South American Freshwater Fishes Research Articles

Fish biodiversity and conservation in South America.

The freshwater and marine fish faunas of South America are the most diverse on Earth, with current species richness estimates standing above 9100 species. In addition, over the last decade at least 100 species were described every year. There are currently about 5160 freshwater fish species, and the estimate for the freshwater fish fauna alone points to a final diversity between 8000 and 9000 species. South America also has c. 4000 species of marine fishes. The mega-diverse fish faunas of South America evolved over a period of >100 million years, with most lineages tracing origins to Gondwana and the adjacent Tethys Sea. This high diversity was in part maintained by escaping the mass extinctions and biotic turnovers associated with Cenozoic climate cooling, the formation of boreal and temperate zones at high latitudes and aridification in many places at equatorial latitudes. The fresh waters of the continent are divided into 13 basin complexes, large basins consolidated as a single unit plus historically connected adjacent coastal drainages, and smaller coastal basins grouped together on the basis of biogeographic criteria. Species diversity, endemism, noteworthy groups and state of knowledge of each basin complex are described. Marine habitats around South America, both coastal and oceanic, are also described in terms of fish diversity, endemism and state of knowledge. Because of extensive land use changes, hydroelectric damming, water divergence for irrigation, urbanization, sedimentation and overfishing 4-10% of all fish species in South America face some degree of extinction risk, mainly due to habitat loss and degradation. These figures suggest that the conservation status of South American freshwater fish faunas is better than in most other regions of the world, but the marine fishes are as threatened as elsewhere. Conserving the remarkable aquatic habitats and fishes of South America is a growing challenge in face of the rapid anthropogenic changes of the 21st century, and deserves attention from conservationists and policy makers.

Transcontinental dispersal, ecological opportunity and origins of an adaptive radiation in the Neotropical catfish genus Hypostomus (Siluriformes: Loricariidae).

Ecological opportunity is often proposed as a driver of accelerated diversification, but evidence has been largely derived from either contemporary island radiations or the fossil record. Here, we investigate the potential influence of ecological opportunity on a transcontinental radiation of South American freshwater fishes. We generate a species-dense, time-calibrated molecular phylogeny for the suckermouth armored catfish subfamily Hypostominae, with a focus on the species-rich and geographically widespread genus Hypostomus. We use the resulting chronogram to estimate ancestral geographical ranges, infer historical rates of cladogenesis and diversification in habitat and body size and shape, and test the hypothesis that invasions of previously unoccupied river drainages accelerated evolution and contributed to adaptive radiation. Both the subfamily Hypostominae and the included genus Hypostomus originated in the Amazon/Orinoco ecoregion. Hypostomus subsequently dispersed throughout tropical South America east of the Andes Mountains. Consequent to invasion of the peripheral, low-diversity Paraná River basin in southeastern Brazil approximately 12.5 Mya, Paraná lineages of Hypostomus, experienced increased rates of cladogenesis and ecological and morphological diversification. Contemporary lineages of Paraná Hypostomus are less species rich but more phenotypically diverse than their congeners elsewhere. Accelerated speciation and morphological diversification rates within Paraná basin Hypostomus are consistent with adaptive radiation. The geographical remoteness of the Paraná River basin, its recent history of marine incursion, and its continuing exclusion of many species that are widespread in other tropical South American rivers suggest that ecological opportunity played an important role in facilitating the observed accelerations in diversification.

Novel, major 2α- and 2β-hydroxy bile alcohols and bile acids in the bile of Arapaima gigas, a large South American river fish

Bile alcohols and bile acids from gallbladder bile of the Arapaima gigas, a large South American freshwater fish, were isolated by reversed-phase high-performance liquid chromatography. The structures of the major isolated compounds were determined by electrospray-tandem mass spectrometry and nuclear magnetic resonance using 1H- and 13C-NMR spectra. The novel bile salts identified were six variants of 2-hydroxy bile acids and bile alcohols in the 5α- and 5β-series, with 29% of all compounds having hydroxylation at C-2. Three C27 bile alcohols were present (as ester sulfates): (24ξ,25ξ)-5α-cholestan-2α,3α,7α,12α,24,26-hexol; (25ξ)-5β-cholestan-2β,3α,7α,12α,26,27-hexol, and (25ξ)-5α-cholestan-2α,3α,7α,12α,26,27-hexol. A single C27 bile acid was identified: (25ξ)-2α,3α,7α,12α-tetrahydroxy-5α-cholestan-26-oic acid, present as its taurine conjugate. Two novel C24 bile acids were identified: the 2α-hydroxy derivative of allochenodeoxycholic acid and the 2β-hydroxy derivative of cholic acid, both occurring as taurine conjugates. These studies extend previous work in establishing the natural occurrence of novel 2α- and 2β-hydroxy-C24 and C27 bile acids as well as C27 bile alcohols in both the normal (5β) as well as the (5α) “allo” A/B-ring juncture. The bile salt profile of A. gigas appears to be unique among vertebrates.

