Aquarium Catfish of the Uruguayan Outback

Author: Ivan Gonzalez and Stan Sung

The avid explorers return! This time, they're delving headfirst into Uruguay, capital of countless catfish, journeying through the exotic countryside and encountering a wide range of catfish varieties that make wonderful aquarium residents.

Searching for Uruguayan Beauty

Our giant seine had been badly shredded by the frantic lashings of captured capybaras from a couple of seasons back, and now the net frequently needs to be repaired after use. While our friends Felipe Cantera and Alejandro Duarte tended to the damaged net, several members of our collecting group decided to explore an idyllic little stream rambling through the Artigas countryside.

John Niemans and Ross Freeman joined me down in the gully where the clear-running creek ran swiftly over waving strands of aquatic plants. As we passed our long-handled dipnets through gardens of submersed vegetation in areas containing the most current, we pulled out a beautiful little gem of a fish—a sparkling little catfish, reminiscent of a shade of emeralds displayed in the bright morning sun. The three of us worked until we had a number of these extremely beautiful aquatic Uruguayan jewels.

An Introduction to the Catfishes

Catfish belong to the order Siluriformes, the most diverse and widespread order of freshwater fish in the world. Encompassing more than 3000 extant species, with many more being described every year, this group is perhaps the most diverse vertebrate order in the world.

Within this massive group, species run the gamut from mini algae eaters to enormous piscivores. There are even strange types that prefer wood or blood to standard catfish fare! Catfish range from diminutive species at under an inch to whopping monsters 7 feet in length. In addition, catfish have incredibly diverse reproductive habits—including egg scattering, nest guarding, mouthbrooding, and even internal fertilization.

Siluriformes is also one of the most widespread fish groups in the world. They have had a global distribution, inhabiting all of the continents except Antarctica (where they also once lived). There are 39 families within the order. All of them lack scales; instead, they possess either a naked body or bony plates that completely or partially cover their bodies. Many have barbels that resemble cat whiskers, which have given them the universal name “catfishes.” In Uruguay, there are representatives of 10 catfish families. In this article, we will focus on the smallest species present in the Uruguay River system, an area belonging to the great La Plata River Basin and the MerÍn LakeBasin.

Fishes That Hear

Together with the Characiformes (characins), the Cypriniformes (carps), and the Gymnotiformes (knifefishes), catfish belong to the superorder Ostariophysi. These fishes are characterized by the presence of the Weberian apparatus, a group of ossicles that connect the swim bladder with the internal ear, creating a resonance chamber for the inner ear that gives these fishes better hearing.

Loricariidae: The  Armored Suckermouths

The loricariids include the plecostomus types, which are beloved aquarium subjects worldwide. Many subfamilies exist within this large family, and they are represented by a multitude of species. Some of the smallest Loricariidae belong to the Hypoptopomatinae subfamily. Like most loricariids, these tiny suckermouth fish feed on a diet of algae.

We have at least three species of hypoptopomatins in the country of Uruguay in two genera, Hisonotus and Otocinclus. They are frequently collected and make outstanding and useful aquarium fish. All of them live within densely vegetated areas in fast currents, whether in small streams or large rivers. Hisonotus species can be easily distinguished from the Otocinclus genus by the shape of the tail. In Otocinclus, a bi-lobed tail shape is present, while the Hisonotus possess a straight-edged tail.

Otocinclus flexilis is a species that appears to be present within the entire Uruguayan territory. There are many times when we had captured these otos alongside dark-brown-colored Hisonotus. In the north of the country, along the Brazilian border, the Cuareim River is home to a bright green Hisonotus. These are the fantastically colored emerald Hisonotus that our collecting party worked hard to accumulate. This jewel-like catfish appears to be sexually dimorphic, as the most splendid males develop maroon-colored leopard spots and a reddish tail, while the females remain a uniform, iridescent green. All Hisonotus are found in areas with clear, rapidly flowing waters. They remain in the shelter of vegetation, grazing on algae attached to the leaf blades of plants.

Both otos and Hisonotus are a great choice for a nano or planted display. They are quite easy to keep, and will clean algae off plants without causing damage to them. In order to keep them healthy, they will need a good source of algae and vegetation, as well as a moderate water current.

Planting Sagittaria varieties with long, wide leaf blades will make them feel the most at home.

