Serrasalmus rhombeus

Black Piranha/Rhom

Classification

Characidae. Subfamily: Serrasalminae

Distribution

This species is widespread in nature although the true extent of its range is an issue for debate. Officially it’s been recorded from Venezuela, Guyana, Suriname, French Guiana, Colombia, Ecuador, Peru, Bolivia and Brazil in various rivers including the rios Amazon, Orinoco, Negro, Xingú and Araguia. However there is evidence to suggest that it may be confined to Guyana with collections at other localities perhaps comprising other species.

Habitat

Inhabits a range of biotopes but as an adult is particularly associated with larger, deeper river channels where it’s typically found hunting in deep water or near rapids. Juveniles are most often collected in quieter areas with dense submerged or marginal vegetation. Interestingly the fish tend to vary in colour depending on the type of water at a given locality with those collected from turbid white waters tending to be much paler than forms occurring in clear or black water habitats.

Maximum Standard Length

Some forms grow larger than others with the largest attaining 400 – 450 mm.

Aquarium SizeTop ↑

This is an active, riverine species. A single, fully-grown adult would need a tank measuring upwards of 210cm x 45cm x 45cm to itself.

Maintenance

Requires a well-oxygenated set-up, ideally furnished with a sandy substrate and some driftwood roots and branches to provide cover. You can attempt to grow aquatic plants if you wish but don’t be surprised if the fish nip at them. Water quality must be of the highest order meaning weekly water changes of up to 50% should be considered mandatory.

If possible the heaterstat should also be situated externally since serrasalmids do sometimes bite equipment located inside the tank. It’s possible to buy external filters with built-in heater elements or inline units that can be installed the filter pipework, or at the very least a sturdy heater-guard should be fitted.

Water Conditions

Temperature: 72 – 82°F/22.2 – 27.8°C

pH: 4.5 – 7.5

Hardness: 1 – 20°H

Diet

Wild fish are opportunistic, feeding on the fins and flesh of other species as well as smaller fishes, insects and crustaceans. They have also been recorded scavenging the carcasses of dead animals including humans although reports of this species attacking live people are mostly attributed to increased aggression during the breeding season.

Some Serrasalmus have been shown to eat nuts, fruits and seeds and small amounts of plant material have been found in the gut of S. maculatus during laboratory analyses. In the aquarium most individuals can be weaned onto dead foods over time although some seem to find it trickier to adjust than others and may refuse to feed initially. A period of starvation may be necessary, eventually giving the fish little choice but to accept what is offered. This is especially true of larger or recently-transported specimens.

Once acclimatised juveniles relish live or frozen bloodworm, Tubifex, Artemia, chopped prawns and similar foods. Adults should be fed correspondingly larger items, such as whole mussels, cockles, prawns, chopped squid, whitebait and earthworms. Once the fish reaches adult size it need only be fed two or three times a week.

This species should not be fed large amounts of mammalian/avian meat such as beef heart or chicken. Some of the lipids contained in these meats cannot be properly metabolised by the fish, and can cause excess deposits of fat and even organ degeneration. Similarly there is no benefit in the use of ‘feeder’ fish such as livebearers or small goldfish which carry with them the risk of parasite or disease introduction and at any rate tend not have a high nutritional value unless properly conditioned beforehand.

Once acclimatised juveniles relish live or frozen bloodworm, Tubifex, Artemia, chopped prawns and similar foods. Adults should be fed correspondingly larger items, such as whole mussels, cockles, prawns, chopped squid, whitebait and earthworms. Once the fish reaches adult size it need only be fed two or three times a week.

This species should not be fed large amounts of mammalian/avian meat such as beef heart or chicken. Some of the lipids contained in these meats cannot be properly metabolised by the fish, and can cause excess deposits of fat and even organ degeneration. Similarly there is no benefit in the use of ‘feeder’ fish such as livebearers or small goldfish. Risks involved with these include the possible introduction of disease or parasites.

Behaviour and CompatibilityTop ↑

Should only be considered as a specimen fish for the enthusiast since it doesn’t make a good tankmate for anything else. Scattered reports of it coexisting with Loricariids, armoured catfish and in conspecific groups should be considered tenuous at best given its carnivorous nature. Similarly there is little point attempting to keep two or more of these together since it’s not a gregarious animal by nature and conspecifics are liable to fight until only one remains.

Sexual Dimorphism

No external sexual differences between the sexes have been recorded although females in spawning condition are usually plumper than males.

