Spined and armoured, pet catfish that are too large are sometimes released into the wild to cause mayhem. Rupert Collins identifies a group that needs plenty of room and responsible ownersâ€¦
Just about every fishkeeper has at some stage kept a Common plec. For some, the ‘plecostomus’ provided the introduction into the mysterious world of L numbers and other exotic catfishes.
For most though, these ubiquitous suckermouths are purchased simply as a cleaner of uneaten food and algae problems.
All too frequently and quickly these 5cm/2” additions become 20cm/8” beasts. In best case scenarios the owner upgrades the tank, the fish are returned the shop — or they simply die in cramped, polluted conditions.
However, some owners, perhaps unaware of such irresponsible or cruel action, release their former pet into a local waterway. In March 2009 a 25cm/10” Sailfin plec, (Pterygoplichthys gibbiceps) floated up dead in a Leicestershire canal. However, when released into warmer climates, these plecs find their new homes to their liking.
In all cases the ornamental fish trade is implicated. As ever more countries report invasions of these armoured catfishes, fishery managers and ichthyologists are only just realising the ecological effect of these introductions. For example, Florida manatees have been observed covered with up to 40 grazing Pterygoplichthys, which may have a serious impact on these rare mammals.
Given an adult size of over 50cm/20”, the kind of set-up appropriate for Pterygoplichthys spp. is beyond the means of the average fishkeeper who often purchases them for tanks as small as 40 l/10 gal.
Many UK stores now no longer stock Pterygoplichthys, and in some, re-homed fish are kept in sump tanks to illustrate just how large and unsuitable they finally become.
Staff at good shops will recommend and stock alternative algae grazers more suitable for home aquaria, such as Ancistrus, Otocinclus, Parotocinclus, Crossocheilus and Garra. Hopefully this attitude will filter through to fishkeepers and retailers in tropical and sub-tropical countries where survival and proliferation of released fish is far more likely.
What is a Common?
To most of us, each nondescript grey/brown plec looks much like any other, which is why they are all lumped together as Common plecs.
The Common plec has, in fact, not been the same species through history, as collection areas have shifted, but people treated these catfishes as the same thing — no matter where they came from.
Early imports from Venezuela, Trinidad and Surinam may have comprised Hypostomus robinii and H. plecostomus, and the latter was probably the first to appear in 1960s hobbyist literature as the Common plec, or just Plecostomus.
Common plecs remain incorrectly identified as H. plecostomus — including that fish in Leicestershire — and it was H. plecostomus that lent its name to the simplistic term ‘plec’ to describe the whole group of these suckermouths.
Subsequent exports from Colombia, Peru and Brazil have seen species, such as H. plecostomoides, H. punctatus and H. sp. ‘L 266’ appear. These brown Hypostomus are infrequently seen in the trade these days though, but can be found as bycatch of other species. Without capture localities they can be tricky to identify.
Since the early 1990s, new Common plecs have arrived and these belong to the genus Pterygoplichthys. P. pardalis and P. disjunctivus are the true Common plecs, but, due to frequent confusions, two other oft-encountered species (P. joselimaianus and P. gibbiceps) are also described here.
In addition, of the 15 valid Pterygoplichthys species, a few others are occasionally seen, including P. multiradiatus (Common plec), P. ambrosettii (Snow king plec), P. weberi (Imperial ranger plec) and P. scrophus (Chocolate, Rhino or Alligator plec).
Looking after Pterygoplichthys is generally simple and much the same for all species, but the biggest hurdle is accommodating them in large enough aquaria. Although these catfishes are not particularly active, their sheer size dictates tanks of many hundreds of litres.
As they are greedy eaters, heavy duty external power filtration is vital. They broadly tolerate poor quality water, but you must avoid extreme conditions and provide clean water. A neutral pH, medium hardness, and a temperature at around 26°C/79°F is ideal.
Best tank mates?
Tank mates can be chosen from almost any fish that thrives in the same conditions, such Neon tetras or Tinfoil barbs.
Pterygoplichthys are often kept with large or aggressive cichlids, as they are armoured and able to withstand the cichlids’ attentions.
Expect skirmishes with other large catfishes though, especially if you provide insufficient hiding places. An uncomplicated décor format works best, so provide bogwood, rocks, and ceramic or plastic pipes for shelter.
What to feed
These are generalist omnivores and scrape biofilm (aufwuchs) from hard surfaces with comb-like teeth. Any commercially prepared staple such as algae wafers are consumed, but provide extra fibre with vegetables daily.
Popular options include courgette, cucumber, mange tout, sweet potato and broad beans. These can be lightly blanched and then frozen for later use.
How do these catfish breed?
Pterygoplichthys spawn in burrows dug into soft clay or mud banks. These are usually about 1m.3.3’ long, straight and with a gentle downward slope. These tunnels are excavated and guarded by the male, although it is not clear exactly how they are dug.
Several spawns can occur each year and have up to 7,000 eggs in each one.
Spawning has not been widely, if ever, reported in the aquarium, as probably few fishkeepers have bothered trying to do so.
Simulating the required biotope would be difficult and obviously a huge tank would be required. They are bred commercially, however, in purpose-built clay ponds and harvested as a food fish in the Amazon, with the eggs being particularly prized.
