Hydrozoans are like the bad cousins of jellyfish. They can deplete your tanks nutrients, cover your tank walls and most importantly, damage or kill your jellyfish.
Earlier this year in march I witnessed a major hydroid attack on my jellyfish. I had no clue what was going on, as I was not expirenced. All I could understand was that something was making my jellyfish very sick. I did nothing to stop the undetected attack and my jellyfish died. My three jellyfish began acting very strangely. They slowed down and began to become very soft and gooey. Obviously moon jellyfish are already gooey, however it seemed like my jellyfish were beginning to melt! The jellyfish spent their time stuck to things such as the substrate or sadly, even the heater. Eventually the jellyfish withered away like a block of melting ice, leaving me with several questions. The tank was empty except for a snail and two hermit crabs. I began to notice this intricate spider web design on the acrylic near the bottom of the tank. I was searching for polyps, and at first I thought I found them, however I later realized that I ran into hydroids.
Hydroids are simply the polyps of hydrozoan jellyfish. They live in a network style colony, forming branches with little buds at the end of the branches.
I watched these strange hydroids for a while, and to my surprise they began to produce small jellyfish. These new jellyfish were very strange. They were very simple and didn't do much. In fact they pretty much latched onto the walls of my tank and stayed their until something disturbed them. They didn't get very big either. At times I would have 20 or 30 of these things. And then their populations began to decrease. They were draining my tanks food and I didn't feel like replenishing it. Then of course I ended up knocking my tank over, and wiped out their population.
A video of some hydroid jellyfish found in my tank in march.
Im sure your thinking this is a nice story and all, but why? Well I realized yesterday that their are hydroids present in my tank again. I think they hitched a ride on my clown fish and were accidentally introduced. My first thoughts were to nuke the tank with bleach. But then I began to question that. nothing in my tank is any different. My jellyfish are healthy, as are my clownfish and snails. These hydroids may produce jellyfish, and possibly a new jellyfish Im unfamiliar with. So I decided I would attempt to remove them and grow them elsewhere. Well I scraped as many as I could see off, and put them in a flask. They grew back almost immediately in the tank. The colony still seems small though, and there is no sign of any hydroids gearing up to produce jellyfish. They just seem to be busy eating. in fact not all hydroids produce jellyfish. These may just stay hydroids. However I hope that they produce something cool while they loiter in my tank.
So I'm just going to watch the colony and intervene if something goes down. My next segment of this will be on the removal of hydroids and what you can do about them.