Larval development of indigenous South American freshwater fish species, with particular reference to pacu (Piaractus mesopotamicus): A review

In recent years, a number of South American freshwater fish have gained increasing attention for their potential in aquaculture, not only because of their excellent performance in farming systems but also to meet the high consumer demand for these species due to declining fishery resources. Many South American freshwater species are migratory and produce altricial larvae, with a small amount of yolk reserves. Unlike precocial freshwater species and altricial coldwater marine fish, these freshwater fish investigated have rapid yolk depletion and metamorphosis. Specific studies on the initial development of South American fish are scarce and fragmented. One of the most widely studied species is the pacu (Piaractus mesopotamicus), farmed in warm continental waters. In the present review we compile new and published data on the initial development of pacu, including morphogenesis of the skeletal, muscle, digestive and sensory systems; compare it to other Neotropical species; and discuss the importance of this information to develop larviculture protocols. When pacu larvae exhaust yolk reserves, they initiate a new form of interaction with the environment, becoming exclusively exotrophic. This type of interaction is made possible by the rapid development of sensory, skeletal, locomotor and digestive structures. In addition to understanding fish ontogeny, studies on larval development are necessary to improve farming systems and larviculture techniques aimed at producing high-quality juveniles in aquaculture.

Myxidium ceccarellii n. sp. (Myxosporea) from the gallbladder of Leporinus elongatus (Anastomidae) from the São Francisco River, Brazil.

During a survey of myxozoan parasites of freshwater fish from the São Francisco River in Minas Gerais State, Brazil, plasmodia of Myxidium ceccarellii n. sp. were found in gallbladders of two out of six specimens (22%) of Leporinus elongatus (Anastomidae). Parasite plasmodia were translucent and greenish, with disporic sporoblasts that develop asynchronously. Mature myxospores were ellipsoidal in frontal and lateral views, with slightly pointed ends. The surfaces of each valve had four to six longitudinal grooves. Spores dimensions were as follows: length 17.7 ± 0.5 μm (17.1-18.1), width 10.4 ± 0.47 μm (9.8-11.3), and thickness 10.1 ± 0.27 μm (9.6-10.4). Two polar capsules, one at either end of the spore, had the length of 6.3 ± 0.5 μm (5.7-7.0) and width of 6.4 ± 0.44 μm (5.7-6.9), with four to five polar filament turns. Some aberrant spores had one or three polar capsules. Partial sequencing of M. ceccarellii n. sp. small subunit ribosomal RNA (ssrRNA) gene resulted in 1,845 bp. This is the first molecular study of a Myxidium species that parasitizes a South American freshwater fish. Phylogenetic reconstruction using ssrRNA gene sequences showed that M. ceccarellii n. sp. was positioned basally in a recognized clade of myxozoans that infect the biliary systems of nonfish vertebrates.

Testes of Astyanax altiparanae: The Sertoli cell functions in a semicystic spermatogenesis

The Astyanax altiparanae (lambari) is a South American freshwater fish belonging to the family Characidae. Although some authors have described reproductive aspects of this species, this is the first study about the morphology of the testes throughout the annual reproductive cycle of A. altiparanae. Fish spermatogenesis differs from that in mammals as it occurs in cysts whose borders are defined by cytoplasmic processes of Sertoli cells, thus creating a favorable environment for spermatogenesis. The functions commonly attributed to fish Sertoli cells were investigated using stereological, light and electron microscopy in A. altiparanae. Results showed that when the Sertoli cells of A. altiparanae are in contact with germ cells, they plan a support function that culminates in the production of spermatozoa. After releasing spermatozoa, modified Sertoli cells form the duct epithelium, transform into secretory cells and release a secretion into the duct lumen where spermatids and sperm are located. Thus, the present study revealed important aspects of the testes of A. altiparanae, and propose a sequence of functions played by the Sertoli cells in this species.

