Taxonomy & naming
Rineloricaria cacerensis was originally described as Loricaria cacerensis by Alípio de Miranda Ribeiro in 1912, from material collected in the Paraguay River near Cáceres, Mato Grosso State, Brazil. Miranda Ribeiro designated a lectotype for the species in 1953. The combination Rineloricaria cacerensis (Miranda Ribeiro, 1912) is recognised as valid by the Catalog of Fishes (Eschmeyer, CAS; FishBase speccode 50567), with placement in the subfamily Loricariinae of the family Loricariidae.
The most recent substantial taxonomic treatment is the revision by Vera-Alcaraz, Pavanelli, and Zawadzki (2012), published in Neotropical Ichthyology (volume 10, number 2, pages 285–311; doi:10.1590/S1679-62252012000200006). That revision examined all Rineloricaria from the Paraguay River basin and established the lectotype as MNRJ 645 (2.5 in SL), with two paralectotypes (MNRJ 30030, 2 in SL) from the same type locality.
The revision's diagnosis distinguishes R. cacerensis from congeners by the presence of five lateral plate series below the dorsal fin with a mid-dorsal series present, combined with a slender area of naked skin on the lateral region of the body posterior to the pectoral-fin insertion (narrower than caudal peduncle width), and five to seven keeled plates in the mid-dorsal lateral plate series extending posteriorly around the end of the dorsal-fin base. The dorsal pattern of large irregular dark brown spots further separates it from related species such as R. fallax, which bears a single predorsal spot.
Morphology
The lectotype of Rineloricaria cacerensis measures 2.5 in SL (approximately 2.5 in), with paralectotypes at 2 in SL. These are the principal published size data for the species; FishBase does not list a maximum size separately from the type series.
The body conforms to the classic whiptail plan: strongly depressed and elongated, tapering to a slender caudal peduncle. The snout tip is raised in lateral view and pointed in dorsal view — a character noted in the 2012 revision as useful in identification. The lateral line carries 30–31 plates. The abdominal surface has six lateral abdominal plates, and the median complex bears three longitudinal plate series.
Teeth are bicuspid with the mesial cusp larger than the lateral. The maxillary barbel is notably long, surpassing the distal border of the lower lip — a feature visible in well-preserved specimens. Coloration is dark brown overall on the dorsal surface, with large irregular dark brown spots; the dorsal and pectoral fins show dark brown bars on the rays; branched rays and membranes of the caudal fin are uniformly dark brown.
As in other Rineloricaria, adult males develop hypertrophied odontodes on the head and pectoral-fin spines during the breeding season. These enlarged spine-like projections serve in male-male competition and are absent or greatly reduced in females. No sexual dimorphism data beyond these inferred genus-level patterns have been documented for R. cacerensis specifically.
Habitat
The entire known range of Rineloricaria cacerensis is the Paraguay River near Cáceres, in the state of Mato Grosso, Brazil. Cáceres lies at the upper reaches of the Pantanal — the world's largest tropical wetland system — where the Paraguay River transitions from a more confined Andean-foothill character to the broad floodplain river that defines the southern Pantanal.
At Cáceres the Paraguay River runs through a landscape of cerrado (Brazilian savanna) and early Pantanal floodplain, with sandy and muddy substrate predominating and extensive seasonally flooded margins. Water levels fluctuate substantially between wet and dry seasons, exposing or inundating vast areas of marginal habitat. The river itself is a large, low-gradient, warm tropical river — very different from the rocky fast-water habitat typical of many Rineloricaria from southern tributaries.
No published temperature, pH, or conductivity data for the type locality have been located in accessible sources. The upper Paraguay at Cáceres is a warm tropical river; temperatures likely range from roughly 72 °F in the austral winter to over 86 °F in the wet-season peak, with soft to moderately hard water chemistry reflecting the cerrado and Pantanal substrate.
Whether R. cacerensis also occurs in the tributaries around Cáceres or is genuinely restricted to the main Paraguay River channel is not established in the literature. The 2012 revision notes the species is known only from the type locality.
Feeding
No diet data specific to Rineloricaria cacerensis have been published. The genus is broadly characterised as an algae, periphyton, and detritus grazer, using the ventral suctorial mouth to scrape epilithic biofilm and fine organic material from substrate surfaces.
In a large, warm, relatively low-gradient river like the Paraguay at Cáceres, available food resources likely include benthic diatoms and algae on sandy or muddy substrate, fine detrital particles, and associated microorganisms. The dietary approach of R. cacerensis is assumed to be consistent with the broader genus, but this cannot be confirmed from the available literature.
Mating
No observations of mating behaviour in Rineloricaria cacerensis have been recorded. The species has not been kept in captivity in any documented account.
Males in Rineloricaria develop hypertrophied odontodes on the head and pectoral-fin spines during the reproductive season, and these structures are central to territorial and mate-competition behaviour across the genus. Males defend cavity or crevice sites — under flat stones, within root mats, or among submerged woody debris — and may engage in contact bouts using the odontode-armed spine against rival males. In the Paraguay River floodplain environment, the availability of suitable sheltered spawning sites in woody debris and root tangles along the bank margins likely governs local breeding density.
Breeding
Rineloricaria cacerensis has not been documented breeding in captivity, and no in-situ breeding observations exist in the published literature. Reproductive biology is therefore entirely inferred from genus-level patterns.
Rineloricaria are cavity and substrate spawners with paternal brood care. The male selects a narrow, sheltered site, and the female deposits adhesive eggs within it. The male remains in or near the cavity throughout incubation, fanning the egg mass with his pectoral fins and pelvic region to maintain oxygenation and removing dead eggs. Incubation duration and fry behaviour have been documented for some congeners in captivity, but no data specific to R. cacerensis are available. Clutch size and fry count remain unrecorded for this species.
In the aquarium
Rineloricaria cacerensis is absent from the aquarium trade as a named, documented species. It carries no L-number, appears in no hobbyist database with keeper accounts or photographs, and has not been bred in captivity in any traceable record. The species is known only from museum type material and the original locality.
If encountered — which would most likely require targeted collecting near Cáceres in Mato Grosso — appropriate aquarium conditions would reflect the warm tropical Paraguay River environment: temperatures around 75–86 °F, soft to moderately hard water, fine sand substrate, and submerged woody debris or flat stones providing shelter and spawning sites. The species' large-river, floodplain origin distinguishes it from Rineloricaria adapted to cooler Andean-foothill or southern subtropical streams.
Any husbandry or breeding record for R. cacerensis would represent a first for the species and would have scientific value.
Conservation
The IUCN Red List assessed Rineloricaria cacerensis as Least Concern in November 2018, with the assessment conducted by Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio), Brazil (IUCN Red List e.T140728315A140728337).
The Least Concern rating may reflect the relatively limited information available rather than demonstrated abundance. The species is known only from the type locality — the Paraguay River near Cáceres — and no systematic survey data on population size, density, or trend have been published in accessible sources. A restricted type-locality distribution combined with an absence of systematic monitoring would not ordinarily support confident assignment to Least Concern, though the IUCN assessors presumably considered the available evidence adequate for the determination at the time.
The upper Pantanal region faces pressures from agricultural expansion in the Mato Grosso cerrado, sedimentation from soy and cattle farming in the headwater catchments, and water management changes in the Paraguay River system. Whiptail catfishes dependent on specific local substrate conditions — particularly those known from only a single locality — warrant monitoring as Pantanal land use continues to intensify.