Those are kubotai, the juvie pattern looks completely different from the adults, and there's a transition period between them. Essentially the dark bands grow wider and connect together, which produces the "polka dot" pattern as the lighter colored band between the dark bands is divided and eventually loses shape completely. I have a group of them in my 220, raised them from very small juvies, and none of them have identical patterns now, but were harder to tell apart when they were small.