My Silver Dollar Sp's

[FJB]

Indeed, post away. We can all learn of both the generalities of sexual dimorphism in various SDs species, and of the nuances of developmental change in individual species.

Last month I got four very small tiger SDs, Metynnis fasciatus, about 2+ inches, too small to have good guesses at genders. I keep looking and them and still can’t tell anything I can believe. They are still in quarantine. I’ll try to post some pictures as well.

 

[fishguy1978]

 

[fishguy1978]

F FJB do the females have the straight blunt anal fin?

 

[fishguy1978]

F FJB do the females have the straight blunt anal fin?

 

[FJB]

The females M. rubripinnis have the big red hook, with relatively straight, or slickly curved outer edge. Usually very red, rimmed with black.
The mal hook is bilobed, not straight, and not slightly curved, and much, much less colorful. Most of the red in males is restricted to the tips of the lobes, rather than on most of the fin as in the females. In addition, the lowest rays of the anal fin in males gave small lateral spines. Normally one does not see these, but if one takes a closeup photo of the anal fin and enlarges it, one can sort of see that.
In your last photo all 6 fish are visible. Although I don’t know their size, I am guessing they are immature fish, still growing and not yet fully expressing dimorphism. I am guessing (could be wrong) that you have possibly 2 males and four females. The two likely males are the uppermost fish, and the leftmost fish. Both have less red (more towards the tip, not the whole fin) and
shorter anal fins. The other four fish appear to have larger hooks, with more red. This is just a guess , and the photos are not that close so as to be certain. Do you agree with my description of your fish, RE. gender?
What is the approximate size of the fish, total length?
Again, they look like beautiful young animals.

 

[xenacanth9]

+1
Nice silver dollars

 

[fishguy1978]

The females M. rubripinnis have the big red hook, with relatively straight, or slickly curved outer edge. Usually very red, rimmed with black.
The mal hook is bilobed, not straight, and not slightly curved, and much, much less colorful. Most of the red in males is restricted to the tips of the lobes, rather than on most of the fin as in the females. In addition, the lowest rays of the anal fin in males gave small lateral spines. Normally one does not see these, but if one takes a closeup photo of the anal fin and enlarges it, one can sort of see that.
In your last photo all 6 fish are visible. Although I don’t know their size, I am guessing they are immature fish, still growing and not yet fully expressing dimorphism. I am guessing (could be wrong) that you have possibly 2 males and four females. The two likely males are the uppermost fish, and the leftmost fish. Both have less red (more towards the tip, not the whole fin) and
shorter anal fins. The other four fish appear to have larger hooks, with more red. This is just a guess , and the photos are not that close so as to be certain. Do you agree with my description of your fish, RE. gender?
What is the approximate size of the fish, total length?
Again, they look like beautiful young animals.

They are all under 3in. and I am not sure about the fins coloring yet.

 

[FJB]

That sounds about right - Too young to tell with any certainty. Gorgeous group!

 

[fishguy1978]

Looks like some males in the group. Couple are of the same fish.

 

[fishguy1978]

F FJB is this a male, foreground, and a female, background.