Supernumerary: additional or added cells, veins or other structures.
One felt one's self a supernumerary hired to fill the scene.
A supernumerary, who thought that he must be dead, had asked for his place.
Placed on the books for victuals and wages; also supernumerary and "for rank."
It is not extravagant to say that any one fish may be considered a supernumerary.