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The Diversity of Catfishes
Although many families of fish have more beautifully coloured representatives than the catfish, (at least to the eye of someone who is not already a confirmed catfish fan), few families of fish show such diversity in their body forms. There are catfish living in nearly every continent, and catfish adapted to utilise a wide variety of living conditions. From the familiar bottom-dwelling loricariids, finely adapted to grazing and holding their place in fast currents, to giant midwater predators, from the gentle and endearing Corydoras to rapacious Wels, catfish have evolved to take advantage of practically every lifestyle available to fish. A tiny catfish, Phraetobius sp., even lives in the leaf litter banks of Amazon blackwater tributaries. The tiny blood-red, wormlike catfish inhabits the damp leaf piles along with various other tiny fishes including a small goby, avoiding the dangers of entering open waters and probably feeding on miniscule invertebrates.
Although to the casual observer catfish may seem to be weird and wonderful shapes, or coloured so as to make them stand out like a sore thumb, once you know how and where the fish naturally live their lives the colours and forms no longer seem so outlandish. In an aquarium the black and white striped zebra plec (Hypancistrus zebra) may be outstandingly obvious, but among the rocks of the clearwater Rio Xingu, with patterns of light and shade from light refracting through the water, such colouring no longer stands out. Given that body form and colouring have evolved so that the fish are adapted to their chosen environments, it is unsurprising that fishes in similar environments have developed similar appearances. Highly contrasting spots and stripes are features shown by a number of loricariids, not just the zebra plecs. Among the Corydoras many species are almost duplicates of each other, sharing almost identical colour patterns, body shapes, and habitats. These ‘twin species’ were first recognised by Nijssen and Isbrucker in 1980, during taxonomic research. The first species examined was Corydoras oiapoquensis (Nijssen, 1972), and an examination of 190 specimens revealed 29 individuals with long straight snouts instead of the blunt rounded snout of C. oiapoquensis. These long snouted individuals then became part of the type material used to describe a new species, C. condiscipulus. Since the original pair of species was noticed, many other Corydoras species have been described sharing almost identical patterns and forms: Corydoras sterbai and C. haraldschultzi from the Rio Guapore, Corydoras adolfoi and C. imitator from the upper Rio Negro tributaries, Corydoras metae and C. simulatus from Colombia, and many more examples.
Although the similarity between some species of Corydoras is very striking, since these are closely related fish sharing the same locations it is not all that surprising. However, another group of fish sometimes share the same habitats as Corydoras species, and have come to resemble their neighbours to such an extent that they are known to aquarists as ‘false Corydoras’. These scaleless pimelodids grow to approximately 10cm, and various species are found sharing colour patterns and habitats with different Corydoras species. The first was described by Myers in 1927 as Brachyrhamdia imitator, which inhabits the Orinoco system in Venezuela along with Corydoras delphax, and is most often imported for the aquarium trade by accident as an individual in a shoal of the Corydoras. Pimelodella rambarrani was described by Axelrod and Burgess in 1987, sharing colour patterns and habitats with Corydoras duplicareus. Brachyrhamida meesi and Brachyrhamdia marthae were both described by Sands and Black in 1985. Myers, who originally created the genus Brachyrhamdia. later decided that they correctly belonged in the Pimelodella genus, and all the ‘false Corydoras’ referred to above are now correctly Pimelodella sp.
Although these Pimelodellas look very like the species of Corydoras with which they share their habitats, they are dissimilar in behaviour. They do not appear to be shoaling fish, and have been noticed to be aggressive to Corydoras when food is around; aquarists who have kept a Pimelodella with a group of Corydoras have also reported minor damage, such as nibbled fins, to the Corydoras. So the Pimelodellas may not only gain the same advantage as the Corydoras from a pattern that makes them almost invisible, when stationary, in their chosen habitat, but also benefit directly in a variety of ways from swimming with the peaceful Corydoras shoal. Advantages gained by such behaviour may include safety in numbers from predation, easier finding of food (whether by chewing their neighbours fins or by passing prey that does not realise the passing shoal of gentle Corydoras contains a predator. The behavioural relationship between the ‘false Corydoras’ and their associated Corydoras species has not been fully studied or explained, and remains an area in which aquarists may well be able to provide valuable information by observation.
It is not only catfish that share the same waters that share body forms and colouration with other species. The Loricariids which have evolved with their sucker mouths to cling onto rocks and food in fast moving currents have counterparts in Asia among the completely unrelated loaches which also have evolved sucking mouths for fast waters; a body form that works in a given environment often can be found in fish that have developed independently in similar circumstances. Similar parallel developments can be seen among the ‘livebearers’ – many of which are completely unrelated, such as halfbeaks and guppies.
Such an example of parallel development between two unrelated catfish can be seen in the ‘glass’ catfishes. The Asian glass catfish, Kryptopterus bicirrhus, is well known to aquarists and is a member of the Siluridae. These shy shoaling fish are midwater swimmers, approximately 10cm long, whose most noticeable feature is the almost complete transparency of their body. They have a long anal fin, no adipose fin, and a forked tail. The dorsal fin is a single small ray, which gives them their latin genus name – kryptopterus means hidden fin. They prefer a diet of live foods, although in the aquarium will take flake foods if it is moved by a current, presumably thus fooling them into thinking it is alive. The internal organs are clearly visible through the body, including the beating heart, although after death the fish turns a milky white. They are atypical catfish, in that they not only prefer life in midwater but are also diurnal, making them ideal for aquarists who prefer not to have to wait until lights out in the hope of seeing their catfish.
A completely unrelated family of catfishes are the Schilbeidae, which are also commonly known as the glass catfishes. Although the Schilbeidae inhabit both Asia and Africa, one of the closest parallels to the Asian Kryptopterus bicirrhus can be seen in the African Physalia pellucida. These fish are about 10cm long, shy midwater shoaling, diurnal, with a long anal fin, forked tail, no dorsal fin, and is also transparent! To finish off the resemblance between the unrelated Kryptopterus and Physalia, both habitually hang in the water with their tails angling downwards in an almost identical pose.
Catfish adapt to a variety of conditions, but one of the most remarkable behavioural modifications must be deciding to swim upside-down. The most well-known upside-down catfish, the African Synodontis nigriventis, swims constantly upside-down, and its colouration has changed to suit its upside-down life. Most fish are dark on the back and light on the stomach. Predators above looking down at the dark water are less likely to spot a fish with a dark back, but a fish with a dark stomach would show up clearly against the light to a predator swimming below and looking upwards. The upside down catfish has reversed this colour scheme so as not to be obvious to predators – it has a dark stomach, visible from above, and a pale back, visible from below. This upside down behaviour is occasionally shown by various other Synodontis species, although only Synodontis contractus swims upside down habitually rather than occasionally. This fish is seen far less often in the aquarium trade, but also appears under the name of upside-down catfish.
The only other fish in the world that also swims habitually upside down is also a catfish, although a completely unrelated one! The Asian Upside Down Catfish, Mystus leucophasis, is rather less desirable as an aquarium inhabitant than the peaceful 10cm upside-down Synodontis. Mystus leucophasis can grow to a foot long, is black in colour, and a quarrelsome predator that will eat any small fish that it finds.
As a family, catfish offer the aquarist some of the most interesting observable behaviour of any family of fishes, given their remarkable diversity. Although such diversity interests the observer, however, such developments have evolved for specific reasons (even if those reasons still remain unclear to us in some cases). It is noticeable, not only in the catfish but in many other fish and other animals, that a good idea is often repeated independently by nature, and a particular habitat and lifestyle often results in similar developments, even when the inhabitants are separated both biologically and geographically.
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