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at that size you may be able to see their organs with a bright light, other than that you will have to wait until the mature to tell, around 2yrs old
 

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at that size you may be able to see their organs with a bright light
Not possible. Larval forms 10-15mm SL the gonads and/or ovaries would be visible under microscope. The rest as to age is roll of the dice to determine sexual dimorphism. I might also add this portion to the seemingly constant application that males tend to color up more than females.

those were attributed to camouflage parameters. In adults, uniform and specific patterns of the pigmentation were attributed to secondary sexual aspects, which are not associated only to the reproductive period, through intensification in the anal and caudal fins and flanks of the body in males.
What this means is that camoflage plays an important part to piranas coloring up not just simply attributed to sexual maturation.

This is quoted from field researchers, Ivan Sazima et al., in their investigation of S. spilopleura.
 

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Frank... what about from making observations on behavior? Is there away to asses a probability to a paticular fish (particulary Serras) as being a male by observing behavior?
 

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Is there away to asses a probability to a paticular fish (particulary Serras) as being a male by observing behavior?
Tough one to quantify since both sexes can behave as male or female including coloration. In your aquario, you may be able to observe patterns of behavior that will lead you to conclude that you might have a pair, but the only way it will be known for certain is if spawning occurs. I have seen such behavior of what appeared to be male and female only to discover later the fishes were male after dissection. That is why I say such things are a roll of the dice and why as a general rule you cannot simply look into a dealers tank or friends and say for certain that is male or that is female. Not even the most dedicated field researcher can do that with any certainty and again, those that claim to be able to do it should publish their observations so that science can review it and ask questions. This will require proof with dissected specimens of before and after, authenticated and supervised by a PhD. Simply taking a person at their word or doctored photos will not suffice.
 

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Another thing... is there a way in which Piranhas may be similar in sexing to thoise of some know cichlids species where it is that:
" its not the sex that determines size, but the size that determines sex"
?
 

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Also, could or would girth around the belly imply a female carrying eggs or can males display this characteristic by simply having a full stomach. I've noticed a few of my piranha's are much fatter in their stomachs to the point of a bulge when I compare them to the others. I know dissection and breeding pairs is the definite way to determine male/female. But was curious if males and females have this same characteristic(the bulge in their stomach) or could this just be that they got more of the meal then the others?
 

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" its not the sex that determines size, but the size that determines sex"
Not really, since piranas are variable on size and age when they are sexually mature. There is nothing in concrete about what size they are ready for this. All studies so far indicate maturation is around 2 years but that could be 2 or more years depending on many factors (ie., water conditions and climate). Size does not seem to factor in much.

But was curious if males and females have this same characteristic(the bulge in their stomach) or could this just be that they got more of the meal then the others?
Again, there are many mitigating factors, one is as you described. Granted some studies, published by people who breed piranas indicate that females have larger girth vs. male which is slimmer, but this is on captive bred piranas where the hobbyists monitors his fishes on almost daily basis. If you put them (the hobbyist) into a situation where he must sort out piranas from a store or in nature, his success of picking out males from females drops to zero.

It is something that can and probably will be argued by people and if what they say is true then everyone should have breeding piranas in their aquario if the sexual dimorphism were that simple.
 

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IF have been around many types of animals with the breeding business from fish, horses, dogs to cats. In general but not always males tend to show much more aggressive behavior and try to show off their dominant power to females and typically stay more to themselves. Females tend to stick together and are more passive
 

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I was generallizing my own experiences with most breeding animals

what I meant is that they stay closer together as a group for support while males prefer to be by themselves
 

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Is that your opinion or what you believe is fact? Cause I've watched a few documentaries on breeding of several different species and it changes from species to species. So with that we can't generalize one breeding or habit to encompass all or most species of fish/mammals. We can state opinions on our observations but they're only opinions.
 

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what I meant is that they stay closer together as a group for support while males prefer to be by themselves
its my opinion, and I said with most animals meaning not all
I'm glad you cleared that up because fish are not the same as mammals, not structurally nor in habits, though instinctive behavior may confuse some for being similar. As for males prefering to be by themselves, I'm not sure how you are formulating this opinion. Based on what? Aquario observations? This is a bit far reaching because you are assuming that your few specimens give you the basis for that opinion, am I correct in this evaluation? If so, then you should reevaluate your opinion because it realistically does not hold water (no pun intended).
 

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Males tend to be much more aggressive and not allow others to get near them except for females during the actuall mating, actually I have found that in cases that I have observed of nearly all fish this comparission holds true, however as I said it is my opinion and experiences can vary
 

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actually I have found that in cases that I have observed of nearly all fish this comparission holds true
You are again, making a broad generalization without providing any data to support your opinion. In otherwords, you are saying you have observed of nearly all fish this comparison holds true, yet how many specimens are we talking about? one? two? three? or five thousand?

