Silver Dollar Questions.

[Adoptedfish13]

I have 2 adopted Silver Dollars that I have spent some time trying to ID and have come up with Metynnis altidorsalis as the closest match. They are about 4 inches, probably longer. I am aware that they would do better in a shoal but do not know how to go about it if it is at all possible.
 
The tank is 125 gallons and these 2 have been colored up and attempting to spawn many times a day. (I think they may be 2 males, but are just doing their thing because there is no one else. lol)
 
Is it possible to introduce younger fish into the shoal? Will they be accepted or be a different shoal?
 
Does it matter if they are the same exact species? 
 
Where could I get the specific species that seems hard (nearly impossible to get answers from a pet store) to ID when young?
 
Finally, If they are some other species, feel free to inform me. 

 
Here is a small clip that shows them that I posted to another thread already.https/youtu.be/teCrAj0-WTM

 

[Byron]

Of the 28 species assigned to the genus Metynnis, only 14 are valid as distinct species; the remainder are conspecific to one of the fourteen.  But that still leaves a number of possibles, and any one of these would likely be seen as "silver dollar fish" in stores.  Sometimes you can obtain the likely scientific name from the invoice, which is fine if the collector/exporter knows to begin with.  M. hypsauchen is the most commonly seen species, though M. argentius is frequently encountered.  The former has a black blotch positioned above and behind the eye, whereas the latter does not.  There are two distinct blotches on both fish in the video, and I am no where near experienced enough to even hazzard a guess.
 
I went through the photos on Fishbase, with no benefit.  I checked a few more sites, and found some that would concur with your thinking on M. altidorsalis, but identifying fish from online photos is often difficult if not impossible.  For one thing, a fish under stress may well change its appearance substantially, which often happens in photo tanks.  For another, many photos are incorrectly named to begin with.  Third, there are sometimes subtle differences due to the geographic location of fish from the same species.  The last major revision of this genus was done by Zarske & Gery (1999 and 2008) in German, with photos.  I have not seen Part I, but Part II is online here:
http/www.senckenberg.de/files/content/forschung/publikationen/vertebratezoology/vz58-2/03_zarske.pdf
As you have the fish "live," you may find the photos useful.  I see the dual blotches on some of the fish.
 
From the behaviour in the video, I would tend to think both fish are males; the interactive behaviour is very typical of males in many species of characin, and cyprinid too.  If this were a pair attempting to spawn, they would almost certainly be doing so over a surface such as a broad-leaf plant.
 
As for increasing the group, this would be advantageous, and with this species (whichever it is) probably not problematic as they are not particularly aggressive within their hierarchy, even though phylogenetic work has now revealed that they are in the "piranha" clade with the genera Serrasalmus, Pristobrycon, Pygocentrus, Pygopristis and Catoprion (Ortí et al., 2008).  However, first you need to pinpoint the species, and then find them somewhere.  As these two have been together for a time (I am assuming this), it might be best to let them live out their lives as they are.  The video is short, but they seem to be interacting well.
 
Sorry I can only suggest and not answer more exactly.
 
Byron.

 

[Adoptedfish13]

Thanks for your reply, Byron. 
 
Unfortunately, I don't have an invoice. I don't know any breeders and it seems that the shops around here don't even label or may not even be able to tell me the species names of many fish they sell.
 
Most of what you said about species ID is what I was thinking. I have made it a hobby to try and identify many arthropoda over the years and there are many misidentified species with photos online. I prefer to have a written description as well as a photo. Taxonomy is often being revised with many species of animal as it is. Anyone can put a photo of an apple online and label it as an orange, and with fish, it seems pretty common with genera that have many similar species.
 
For these fish, I rely on the shape of the fish more than the colors because the colors are a display that they turn on and off. Only the faintest amount of the uppermost spot is visible when they are pale, and one fish, I call it "S Dollar" (as in submissive) is usually not displaying the lower spot until it is physically interacting with the other, "D Dollar" (as in dominant) who is usually displaying both spots as well as darker coloring in the body and fins. I doubt that my fish's colors would even be visible on a dead specimen. The slightly elongated shape and the shape of the fins is very similar to what I see in photos of Metynnis lippincottianus but my fish are not polka dotted as a base color. When they color up and display, the whole fish darkens and there is a faint pale banding that doesn't darken with the rest of the fish. Nothing even close to all the spots I see on those. Well at least I know they are Metynnis sp. for now.
 
I do not use a flash when photographing the fish because it us usually unflattering to begin with. In the video, those 2 were busted while I was folding laundry on the couch and then I left the room to grab my phone. It was not the best capture of the behavior that they do. Most of the time, it is for longer with one fish curling the anal fin under the other, making a scoop and even listing to one side as they do, and they like to do it in that same area all the time. They also do the more macho appearing "side bumping" and chasing eachother with open mouths that does not look as nice (those teeth!).  The tank interior has been changed and the skull removed (thank goodness my husband finally took it out!) I would rather it be in a corner covered with plants if it were to go back in there as a small cave.
 
Oh well, I still don't know if it would be OK to introduce youngins into the tank. hehe 
I wonder if some of these Silver Dollars already are hybrids upon purchase. It seems like the Metynnis lippincottianus would fit right in by shape alone as the shape is similar. Also, they would be easier for me to obtain as I have already seen a couple different Spotted Silver Dollars in fish shops. If I were willing to risk the possible tank upset of introducing some young ones...

 

[Byron]

The additional description of their behaviours suggests males even more.  I have no direct experience with these fish, as they eat plants and my larger 4 and 5-foot tanks are planted for shoals of smaller species and these larger plant nibblers would not work..  But the behaviours are certainly typical of males of many characins.
 
You are using the better characteristics to ID the fish.  If we could track down Part I of the Zarske & Gery work we might find something, though my German is no where near adequate to fathom scientific literature.  Another thought came to me, and that concerns the two closely-related genera Myleus and Mylossoma.  These fish we are discussing are most likely wild caught, so we cannot know their origin and they might be almost any species that just happens to have been captured once.  Hybrids in this case would be rather unlikely, but I am only surmising.
 
Byron.

 

[Adoptedfish13]

Thanks again. I am almost positive that they are both males by seeing a video with the same types of fish in a tank with females that look very different (but I have some practice with sexing arthropoda so I notice differences when they are there).
 
However, here are some better clips that I just got of the boys' "friendly" behavior and appearance:
 
https/youtu.be/xz2r70wWD8g
 
https/youtu.be/Ynd3LDnra9M