH 6.0–7.0) at tropical temperatures listed as 82–90 °F (82–90 °F). The IUCN assessment describes it from rivers, streams and temporary pools, noting that although it is not a seasonal killifish it can persist for more than a year where a water body is permanent. The assessment also maps a broader range than the taxonomic literature, extending through the Madeira mainstem and into the Purus basin, with occurrence listed for Bolivia, Brazil (Rondônia) and Peru.
Feeding
No dietary study exists for this species. FishBase estimates a trophic level of 3.1 ± 0.4, derived from body size and the trophic data of its closest relatives — the profile of a small invertebrate feeder rather than a piscivore or a grazer. That accords with the genus at large: Anablepsoides are surface-oriented micropredators that pick aquatic invertebrates, insect larvae and terrestrial insects from the upper water layers and the water's edge.
In captivity, congeners are maintained on the standard killifish fare of live and frozen invertebrates — Daphnia, mosquito larvae, brine shrimp — and there is no reason to expect A. beniensis to differ, though nobody has published first-hand feeding observations for the true species.
Mating
Courtship and spawning have never been described for genuine Anablepsoides beniensis. The one detailed behavioural and ecological account published under the name — Lüling's field studies at Tingo María, Peru, in the 1960s and 1970s — was shown by Fels and Huber in 1985 to concern the separate species now called Anablepsoides derhami, and cannot be transferred to this fish.
What can be said with sources behind it is limited: the species is not an annual. FishBase (following Huber's 1992 review of Rivulus) records it as not a seasonal killifish, and the IUCN assessment repeats the point, noting individuals can live beyond a year in permanent water. On the pattern of its non-annual congeners, reproduction would be expected to proceed by repeated small spawnings of adhesive eggs on vegetation rather than by a single seasonal effort with eggs buried against drought — but for A. beniensis specifically this remains an inference from the genus, not an observation.
Breeding
There are no published egg measurements, incubation times or fry-rearing accounts for this species — a direct consequence of its absence from the hobby and the long shortage of live material. Breeders who obtain it would have to work from the general Anablepsoides method: a pair or trio over spawning mops or fine-leaved plants in soft, slightly acidic water; regular collection of the adhesive eggs; incubation in shallow water for roughly two weeks; and first foods of newly hatched brine shrimp, which fry of most members of the genus can take immediately. Whether the species-level details — egg size, incubation tolerance, growth rate — match its better-known relatives is simply unrecorded.
In the aquarium
Anablepsoides beniensis has almost no genuine aquarium history, and what history the name has is mostly mistaken identity. The 'Rivulus beniensis' written up for aquarists by Klee in 1963 was a fish of unknown origin later identified as Rivulus rectocaudatus; the Tingo María fish kept and redescribed by Meinken in 1971 was Anablepsoides derhami. When Fels and Huber untangled the confusion in 1985 they noted that the true beniensis was unknown alive — every specimen then in existence was preserved museum material. FishBase nonetheless flags the species as of potential aquarium interest, and its home region is reachable: the redescribers pointed out that a Bolivian road runs through the type region at low altitude, so fresh collections would not be difficult for a determined expedition.
A keeper who did obtain it would sensibly start from the water FishBase records — very soft (0–1 dH), pH 6.0–7.0, warm lowland temperatures with a listed range of 82–90 °F (82–90 °F) — in a small, densely planted species tank with a tight-fitting cover, since rivulines as a group are accomplished jumpers. Every husbandry parameter should be treated as provisional: this is a fish for which the aquarium literature still needs to be written.
Conservation
The IUCN Red List assessed Anablepsoides beniensis as Least Concern, with an assessment date of 10 December 2020 (published in 2024 by G. L. Brejão as assessment e.T49829693A58730798). The assessment finds a wide lowland range — the Mamoré-Guaporé, Madeira and Purus river basins across Bolivia, Peru and Brazil (Rondônia) — identifies no current threats, and records no population data at all; its main recommendation is research into the species' ecology and population trends. FishBase classes resilience as high, with a minimum population doubling time under 15 months.
Two caveats attach to that clean bill of health. First, the long history of misidentification means older occurrence records under this name — particularly Peruvian ones — need checking against Anablepsoides derhami and other look-alikes before they anchor a range map. Second, a species nobody has studied alive can hide almost anything: the Least Concern category here reflects a broad mapped range and an absence of documented threats, not demonstrated population health.