peacock mantis with splotchy carapace

[reeferious]

couple days ago while watching my peacock mantis shrimps chasing feeder crabs around i noticed one of them had what looked like splotches on its back and so today i spent considerable effort luring it out and trapping it. closer examination shows the splotches are actually pitted holes in it's shell. i've heard of shell rot but far as i can see none of the others suffers the same condition. considering they're living in the same tank why just one and not any others? he's been with me for couple years just hate to see him affected so.