Mysterious Red Line

[PrairieSunflower]

One of my male guppies has a zig zaggy bright read line down its belly... like the width of a thread... looks much like the tiny veins in your eye in color, width and size. So... I was baffled and removed him into a jar to take a picture... and... it vanished! Put him back in the tank and 5 minutes later... its there again! I just can't manage to get a picture...

Any idea what this could be?

 

[phurn]

Parasite.

 

[PrairieSunflower]

Oh no! What kind of parasite could this be? It seems to be apart of him rather than being on/attached.