This is really interesting. All the sites about tank critters I've come across say hydras are harmless, BUT I always noticed how my apple snails seemed to be 'stung' when they touch/slide over a hydra. I've seen it close up; I have had lots of hydra in my tanks. Sometimes the snails got a little swelling on their antennae where they touched the hydra that went down after a few days.
Hmm, if it hurt an apple snail... think of what it could do to a sensitive little shrimpy?
I dont actually have hydra anymore. I had so many so I just scraped the glass everyday, which pretty much got rid on them? It might of been other things too. Now I have HUNDREDS of copepods/ostracods, little white wormy things, and some planaria.