The two 'true' Commons
Pterygoplichthys pardalis: In recent years P. pardalis has been the most common Common plec seen on sale, due to some competitive prices attached to farmed fish then imported in bulk from South-East Asia.
As is the trend with mass-produced fishes, albino and longfin varieties are also available. The albino can look similar to P. scrophus at first glance, and may even be sold under the same trade name of Chocolate plec.
P. pardalis was described in 1855 by Castelnau and the type locality was recorded rather ambiguously as ‘the Amazon’.
There exists Liposarcus varius and L. jeanesianus, two synonyms of this species, and these were discovered from the Ambyiacu and Marañon rivers in the upper (western) Amazon basin.
Some sources may also list the generic synonym of Liposarcus, but Dr Jonathan W. Armbruster, in his study of loricariid intra-relationships made in 2004, could not find reliable characters to diagnose Liposarcus and Glyptoperichthys, so for now he treated them in the synonymy of Pterygoplichthys.
Ingo Seidel, in his Catfish Atlas, reported habitat data from an expedition to the Yarina Cocha, a huge oxbow lake of the white water Río Ucayali in Peru.
P. pardalis were in abundant numbers there and found alongside angelfish, Festive cichlids, several doradids, Pterygoplichthys scrophus, Hypoptopoma sp. and Loricaria simillima.
Dry season water parameters of a tributary creek there were recorded at 28°C/82°F, pH 7.0 and conductivity of 74 µS/cm.
P. pardalis appears to have a wide distribution and has been documented from the upper, middle and lower Amazon river, as well as numerous tributaries.
A population collected from the confluence of the Rio Araguaia and Rio Tocantins was presented by DATZ as L 021, but this variety is highly unlikely to be a viable export commodity as farm-raised juveniles cost a fraction of the wild-caught price.
Pterygoplichthys disjunctivus: While P. pardalis has been reported throughout most of the Amazon basin, P. disjunctivus is known from the Rio Madeira drainage in Brazil.
While the majority of Common plecs appear to be P. pardalis, P. disjunctivus are also farmed and sold with regularity, although the differences do not often manifest themselves until the fish are sub-adult.
No distinction is made between the two species in the trade and reports, however unconfirmed, suggest that they may be hybridised at commercial farms.
Pterygoplichthys joselimaianus: This has only recently been seen in stores, but almost as ubiquitous as P. pardalis. Described from a tributary of the Brazilian Rio Tocantins system, P. joselimaianus grows smaller, at around 30cm/12” SL, than the other Pterygoplichthys described and is a better proposition for home aquaria.
Rarely sold as Common plecs, they are more often traded as Gold spot plecs or as L001.
Pterygoplichthys gibbiceps (pictured above): From a wide distribution throughout the Amazon basin and several colour/pattern forms exist. Commonly known as the Gibby or Sailfin plec.
These can be truly impressive and large specimens are hard to confuse with anything else, due to their enormous dorsal fin.
A mature fish makes a fantastic display in an appropriate tank.
Pterygoplichthys multiradiatus: Described from the Essequibo drainage of Guyana, this has also been recorded from the Orinoco. There’s no reason for its absence in the trade, but of the confirmed identifications most appear to be P. pardalis. Due to similarity to P. pardalis and P. disjunctivus, a diagnosis on these pages should prevent misidentifications.
Expert Q and A
Krista Capps is at Cornell University, New York, studying freshwater ecology, focussing on the effects of introduced suckermouth catfishes in Mexico.
Pterygoplichthys are found in what countries?
Exotic plecs have been officially documented in several countries, including Mexico, Puerto Rico and several states of the United States. Several Asian countries have also been affected, including Singapore, Peninsular Malaysia, Java, Sumatra, Philippines, Vietnam and Bangladesh. I’ve also been told that exotic plecs have invaded Thailand, Australia, China and South Africa.
How have they affected natural biota and various fishing communities?
Exotic plecs have been linked to freshwater fishery decline in Mexico and certain other countries, but the causes are yet to be fully understood.
If allowed to do so, these fishes become highly invasive and can reach incredible densities, so they may be crowding out the native residents.
The plecs feed on sediment and algae, so do not represent a predatory threat to adult fishes.
However, they may disrupt the nests of other species or consume the eggs of other fishes.
Feeding plecs may also indirectly compete with native fishes by displacing their invertebrate prey, or directly competing with native grazing fishes via the consumption of algae and sediment.
How have they become such successful invaders?
They are medium-sized, bony-plated fishes covered in spines and Pterygoplichthys, the predominant invasive plec in Mexican waters, breathes air and can live out of water for long periods.
Additionally, the males excavate nests and guard fry until they have reached larger sizes and can potentially avoid predation themselves.
In many countries, exotic plecs are not caught to be consumed by humans as many fear they are poisonous. Populations of these fish are therefore not monitored or controlled.
How can they be stopped?
Controlling invasive plecs can be difficult. Because they can breathe air and tolerate environmental changes, chemical control methods may not be effective. Fishing can be effective, but not economically viable for subsistence communities without governmental support.
The best method is to prevent introduction through policy making and education. Governmental agencies, policymakers, and resource managers should consider the invasion potential of these fishes and create policies that regulate their import and sale.
Krista Capps also runs plecoinvasion.org, a website raising awareness about the risks of non-native loricariid introductions.
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