I left a bottle trap in last night and came out this morning to find everything inside but the scuds were dead, casualties: 7 sakura/pfrs, 1 cbs, 2 c. pygmaeus. I have tiny needle poked holes all over the top of the bottle trap so I don't think it's stagnant water.
I had previously trapped c. pygmaeus with rcs before for the same amount of time ~6hrs, and never had a casualty. Only difference is the hundreds of scuds. So they either (1) maliciously clawed the victims to shreds, or more likely they (2) scared/harassed them to death wiggling all over the bottle.
(1) if we look at the anatomy, the first 2 gammarus species is capable of shredding into prey with its large gnathopods/claw
(2) whereas the pulex only has prehensile legs
The ones that weren't mating were feasting on the corpses.
I'm not certain which species I have, the macro shot above was the best I have and the front pods are tucked in.
On a happier note, my filter box is blooming