Conserving the freshwater fishes of South America

South America harbours the most diverse freshwater fish fauna in the world, and recent estimates point to between 6000 and 8000 species in the Neotropical region. Such fauna has diverse historical origins, either having invaded and diversified in fresh water from marine ancestors during the Palaeogene or being isolated on the continent since the end of the break‐up of Gondwana in the Cretaceous. Taxonomic, morphological and ecological diversity of South American freshwater fishes is dramatic, as are the myriad freshwater habitats they inhabit. Unfortunately, many of these habitats are severely threatened by deforestation, water divergence for irrigation, industry and other uses by humans, hydroelectric damming, mining, pollution and invasive species. Despite these multiple threats, there are very few on‐the‐ground conservation initiatives in South America, although assessments of species‐extinction risks have been produced at regional and subregional levels in different countries.

Endocrine disruptive potential of endosulfan on the reproductive axis of Cichlasoma dimerus (Perciformes, Cichlidae)

Endosulfan (ES), a persistent organochlorine pesticide, is widely used despite its toxicity to non-target animals. Upon reaching water bodies, ES can cause negative effects on aquatic animals, including disruption of hormonal systems. However, the action of ES on fish reproductive axis has been hardly studied thus far. The aim of the present work was to assess the endocrine disruptive potential of endosulfan on the pituitary gonadotropins levels and on the testes function due to ES in the South American freshwater fish Cichlasoma dimerus, using in vitro and in vivo approaches. In vitro experiments showed that ES inhibited the LH-stimulated steroidogenesis in gonads; no change was observed in gonadotropins release from pituitaries in culture. Laboratory waterborne ES (0.1, 0.3 and 1μg/L) exposure for two months caused decrease in βFSH pituitary content and γGT activity in the testes (Sertoli cell function marker). Testicular histology revealed pathologies such as scarce intermediate stages of spermatogenesis, release of immature germ cells into the lobular lumen, presence of foam cells and interstitial fibrosis. As FSH and FSH-mediated steroidogenesis regulate spermatogenesis and Sertoli cell function, the effect of ES on FSH could be responsible for the morphological alterations observed in testes. In vitro, ES disrupted steroidogenesis in gonads, therefore similar effects in vivo cannot be ruled out. Based on this evidence, ES exhibits an endocrine disruptive action on the reproductive axis of C. dimerus, causing disruption at the pituitary and/or at the gonad level. These effects could acquire ecological significance under prolonged exposure to the pesticide in nature.

Seasonal changes in reproductive endpoints in Trichomycterus areolatus (Siluriformes: Trichomycteridae) and Percilia gillissi (Perciformes, Perciliidae), and the consequences for environmental monitoring

As protection of the natural reproductive pattern is necessary to ensure adequate conservation of natural populations, we examined some typical reproductive endpoints throughout a reproductive season in two Chilean species that lack this baseline information. Trichomycterus areolatus and Percilia gillissi both have maximum gonad development for an austral spring spawning period beginning in October/November. Trichomycterus areolatus appears to spawn many times, as there was no significant correlation between gonad size and body size, a characteristic of asynchronous spawners. Percilia gillissi appears to be a multiple spawner, with larger individuals spawning into January, although most fish completed spawning by December. To design a study for monitoring purposes with these species, the best sampling periods would be late fall (June) and spring (October) for P. gillissi, as power analysis indicated a target sample size of 20 females in June and 34 in October; the sample size in October could be reduced to 12 by selecting females > 50 mm in length, thereby reducing the variability. Trichomycterus areolatus gonadal variability was much higher, requiring a sample size in excess of 80 individuals for adequate statistical power. An understanding of these basic reproductive and metabolic patterns in South American freshwater fishes will help ensure better management of the water resources.

An overview of freshwater fish aging in South America: the science, biases and future directions

South America harbors the largest freshwater diversity in the world (about 4,475 valid species). Aging studies of 153 species published in 92 papers were reviewed for this publication. They were categorized according to the journal where they were published, decade, number of researches per river basin, methods and structures most used to estimate age, validation methods, period and causes of the formation of the age ring and age of the fish in South America. Our results showed an increase in the studies of age and growth, especially in the Parana river basin. Scales were the structure most used in the studies of aging. Most of the researchers did not validate age. On a continental level, there was no specific period for growth ring formation, but in general, it was related to reproductive activity and flooding period. South American freshwater fishes did not present high longevity (maximum of 15 years). The ongoing governmental support to the fishery in South America, such as in Brazil with the creation of the Ministry of Fishing and Aquaculture, will certainly enlarge the fishing industry. Thus, it is necessary to know and to evaluate the fishery stocks correctly in order to guarantee sustainable use.