Reproduction

This species has only rarely been bred in captivity with the main restriction appearing to be the volume of water required to avoid the fish murdering one another. Breeding is not known to have occured in the hobby but one particularly well-recorded event occured at the Duisburg Zoo in Germany in the 1970s. The fish spawned in a 1200 gallon, heavily planted tank, deposited their eggs among a patch of aquatic moss and were subsequently observed to defend the surrounding area. The eggs were removed and the fry raised separately with some returned to the adults’ tank after a year. Apparently these were tolerated until they reached about 6″/15cm in length at which point their parents began to attack them.

NotesTop ↑

There are in excess of 30 described species of Serrasalmus many of which appear superficially similar and have confusing taxonomic histories. S. rhombeus is among the most convoluted of all with numerous synonyms of which S. niger is the most well-known. There has also been a muddle surrounding the common names “white” and “black” piranha, both of which have been used to describe the species.

Of greater interest to both scientists and hobbyists has been the discovery of numerous geographical forms, some of which vary considerably in size, body form and colouration. The primary feature linking these is the deep red eye that characterises mature specimens and the existence of a species complex has been suggested by several authors. In addition some species in the genus (e.g., S. aureus, S. hollandi) were described using what is now considered to be insufficient data and may turn out to be synonymous with S. rhombeus.

Juveniles are particularly tricky to identify correctly as they lack the red eye until they are around 5″/12.5cm in length, and share their spotted patterning with some other species. We’ve seen young S. rhombeus mislabelled as S. marginatus, S. sanchezi and S. compressus among others. To illustrate the difficulties involved, the specimen in the second image above was sold as and confirmed by several experienced piranha keepers as S. marginatus but turned out to be an S. rhombeus variant after more than a year in captivity.

Some of the forms are highly-prized in the hobby and have been assigned specific vernacular names. Ostensibly implemented to ease identification, these have instead added to the confusion, with exporters/dealers frequently attaching false locality details to fish in order to increase their value. The more commonly seen ones include the rio Xingú form, the so-called “Diamond” forms and the “Jet Black/High-back” Peruvian fish with its characteristically deep body. Fish from Venezuela are often referred to using the nickname “Vinny” by enthusiasts.

Most experts agree that a detailed revision of Serrasalmus is necessary since historically the genus has been viewed as something of a “catch-all” for similar-looking fin-biting/predatory characins for decades and has long-been regognised a polyphyletic grouping. There have been several attempts to reorganise the group with the most recent major revision published by Géry in the late 1970s but a handful of new species have been described since then and an exhaustive molecular analysis of member species remains lacking.

The family Serrasalmidae consists of over 60 species in 16 genera and also includes the seed and fruit eating tambaquis and pacus among others. Their characteristic features include a compressed body shape, long dorsal fin with 16 or more rays and possession of sharp serrae on the ventral surface formed by modified abdominal scales. The number of the latter is variable, ranging from 6-9 in Acnodon to over 60 in Piaractus spp..

Members display three main feeding habits: carnivory (flesh-eating), frugivory (fruit and seed-eating) and lepidophagy (eating the scales and fins of other fishes). Carnvirous species normally possess a single row of tricuspid teeth on each jaw, frugivores tend to have two series of incisor or molariform teeth on the premaxilla, one row of teeth on the dentaries, and often a pair of symphyseal teeth, while in lepidophages the teeth are tuberculated and located on the outer side of the premaxilla.

Based on the results of their mitochondrial DNA-based phylogenetic analyses of serrasalmids Ortí et al. (2008) recommended that member genus Pristobrycon be restricted to contain just the single species P. striolatus with all other members placed in Serrasalmus, to which they are more closely-related, but this concept appears not to have achieved general acceptance at time of writing.

In South America only Pygocentrus species are referred to as “piraña“, with Serrasalmus and other related genera being referred to by other names such as “Pirambeba“. Serrasalmus species are therefore not considered to be “true” piranhas, the name being applied to them by the English-speaking world and aquatic hobby.

It should also be said that serrasalmids are not the fearsome “monster fish” as often depicted by the media. Obvious care must be taken when performing tank maintenace but these fish will usually only bite when threatened, and they can be quite skittish in an aquarium setting unless their rather specialist needs are catered for. Most also live in excess of ten years and become less active as they mature so careful thought is required before purchasing one as they represent a considerable investment.