My friends and S.Korea breeders know that is growth spot.
(Growth spot isn't scientific name, just...nickname...)
Anyway, We have experience it many times for several yearl.
But we don't got any probelm and think that is totally non-harmful.
As a similar case , Do you have seen guppy tail's edge in growth period?
Guppy tail's edge in growth period is colorness and transparent.
Pigmentation is slower than cell divsion when the fish grow up fast.
So~ The faster grow up The more colorness.
I guess this is the why.
http://www.monsterfishkeepers.com/forums/showthread.php?t=127128
I think this spot isn't growth spot. I don't kow what it is.
Growth spot is only white but this spot is black.
I have a pic of ray with growth spot. I think this is growth spot.
Do your ray have growth spot? Don't worry about it.
My Englis is not so good.