Dermal lip protuberances associated with aquatic surface respiration in juveniles of the piscivorous characid Salminus brasiliensis (Actinopterygii: Characidae)

Some South American freshwater fishes can improve their capability of aquatic surface respiration (ASR) by developing dermal lip protuberances in the lower jaw. This adaptation was thought to be limited to omnivorous or herbivorous fishes. The present work provides the first evidence that juveniles of a piscivorous characid, Salminus brasiliensis, develop dermal lip protuberances during periods of hypoxia in floodplain ponds of the Salado River, in Argentina. The protuberance of S. brasiliensis involves dermal portions of both jaws exhibiting lateral lobes on the sides of the mouth arranged in the vertical plane. Water dissolved oxygen concentrations less than or equal to 1.05 mgl-1 were associated with a remarkable increase in lip protuberance. The lateral lobes of the protuberance in this species may limit the access of water to the anterior portion of the mouth which is positioned nearer to the oxygenated surface water during ASR. Finally, ASR, complemented with the development of dermal lip protuberances, can be considered a valuable strategy to survive in hypoxic environments, even for carnivorous fishes with elevated oxygen requirements.

Braga cigarra comb. nov. for Philostomella cigarra (Crustacea: Isopoda: Cymothoidae) with a redescription of the species based on specimens from Galeocharax kneri, a freshwater fish of Minas Gerais State, Brazil

Specimens of a cymothoid isopod from a freshwater fish of Minas Gerais State were studied. These were found to be morphologically comparable to Philostomella cigarra Szidat & Schubart, 1960, except for having multilaminate pleopods which is the distinguishing characteristic of the genus Braga Schiödte & Meinert, 1881. The first pleopod is bilaminate while the second is quadrilaminate. Pleopods three to five are trilaminate. It is herein proposed that the generic name Philostomella Szidat & Schubart, 1960, should be considered invalid and that the species P. cigarra should be transferred to Braga. Braga now contains eight species, including B. cigarra comb. nov., all from South American freshwater fishes. The principal difference between this species and the others in the genus is the form of the female which has a narrow pleon and a small pleotelson. A redescription of this species is presented here because previous descriptions in the literature are inaccurate or incomplete.

Isolation and characterization of 13 microsatellite loci for Percichthys trucha (Percichthyidae)

Thirteen polymorphic microsatellite loci are described for the South American freshwater fish Percichthys trucha. Number of alleles per locus ranged from two to 21 and observed heterozygosities ranged from 0.304 to 0.915 in a sample of 47 individuals from four different sampling locations.

Threatened fishes of the world: Brycon nattereri Günther, 1864 (Characidae)

Brycon nattereri (Ostariophysi: Characiformes: Characidae), a threatened South American freshwater fish, occurs in the Parana, Tocantins and Sao Francisco river basins in central Brazil. It is a middle-sized (up to 50 cm SL), omnivorous species, which occurs in swift, clear-water rivers with well-preserved riparian vegetation. Main threats to the species are water pollution, dam building, and deforestation.

Nilonema gymnarchi Khalil, 1960 and N. senticosum (Baylis, 1922) (Nematoda: Dracunculoidea): Gondwana relicts?

Data-base searches of records of the helminth parasites of South American and African freshwater fishes revealed only two pairs of species from genera endemic to the two regions. One pair, species of the primitive amphilinid cestode genus Nesolecithus Dönges & Harder, 1966, has already been designated as likely Gondwana relicts. The second pair are the philometrid nematodes Nilonema gymnarchi Khalil, 1960 from Gymnarchus niloticus Cuvier (Gymnarchidae) in Africa and N. senticosum (Baylis, 1922) from the South American fish Arapaima gigas (Cuvier) (Arapaimidae). Both species are partly redescribed on the basis of light and scanning electron microscopical observations of the type-specimens, and their relationships are discussed. In view of the fact that both hosts are basal teleosts of the order Osteoglossiformes and they are the same two fish which harbour the amphilinid cestodes already indicated as Gondwana relics, the nematode pair is also deemed to be relictual. However, these species are suggested as being of limited potential value for calibrating a molecular clock.

