The below article from Advanced Aquarist mentions that these are not common amphipods. I know I’ve never seen one in my tank. If I had I would have been quite amazed. Now that I’ve seen this article I at least know they are out there. If you happen to see one of these running around your sump (I spend time starting at my sump by the way) take a macro shot or video and let me know. I’d be very interested to see.
A tiny marine crustacean with a great big claw has shown that not only does size matter, but left or right-handedness (or in this case, left or right-clawedness) is important too.