So are hydra my enemy?

[JoshBA]

A little while back a hydra population became established in one of my newt tanks. The tank is heavily planted, and there are a variety of crustaceans/microorganisms, with regular boom and busts of moina. Seeing this as indicitive of a balanced ecosystem, and having read of their harmlessness toward newts, I assumed they would make a positive addition to the aquarium community. The events that followed my first observations made me think otherwise. The newts became very hydrophobic, and now only go into the water briefly, leaving it frantically as if it all suddenly turned into battery acid. Furthermore, I've even seen the newts brush up against hydra on the glass, where they jerk their body away in response to the stinging cells. Should I make enemies out of these evil cnidarian invaders, or is something else behind all of this?

 

[otolith]

I would think if the hydra got to a high enough density they could be problematic to adult newts. I've seen newt larvae do the same touch and jerk action you've described. I tend to remove them whenever I find them in my larvae tubs. I'd rather my tank's microfauna become snacks for newts.

 

[Blackbun]

Your post made me read up on these inverts. I had them for a while in a new tank set up years back. They seemed to come from no where (probably from the hornwort I got from the local lake), multiplied to quite a high density as could be seen on the glass and then vanished.
I'd not trust them with newly hatched newts.
Any way.....in my reading.....I hadn't realised they have different types of nematocysts. They have them for catching prey, for moving and importantly for defence. Maybe it is the latter nematocysts which could cause issues with an animal too large to kill.