Ailinella mirabilis gen. n., sp. n. (Eucestoda: Pseudophyllidea) from Galaxias maculatus (Pisces: Galaxiidae) in the Andean-Patagonian region of Argentina

Ailinella gen. n. (Pseudophyllidea: Triaenophoridae) is proposed to accommodate Ailinella mirabilis sp. n. from Galaxias maculatus (Jenyns, 1842), a freshwater fish inhabiting the Andean lakes in Argentinean Patagonia. Ailinella belongs to the Triaenophoridae because it has a marginal genital pore, a follicular vitelline gland, and a ventral uterine pore. The new genus can be distinguished from other triaenophorids by the following combination of characters: a small body size, a low number of proglottides, which are longer than wide, a truncated pyramidal to globular scolex, a rectangular apical disc, presence of the neck, lack of internal longitudinal musculature separating the cortex from the medulla, testes distributed in one central field surrounding the ovary laterally and posteriorly, the vagina predominantly anterior to the cirrus sac, vitelline follicles circum-medullary, the genital pores post-equatorial, a saccate uterus, and operculate eggs. Blade-like spiniform microtriches were present on all tegument surfaces, and tumuli on all surfaces of the scolex and the anterior surface of the neck. Microtriches were characterized according to their size and density, and tumuli according to their size, inter-tumulus distance and density. Ailinella mirabilis is the first cestode described from G. maculatus and the second triaenophorid species recorded from a South American freshwater fish.

Historical biogeography of South American freshwater fishes

AbstractAim To investigate biogeographical patterns of the obligate freshwater fish order Characiformes.Location South America.Methods Parsimony analysis of endemicity, likelihood analysis of congruent geographical distribution, and partition Bremer support were used.Results Areas of endemism are deduced from parsimony analysis of endemicity, and putative dispersal routes from a separate analysis of discordant patterns of distribution.Main conclusions Our results demonstrate the occurrence of 11 major areas of endemism and support a preferential eastern–western differentiation of the characiforms in the Amazonian region, contrasting with the southern–northern differentiation of terrestrial organisms. The areas of endemism identified seem to be deeply influenced by the distribution of the emerged land during the 100‐m marine highstand that occurred during the late Miocene and allow us to hypothesize the existence of eight aquatic freshwater refuges at that time. The raw distribution of non‐endemic species supports nine patterns of species distribution across the 11 areas of endemism, two of which support a southern–northern differentiation in the eastern part of the Amazon. This result shows that the main channel of the Amazon limited dispersal between tributaries from each bank of the river. The levels of endemism further demonstrate that the aquatic freshwater refuges promoted allopatric speciation and later allowed the colonization of the lowlands. By contrast, the biogeographical pattern found in the western part of the Amazon is identified as a result of the Miocene Andean foreland dynamic and the uplift of the palaeoarches that promoted allopatric divergence across several sedimentary basins by the establishment of disconnected floodplains. The assessment of conflicting species distributions also shows the presence of seven putative dispersal routes between the Amazon, Orinoco and Paraná rivers. Our findings suggest that, rather than there being a single predominant process, the establishment of the modern South American freshwater fish biotas is the result of an interaction between marine incursions, uplift of the palaeoarches, and historical connections allowing cross‐drainage dispersal.

A remarkable sand-dwelling fish assemblage from central Amazonia, with comments on the evolution of psammophily in South American freshwater fishes

