Measuring Piranhas and Related Forms Scientifically

By Frank Magallanes, November 3, 2002

Lower Jaw is not measured on piranhas because it projects and is variable in size

Standard Length (SL) measurement.  S. sanchezi Photo by Frank Magallanes








The average home aquarist doesn't realize that Serrasalminae fishes are measured by standard length. This is a scientific method of measuring fish from the tip of the snout to the edge of the caudal peduncle. Why you might ask? It is because neo-tropical rivers, lakes, and streams (and African continent too), have parasitic fishes that feed on fish fins, including piranhas. This type of diet can obstruct natural fin growth in the caudal fin when it is bit off. Abnormal growth develops over repeated bites in that area that prevent the caudal fin from developing into a normal  sized fin.


Piranha are found almost everywhere in South America where they have access and the water is warm enough.  In the Venezuelan Llanos, at least 8 species are found in mostly all aquatic habitats, of the approximately 35 that are known in South America (Pygocentrus cariba, Serrasalmus irritans, S. rhombeus, S. medinai, S. altuvei, S. elongatus, Catoprion mento, Pristobrycon striolatus). (Nico and Taphorn 1988).  These piranha range in size from approximately 16 inches for SL for S. rhombeus to under 10 inches adult size for most of the rest (Nico and Taphorn 1986). All measurements begin at the anterior (snout) end of the fish, but each ends at a difference location. Standard length (SL) technically measures to the posterior end of the hypural bone. Realistically, SL often ends at some visible external feature such as the last lateral line scale or the end of the fleshy caudal peduncle. As shown above. You never want to eyeball measure your fish since this will definitely give you the fisherman's fish tale or in this case; hobbyist fish size, which means, larger than real life in the aquarium. Optical illusions are common when eyeballing fish in a tank, they always appear larger than out of the tank. Each person often has a "preferred" method in measuring an object, whether it is to always round up to the nearest millimeter, to always round down, or to always measure to the nearest millimeter. Further, many individuals have a subconscious "digit bias", generally measuring to either the nearest odd or even number.


With the piranhas studied, it was found a historically coupled increase in developmental integration caudally and a decrease in developmental integration cranially (Fink, W. L. and M. L. Zelditch 1996).


In the aquarium, fish always appear larger than what they really are and I can't even begin to tell you how often I have misjudged a fishes true size in the aquarium. Even hobbyists who have emailed me and swore their fish was 16 inches Total Length (TL) would later discover (after measuring out of the aquarium) their fish was actually 3 to 4 inches shorter in size using the scientific method. On average if you think your piranha is 14 inches long (TL), realistically, your fish may only be 10 1/2 long (SL) using the correct method of measurement. I have not seen any report (wild caught) that records red bellied piranhas larger than 12 inches (total length). Larger ones may exist but the record has not been broken using standard length. World records are based more on weight than length of a fish. In the home aquarium is another matter. (See below for an explanation).


Weight (or mass) is measured using either a mechanical or electronic balance. Weight is generally measured to the nearest gram. If an electronic balance is used, weight is generally subject to less measurement error than length. Recording error, however, is a human problem, and this is always possible.




All fish scales, however, are not alike. There are four major groups, one of which is comprised of cycloid and ctenoid scales. The ctenoid variety have a comb-like, spiny posterior edge, their name developing from the Greek word cteno, meaning comb. Conversely, cycloid scales have a smooth posterior edge as their name from the Greek cyclo, meaning circle, implies. Both types of scales, however, consist of two main regions; a rigid surface layer chiefly composed of calcium-based salts, and a deeper fibrous layer consisting mainly of collagen. Fish scales were once used to estimate the age of a fish. However, this method is now regarded as unreliable. Instead when the fish die, the bones in their heads called otoliths (oto’ meaning ear and lith’ meaning stone) are removed. These bones help the fish to keeping its balance in the water. When an otolith is removed from a fish, sectioned into thin slices and viewed through a microscope, it reveals a pattern of light and dark concentric rings. The only other way you can determine a piranhas age is to hatch it from an egg and record its growth.




In the aquarium, a most unnatural environment for piranhas, especially large ones, the fish can grow almost infinitely in size solely because of 1) food availability 2) care 3) no active predators of piranha available. In a situation like this, the fish can exceed known wild captured records only if the aquarists takes good care of their fish. Would this standard of measurement be worth anything to science? Yes and No. Home aquarists perform an invaluable service to science just by taking care of there fish and sending information to people of science. All that is required is a basic knowledge of protocol in providing information that can be tested and substantiated. Visual examination of a fishes size is not a valid method because as stated above, the optical illusion effect will take over good judgment (along with embellishment of their fish).








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