Mimicry and Fishes

etragonopterus argenteus

Serrasalmus species

As a young boy first learning about piranhas, I was amazed how close certain species of little tetras resembled those ferocious piranhas. Gymnocorymbus ternetzi, was the first species of black tetra that I thought looked the closest to a piranha-like fish. Later, as I became more involved in studying piranhas, I realized many of these juvenile piranhas took on a type of mimicry of their harmless relatives. Who hasn't seen a chameleon? You know this reptile is capable of viewing in almost all directions with its eyes. It is also capable of changing its body color to blend in with the surrounding. Some fishes are capable of doing the same thing. To be "seen or not seen" is a matter of life or death in the underwater world of fishes. Piranhas and their associated forms use a form of mimicry to keep from being eaten. They also us this ability along with stealth to snack on a fishes fins. Juvenile piranhas are able to blend in well with small characins (tetras) that it sometimes takes an expert to separate the harmless from the wolf in sheep's clothing! They take on the colors and fin markings common to many of our smaller favorite tetras. Juvenile Piranhas also play dead on occasion. Many hobbyists have been startled to see their new baby piranha float motionless on the bottom of their tank. They believe their fish has just died from introduction to its new environment. A slight tap on the fish and it moves. Leave it alone for a few hours and the fish will be swimming around as if nothing was wrong. They use this for a defensive purposes, whether or not it works depends on the type predator they are avoiding.

Pacus/Tambaqui in genera Piaractus, Colossoma, Metynnis, Mylossoma, Acnodon et al., use mimicry to keep from being attacked and eaten by their carnivorous relatives. C. brachypomus, the red-bellied pacu, is known to look nearly identical (save the lower jaw) to its cousin, Pygocentrus cariba (the Orinoco red-bellied piranha) from Venezuela. Other types of piranha could be mistaken for the more common, ordinary tetras. Some like those in genera Poptella, Stichonodon, Stethoprion, and Tetragonopterus, look like some of the juvenile pirambebas in genus Pristobrycon. Even some of the genus Ctenobrycon could pass for the more slender piranhas in genus Serrasalmus, for example.

The purpose of having these common tetra juvenile patterns is to allow piranhas to feast on fins and whole fish on the unsuspecting tetras. They are able to swim in large tetra schools and their neighbors are unaware that these little piranhas are there for just one purpose. K. O. Winemiller (University of Texas) remarked how certain cichlids would feast on aquatic organisms that were on rocks and their tail fin would be exposed to certain attack by piranhas. Evolution provided "eyes" on these cichlids tails to make the piranha believe it was being watched. The cichlids also involved into creating a defensive perimeter as a group to keep certain species like S. spilopleura from biting them as well. The pronounced caudal ocelli with its ring of bright orange scales against an otherwise drab and cryptic background, greatly resembles the bright orange iris of the eye and the other end of the fish. One can easily see why a piranha might become confused (Winemiller 1990) (Winemiller and Winemiller 1993).

Winemiller reports that in lab experiments, the fusiform-elongate Serrasalmus elongatus seemed to attack by stalking and ambushing its prey. Winemiller reports a similar behavior for the intermediately shaped S. irritans, which exhibited more of a stalking method rather than ambush. And for the most deeply bodied species, S. medinai, Winemiller reported none of the stealth exhibited by the previous two species. Winemiller states that S. medinai seemed to be an active swimmer and prefer roving in the open water to hovering near cover. (Winemiller and Winemiller 1993) . However, Winemiller did find small juvenile piranhas (P. notatus = P. cariba) in the belly of some of those cichlids (Cichla ocellaris). Of course none of this explains why this would cause the piranha to reconsider an attack on Cichla ocellaris, other than confusion. Other species like S. medinai has been known to have a body pattern very similar P. cariba (Machado-Allison and Fink, 1996) which would help it travel within the groups of these true piranhas. What better way to grab a bite of fins from its sisters without itself being made into food!

The species S. geryi and Catoprion mento have frontal markings that resemble racing stripes. With S. geryi, the stripe is violet very distinctive, prominent feature found in juveniles and adults. With Catoprion mento, the feature is less distinct and colorful, but nevertheless present with a different color tone. While science does not know the exact reasons why these stripes are there, we can speculate why. These fish by their body shape are very compressed and built to be able to maneuver in reeds and plants along sandy banks. It also makes them fast swimmers since their shape allows them to cut through water faster. It also allows them to inhabit deeper water, though C. mento is found mostly in creeks. From a head-on view, this racing stripe would likely give prey fish a non-threatening presence. The fish may think it is just a twig or some other non-dangerous stationary object in their mind, such as a floating plant stem. Or the racing stripe could make the prey fish inquisitive just like some deep sea angler fish use luminous lures to draw fish in to be eaten. It certainly could help with stealth. In either case, it is an interesting body marking these fish have and very likely tied to mimicry of some kind.

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