btw, I don't know about how they play with worms in an AP grow bed, but trout love to eat 'em.
As these amphipods are completely aquatic, I might guess that your bed is on continuous flood rather than flood and drain? I think the former conditions could favor scuds over worms.
If the picture is typical of your beds, you have a considerable snail population as well.
I would recommend looking through the different varieties of amphipoda on wiki at http://en.wikipedia.org/wiki/Amphipoda