A fishkeeper's guide to Disc characins
When housed in a shoal, Disc characins, like the Silver dollar, are a truly stunning sight, says Richard Hardwick, who also warns that they will strip your tank bare...
really are the aquatic version of locusts. These attractive fish form huge shoals in the wild, and with their laterally compressed silver bodies, cut through the water at high speed and scavenge on basically anything - though aquatic plant life is preferred. I am referring to the Disc characins from the Amazon and its tributaries. Not surprisingly, perhaps, these disc-shaped fish are the closest living relatives of the piranha, and their genetic make-up is so similar that they are often mistaken for the carnivores. However, their jaws - although powerful - are largely made up of rows of molars; more appropriate for its omnivorous life. Apart from their obvious body shape, their eyes are large, round and clear, and their extremely fine scalation glistens through a heavy mucus membrane. They are gaining in popularity for they also co-habit well in large set-ups. If you are tempted to keep these fish, note that they are only suited for tanks of 136 l./30 gal. or more. Anything smaller and you run the risk of injury as they are very skittish. So jumpy are they that before switching on their tank lights, it is better to turn the room lights on to prepare them. Despite this tendency to panic, these are remarkably tough cookies. However, because of their huge appetites, water parameters can quickly change - sudden high levels of nitrate coupled with a low KH will prove disastrous. Symptoms of distress include a loss of appetite, cloudy eyes and rapid gill repetitions. Frequent water changes and close monitoring of parameters are critical.All species of this family do well on a diet of floating food sticks, preferably with a vegetable content of spirulina. This should be supplemented with some animal protein such as bloodworm, chopped earthworms, mussels, prawns, etc.As far as the tank goes, apart from being large, you can simulate an Amazon biotope using twisted roots or beech. Sand looks best, with the occasional smooth boulder to complete the theme. As for plants, you'll have to go for the artificial variety or you'll just be providing free snacks. However, before you wince at the thought of fake plants, have a look at what's available in shops - I think you'll be pleasantly surprised.These fish need good water quality and plenty of oxygen. Use externals as opposed to internals to give that extra biological capacity. Keep temperatures to 24-25.5?C/75-78?F, and ensure the water is well oxygenated.
Silver dollar, Metynnis argenteusThis is by far the most commonly available member of its genus - it's so popular that it is spawned and raised in fish farms purely for the aquarium trade. Available in sizes from as small as a ?1 coin, they can grow alarmingly if fed the correct diet. You'll really need to buy three or four and house them in a tank minimum 120cm/4'. Very often, fish of about 13cm/5" are brought into the shop as trade-ins for smaller species.The Silver dollar is found in the Rio Negro and its tributaries among weedy backwaters. Males have a long, lobed anal fin which turns red with sexual maturity. The body, as its name implies, is plain, silvery white with clear finnage, though a few black flecks are sometimes present in the dorsal fin.A very similar species is M. hypsauchen, though this species has a less rounded body and the males a yellow-orange tint around the operculum when adult.
Spotted Silver dollar, Metynnis lippincottianusThese share exactly the same characteristics as the above, but also have beautiful body spots which give it its common name. It is sometimes called M. maculata. However, it is very rarely imported and sports fewer and larger spots. It is found in Brazil and French Guiana and reaches 12cm/41/2".
Red shoulder spot dollar, Metynnis molaThis is another gem to reach these shores. It grows to the same size as its spotted cousin, but its body is deeper and more rounded. Its colour is so bright that it almost looks white. It is found in the Rio Paraguay-Parana basin.Studying these fish in our shop recently with some colleagues, we came to the conclusion that males, with the broader, more round anal fin, had a darker red spot than the females.It gets on alright with its own kind, but is said to be slightly quarrelsome with other species of Disc characins.
Striped silver dollar, Metynnis (hypsauchen) fasciatusI have been very lucky to have kept these fish a couple of times in recent years as these are rare imports - grab them if you ever see them in shops!It grows slightly larger than the others here, to about 14cm/51/2". Males can be distinguished as they have a longer anal fin which finishes with a hook at the end.Let's now look at the larger members of this fascinating family, from the genus Myleus. These are larger than Metynnis and grow to 18-20cm/7"-8", though the main differentiating aspect is that with Myleus, the dorsal is long with a tiny adipose fin whereas with Metynnis, the dorsal is shorter but with a longer adipose fin.
Red hook, Myleus rubripinnusHailing largely from Guyana in the Amazon river system, this species has mighty jaws, indicative of their preference for seeds and nuts. I raised a group of Red hooks over a four-year period. For the first two years, there was no sign of orange spotting on the flanks. However, since becoming fully fledged adults, the bodies of both sexes have become heavily decorated with orange spots. Sex is determined by a wave-like anal fin in males, referred to as a bilobed anal fin.
Myleus schomburgkii
This has a single vertical black bar that starts in the middle of the dorsal and finishes in front of the anal fin. A shoal swimming among twisted roots and branches creates the most wonderful of Amazonian themes along with a shoal of Pimelodus pictus and Leporinus.There are currently three regional variants of the species. Two to me look very similar, but I suspect that the first will soon be described as a different species. This variant sports blue-white finnage with an extremely broad vertical vertical band while the other two have a red anal fin and a narrower black band.The species is however only imported from time to time, and is collected from Brazil, notably the Rio Negro, Xingu, Araguaia, Venezuela and the Orinoco.It is common for these fish to be imported with what looks like black spot disease. These black spots are scattered over body and fins and are smaller than a pinhead. This is also common in wild-caught Angels and geophagines. Although unsightly, these aren't really a problem - they are probably just "glochidia" - a larval stage of a freshwater mollusc. What happens is that free-swimming larvae attach themselves to a host (the fish) and wait until it is swallowed whole by a particular bird. The mollusc larva then travel through the bird's digestive system, which triggers metamorphosis, before finally being excreted in the bird's faeces back to the river bed... So you don't have to worry about clams appearing in your tank! As the fish grow, these spots will become less as layers of skin tissue flake off. So don't let this problem get in the way of your enjoying this fish - unless you have a kingfisher or fish eagle in your living room.
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