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Wild Piranhas and How They Compare to Aquarium Specimens
By Frank Magallanes
Piranha morphology is something that tosses even experts when it comes to wild fishes and where they are collected. You have to be very careful as a hobbyist about making assumptions on species found in your aquarium or pet store. Particularly when it comes to wild fish. Of course they would look different just like many of the same species look different when they are collected in large quantities and a few standout. Just like you will see in a fish tank that some have some traits that set them away from the majority. Some will appear more elongated, others might have a bigger jaw etc. It doesn't make them a different species anymore than humans which can also vary in size, color and temperament. Fishes found in the compressus group go through morphological changes that are somewhat similar depending on age, size and water chemistry. That is one reason why it is so difficult to separate visually S. compressus from S. altuvei. They share common characteristics, though they are indeed a separate species. Field expert and scientist Dr. Paulo Petry provided OPEFE photos of 2 exact same sized S. rhombeus that were pulled seconds apart. Both, not only had different body shapes, but also had different colors.
Below are 2 examples of this type of morphology. Adrien Leroy reported the same observation with this two species that were collected moments apart in the same river.
Piranhas, as mentioned above are certainly individualistic and even DNA can be scattered all over the place and the answers will be even more vague. S. medinai and S. neveriensis come to mind in this regard. Both fish are externally similar with just some minor spotting differences, yet internally they are different. S. medinai is found in open rivers. S. neveriensis is found naturally blocked in. So this latter population evolved due to in-breeding in its present form. There is also suggested evidence via DNA a population of fishes which resemble S. rhombeus have also developed in closed in lakes yet are distinctive enough to be a separate species by DNA.
The topic itself is indeed very controversial and complex on top of it..
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