Identificaciones de cannizag

4mm nose to body rear. Found in water dipped from a vegetated area of Watermans Pond. I wonder if the orange spot inside this amphipod is the Pseudocorynosoma constrictum parasite. Can anyone confirm? The swimming behavior of this individual seemed to be much more erratic than most amphipods. If a parasite is present, could it be the cause of that?

i have no idea what these guys are but they are everywhere on the rocky beach. they were mostly under rocks in the intertidal zone, but not in the water. they jump so high and really prepare themselves for the launch by tucking their tails in and then catapulting!