These droll little fish will accept a wide variety of prepared foods and will usually learn to rasp at veggie wafers. Plants are essential for these cats to spawn because they lay their eggs on the leaves. They only produce a few eggs, but many spawnings are nevertheless successful due to parental care.

Callichthyidae: The Armored Tanks

This group comprises the armored catfishes. This armor is formed by a series of two bony plates on the sides of their bodies. In Uruguay, we can find representatives of CorydorasCallichthys, and Hoplosternum. The family Callichthyidae are divided into two subfamilies: Corydoradinae (which is represented solely by the genus Corydoras in Uruguay) and Callichthyinae, which includes our Callichthys and Hoplosternum cats.

Corydoradinae: The Loveable Cories

Corydoras is the largest South American freshwater fish genus, with about 142 species described at the moment. These immensely popular aquarium fish are primarily bottom feeders. In aquariums, they are more than just beautiful fishes; they are very useful for consuming uneaten food. Despite their helpful feeding habits, they should not be kept solely for the benefits provided by their scavenging talents.

Cories will appreciate any small, sinking aquarium foods offered. Many wild-caught Corydorasarrive in a weakened and emaciated state, and generous feedings of sinking, meaty foods will be required to nurse them back to health. They have sensitive barbels and should be placed in aquariums containing a soft, fine-grained sand substrate. Sharp gravel will definitely injure their delicate mouths. There is a plethora of fantastic patterns to choose from, such as C. pantanalensisC. weitzmani, and C. gossei. Among the classic species, few are better known then C. paleatus and C. aeneus. Both species are quite variable due to their wide range, and they can be collected in Uruguayan waters.

Corydoras paleatus

Corydoras paleatus is quite abundant within the entire Uruguayan territory. Some populations of freshly caught specimens may be a brilliant, iridescent green with black-and-white banded fins. Individuals will sometimes have a greatly enlarged dorsal fin. The Uruguayan populations of C. paleatus are some of the most attractive forms of this fish.

This cory can be found at night in large congregations. It is quite a spectacle to see these giant schools under the illumination provided by a camping lamp, which is particularly enjoyable when camping next to the river during the summer season. During the day, these great concentrations disperse, and they are usually found in small groups of around five or ten fish. It is obvious that these (and all cories) are highly social creatures that should never be kept individually in the home aquarium. A minimum of five cories should be purchased in order to maintain their social habits.

Three additional cories can be found in the north of Uruguay. These are C. aeneusC. undulatus, and C. hastatus. All of these species, as well as C. paleatus, can be collected in the wetlands and shallow lakes along the banks of the Rio Uruguay.

Corydoras aeneus

The bronze cory C. aeneus is a highly variable catfish. Depending on the population, this species can be white, yellowish, or gleaming bronze—with or without metallic green, orange, or red designs along their flanks. We have sampled specimens from Bella Union in the north and found that these C. aeneuslook similar to the standard types found in the United States aquarium trade. Here, the little catfish have a bronze base color with dark charcoal flanks. We have also found gigantic specimens in the Caebal wash site by the town of Salto. These were extremely robust 3-inch fish with spectacular bronze, orange, and gold coloration.

In this cory species, a new reproductive method was first described. This still-controversial finding was that of an oral insemination process thought by some to exist in the entire Corydoras genus. It consists of the female ingesting the male’s sperm during mating. The female attaches her mouth to the male genital opening, and the sperm passes through the female’s digestive system. Fertilization occurs when the female vents the sperm along with her eggs.

Corydoras undulatus

Here is a very rare fish to find in Uruguay. We collected this species in Bella Union in the summer of 2008. Attesting to the rarity of this species in Uruguay, only a single specimen was caught during the most recent expedition. C. undulatus is an aptly named species as it has wavy, undulating patterns over a light greenish-gray body.

Corydoras hastatus

The smallest cory species found in Uruguay is C. hastatus. This is a tiny one-inch schooling species from the northern lakes and wetlands. C. hastatus frequently form great schools with the similarly patterned Cheirodontinae tetras; both species have clear bodies with a large black spot on the caudal peduncle. It is a treat to observe these fish schooling through the plant thickets of their natural environment. These dwarf cories spend more time swimming in the mid-water column and should be kept in groups of at least 10.

[Next month Ivan and Stan continue their report on Uruguayan callichthyids and complete this account with a discussion of the remaining catfish groups, including the infamous trichomycterids!—Eds.]

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