We studied a specialised assemblage of sand-dwelling fish species from a streamlet in central Amazonia. The five sanddwelling species comprised 15.2 % of the total number in the streamlet (33 species). Two of the sand-dwellers are daytime foragers, Characidium cf. pteroides (Crenuchidae) and Stauroglanis gouldingi (Trichomycteridae), whereas three ones are night-time foragers, Gymnorhamphichthys rondoni (Rhamphichthyidae), "Imparfinis" pristos and Mastiglanis asopos (Heptapteridae). With the exception of C. cf. pteroides, the remainder species bury in the sand during their resting period. All five species bear a cryptic, sand-colour pattern, and some are translucent, traits which we regard as advantageous both for evasion from predators and for approaching prey (for the daytime foragers). All five species are microphagous carnivores and their foraging tactics range from generalised sit-and-wait (C. cf. pteroides) to active searching for interstitial prey (G. rondoni). A unique "drift-trap" system made up by the extended barbels and filamentous first pectoral-fin rays is employed by M. asopos. We regard the small size and low number of vertebrae (which is functionally expressed by fast displacements in the sand) as additional features advantageous to explore the sand habitat, allowing diving quickly in the substrate to evade predators and to forage for small prey in sand interstices or plant debris. The relationship between morphological and behavioural characters suited to life in sandy substrates is examined under the perspective of the current phylogenies for each group of psammophilous fishes here studied. The mapping of these characters demonstrates that not all of them evolved specifically for strict psammophily. Instead, some of them may represent the final step of a transformation series or synapomorphies of higher hierarchical levels. Several characters linked to psammophily, such as small body, large eyes, and scarce pigmentation are probably paedomorphic traits. We consider that the little variable specific composition of fish sand-dwelling assemblages and the wide (although patchy) geographical distribution of their species in the Amazon are due to the widespread occurrence and low complexity of this specialised habitat. The pre-existence of benthic habits, small size, and the broad distribution of rivers with sandy beds, mainly in the Amazon and Orinoco basins, possibly played a determinant role in the origin and diversification of psammophilous assemblages among freshwater fishes in South America.

GALAXITAENIA TOLOI N. GEN., N. SP. (EUCESTODA: PSEUDOPHYLLIDEA) FROM GALAXIAS PLATEI (PISCES: OSMERIFORMES, GALAXIIDAE), IN THE PATAGONIAN REGION OF ARGENTINA

Galaxitaenia n. gen. (Eucestoda: Pseudophyllidea) is proposed to accommodate Galaxitaenia toloi n. gen., n. sp. from Galaxias platei, a freshwater fish inhabiting Andean lakes in Argentine Patagonia. Galaxitaenia belongs to the Triaenophoridae because it possesses a marginal genital pore, a ventral uterine pore, and a follicular vitelline gland. The new genus can be distinguished from other triaenophorids by the following combination of characters: a scolex with a prominent rectangular apical disc without hooks, grooves, or indentations; neck present; segments wider than long to quadrangular; testes in 2 lateral fields, often connected anteriorly; cirrus unarmed; an ovary situated posteriorly; a vagina posterior to the cirrus sac; vitelline follicles medullary, in 2 ventral fields forming 2 wings interrupted medially; a uterus saccate to branched; and eggs without operculum. The types, distribution, and density of microtriches were analyzed on the surface of the tegument of scolex, neck, and immature segments. The presence of tumuli were observed on the apical disc of the scolex. Galaxitaenia toloi n. sp. is the first triaenophorid species recorded in a South American freshwater fish and the fourth helminth parasitizing the intestine of G. platei.

AURICULOSTOMA ASTYANACE N. GEN., N. SP. (DIGENEA: ALLOCREADIIDAE), FROM THE BANDED ASTYANAX, ASTYANAX FASCIATUS (CHARACIFORMES: CHARACIDAE), FROM NICARAGUA, WITH A REEVALUATION OF NEOTROPICAL CREPIDOSTOMUM SPP

Auriculostoma n. gen. (Trematoda: Allocreadiidae) is proposed for Auriculostoma astyanace n. sp. from the intestine of the characid fish Astyanax fasciatus in the Atlantic coastal drainages of Nicaragua. The new genus differs from all papillose allocreadiid genera, except Bunoderella, in possessing 2 pairs of muscular oral papillae (instead of 1 or 3), of which the ventrolateral pair is moderately developed and the dorsolateral papillae are long and auricular. Auriculostoma differs from Bunoderella Schell, 1964, in having lateral vitelline follicles, completely separated or confluent only in the posttesticular region, a uterus limited to the pretesticular region or with a few eggs at the level of the testes, and a long cirrus sac that overlaps the acetabulum or usually reaches posteriorly to the ovarian level. Three other allocreadiid species, all from South American freshwater fishes and each of which had previously been placed in Crepidostomum, are transferred to Auriculostoma based on the presence of the diagnostic muscular oral papillae. These include Crepidostomum platense Szidat, 1954, C. macrorchis Szidat, 1954, and C. stenopteri Mañé-Garzón and Gascón, 1973. Diagnostic features for Auriculostoma also include mainly pretesticular uterus, lateral vitellaria with variation in posttesticular confluence, and tandem testes. The genus appears to be typically associated with neotropical siluriforms (catfishes) and characiforms (tetras).