Are you saying that is true of wild fish and of aquario specimens?

however as I said it is my opinion and experiences can vary
Your entitled to your opinion as I am entitled to examine and dissect it for accuracy and substance in piranha science. Your experiences while interesting to some, is still very limited to simple home study analysis with a few fish. You might consider digging a bit deeper in evaluating opinions before stating them so that when you present an argument it is based on strength of evidence than just weak opinion's without credibility. You will note, I allow certain remarks to stand by other members when it is factually written and supported by evidence. In your case, I tend more to reply to you because your opinions are a bit misguided and warrant a bit more of my attention because I do see that you desire to learn. How much you can learn is entirely up to you and what you are getting out of piranha science.
 

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indeed I do wish to learn as much as possible

I would be able to conger up a number of fish, but I have kept fish for well over 12 years now so I would guess that at would at least be into the 1000's, not all my fish though friends, exhibits and so forth

how can you really provide evidence for your own opinion when it is based upon observations though, I guess I could have mentioned that I have kept tropical fish for 12yrs or something,

something I have been meaning to ask you Frank, dont take this wrong or anyhting

You always tell me that my observations in my own tank hold no meaning to the fish in the wild. So what you are saying is that since a fish is in a much smaller zone it will act completely different. I would like to know how you can support this hyppthesis?

My way of looking at it is................

Why would a fish say hey, Im in a smaller tank so I will act different, I will agree that they certainly will not do everything the same due to limitations such as traveling and hunting and such.

Again I am not trying to be a jerk, I just would like to see your view point
 

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but I have kept fish for well over 12 years now
If you wish to compare years of experience, I have well over 45 years of experience dealing with piranas in the aquario and scientific examination and I have not seen a single example of what you profess to be fact substantiated by literature and science.
You always tell me that my observations in my own tank hold no meaning to the fish in the wild. So what you are saying is that since a fish is in a much smaller zone it will act completely different. I would like to know how you can support this hyppthesis?
Very simple, you are holding your fish in an unnatural state of environment. Any scientist worth their weight in salt will tell you that is not a serious evaluation of how your fish would behave in the wild. That is supported by a long list of references at my web site. Suggest you look it over and see how references are used to support arguments. I have yet to see any from your arguments.

Why would a fish say hey, Im in a smaller tank so I will act different, I will agree that they certainly will not do everything the same due to limitations such as traveling and hunting and such.
And you have answered your own question, they will not act nor behave the same in an enclosed environment. You can only simulate what you think is an natural environment in hopes the fish will adjust. Some do, some don't. Disagree? then go back and read the unsuccessful stories of keeping captive piranas and those that had little luck in keeping them alive. As for this last remark:
Again I am not trying to be a jerk, I just would like to see your view point
You are making to much of a personal issue of this where in reality I'm pointing out to you that your experience in such matters is lacking any real perspective. And you are guided by personal opinions on matters that I question to relevancy and accuracy of execution, because you have yet to provide me one single published example of your information supported by science. It is exactly what you make it out to be, personal opinion with a hint of prejudice without merit.
 

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So basically what you are saying is that anybody that does research or makes observations, and then reports them is of no importance. All you ever say is published articles yada yada. This is what I was trying to avoid with you. I never intended to measure up the fish keeping years with you.

Just because somebody else publishes something doesnt make it right. I really think you need to come to reality that not everyone spends their time reading over articles, we simply learn from observing our own fish. You believe that fish act different in aquariums that is fine but you offer no proof of that, where is your references.

Nor did I ever say in my post this was fact, I said its my observation, let me say that again My Observation!

How do you want me to provide publications on that, you expect me to write a report every time I post


My proof of them acting fairly natural is that you cannot prove otherwise, plain and simple. Your approach to science is you must prove it to be so.

My approach is that you must prove it isnt so, you a pecimist while I am a optimist.

As far as I see it I have indeed figured out your motives here. You always try to make it look like you are soo much better than everyone else, perhaps you should learn some manners and not gloat and brag, its called public decency, I have it and I am less than a third your age. Its a shame to see you act so authoritative towards others. When we chated before via email about 5 months ago you seemed like at totally different person, its a shame to see you act so childish, and try to stir up problems, perhaps this is the type of thing that got you flamed on boards in the past. I really tried to make a mends with you but it is very obvious to me that you have no intentions of doing so. Good day, and good luck to you Sir.
 
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