Piranha Anatomy 255n55

Piranha anatomy

Special thanks to Frank Magallanes (OPEFE) for helping me out with this image

As mentioned, piranha's can be divided into a number of different genera, all with different visual traits. from the genus Pygocentrus are all recognizable by the convex shape of their head and massive bulldog-like lower jaw (more powerful and muscular than most Serrasalmus-species). This reflects their diet: besides being scavengers, if necessary all Pygocentrus-species are full blown predators as well, that actively give chase to their prey. Serrasalmus-piranha's have a more concave head shape, and less powerful lower jaws. A number of species feed themselves mainly on fins and scales of other fish, and even nuts and fruits, and therefore they do not need the same muscle packed lower jaw to rip through skin, muscle and bone. This does not apply every species of this group, however: Serrasalmus rhombeus, manueli and elongatus, to name a few, are true predators when adult, and do have very massive and powerful jaws as well. But regardless of their diet, all Serrasalmus-species share the piranha's unmistakable trademark feature, the triangular, razor sharp teeth: large ones in the lower jaw and smaller ones in the upper jaw. When the mouth is closed, the teeth from both jaws fit exactly, comparable to a bear-trap. This enables them to slice off pieces of meat or fins or scales, literally taking apart their prey piece by piece.

Convex head, typical for the Pygocentrus group

Concave head, typical for the Serrasalmus group

All piranha-species have a powerful, high, thick but laterally compressed body shape, with keellike edges running over the upper part of the body from head to dorsal fin, and on the lower body running over the belly. Together with a large and powerful large tail and a body covered with very small scales, their streamlined bodies make them very fast and agile swimmers. The oddball in the family, shape wise, is Serrasalmus elongatus, the Elongated or Pike Piranha. This species has a salmon-like, elongated and slender body, but with the same powerful, well developed tail. It is said to be the fastest swimmer of all piranha species. Unlike many fish-species, piranha's have a small adipose fin between tail and dorsal fin. This feature is characteristic for the Characin-family, although from some other families, like catfish, have an adipose fin as well. Their predatory lifestyle is reflected by large eyes and a large nose with big nostrils to maximize the water inflow. They have a very acute sense of smell: in their natural habitat, murky rivers in South-America, even more darkened by overhanging vegetation, scent is their main way of tracking down their prey. Determining the gender of piranha's is considered almost or all together impossible by most piranha experts, because there are no visible differences between the genders (in other words, piranha's are not sexually dimorphic). The general consensus is that the only more or less foolproof method to sex piranha's is to observe them during spawning. It is true that adult female specimen tend to be thicker due to the eggs they carry, it nonetheless is an unreliable method to tell both genders apart, as well-fed males are often just as thick. The only known exceptions to this sexual dimorphism are the Wimpel Piranha (Catoprion mento), of which males and females are easy to tell apart, and possibly the Five-Cusped Piranha (Pygopristis denticulata).