Hydra

[shmohney]

Has anyone else observed hydra (or hydroids?) in their opae tanks? I've noticed several in mine lately, if I'm right about what they are. They are attached to a substrate by a short stalk, and have numerous very fine tentacles radiating from the end of the stalk opposite the attachment point. They're very tiny - if stretched fully out, the biggest one would be less than a centimeter long, and they're so small, I can't be sure they're not some sort of algae or fungal growth. Some are attached to the sides of the aquarium, and some seem to be attached to the scum floating on the water's surface, hanging upside down. It's easiest to see them by turning off the room lights and illuminating them with a flashlight from the side. I've not seen their tentacles move, but if I observe them at different times, the tentacles will be in different positions each time. If I've got hydra/hydroids, I'm a little concerned, because I've read that they can hurt or kill dwarf seahorse fry, which makes me think they might possible also hurt newly hatched opae (if I ever get any).

[shmohney]

So, barely five minutes after I posted the above, i went and took another look at my tank. I have middle-aged eyes so I doubled up on two pairs of reading glasses in order to look closely at the biggest hydra. Just then, a shrimp crawled right over the hydra, and it retracted dramatically! So it's not an algae or fungus. It really is a hydra/hydroid-type critter. I need to devise a way to look at them even closer.

[Mustafa]

I've only ever seen hydra in freshwater tanks. I would just leave them for now. They usually starve to death after a few weeks. I don't think they would be able to catch any shrimp larvae or postlarvae.

[shmohney]

With a little more googling around, I've concluded that they are probably hydroids. Seems that they sometimes infest conventional saltwater tanks. I've had my shrimp for about five months now, and noticed the first hydroids about two months ago. I don't mind them if they don't multiply too much. They're interesting - I like having variety in the tank. I managed to look at a couple under high magnification last night and saw a nematode-like critter struggling among the tentacles, but i think it escaped.

[Mustafa]

Is it possible to get some pictures?

[shmohney]

I can try. They're very small and need side lighting to see. Rey well at all. And my camera's not that great. But I'll see what I can do.

[Mustafa]

If it works, it works...if not, then...no big deal.

[shmohney]

I managed to get a couple of photos of these critters. Low quality but maybe useful. The first one is the better one. This one is attached to the left side tank wall near the corner, and I'm viewing it from the front. A small flashlight provides sidelight that makes the tentacles visible. From its base to the tip of its tentacles, this guy is probably less than a centimeter. This is the largest one I've noticed. I probably started seeing it in that location a couple of months ago, but could only see the stalk until I happened to sidelight it one day. This one disappeared from this location several days ago.

The second photo is of one hanging down from the glass near the surface of the water - in fact, it's so close to the surface that when I allow a little evaporation, the higher parts are out of the water and only the tentacles are dangling in the water's meniscus against the tank wall. It's lighted by the flashlight from above.

Others are attached to rocks and coral in the tank, and a couple were attached to the water surface scum, hanging straight down. They are always single and I haven't seen any budding. I have a little lighted 10-power pocket magnifier that lets me see a lot more detail than I can photograph. I can see a structure at the top of the stalk where the tentacles are attached, and I can even see tiny barbs on the tentacles. I can see the tentacles moving. Very neat. I'm terribly curious as to what exactly they are..

[Mustafa]

Thanks for the pictures! Fascinating animals. They seem to have more tentacles than most freshwater hydra I have seen. If they another type of hydroid, then they are either a larval form of something like a jellyfish, or just a species similar to freshwater hydra. If you see any tiny jellyfish floating around (probably even harder to see than the hydroids), they you know the answer. Either way, it doesn't look like they go for anything other than microscopic creatures, so just keep them around and enjoy the variety!

[shmohney]

I'll watch for jelly fish! That would be cool. Several of these hydroids have disappeared from the spots where I was observing them. Don't know if that means they're dying, or becoming adult jellyfish, or what. But there are numerous others scattered around. I just found a nice group of them at the surface on the tank's back wall. I like to watch them react when a shrimp bumps into them - the tentacles contract and the stalk jerks. I'll report back if I see any jellies.

[Mustafa]

Cook, keep the updates coming. As for the disappearing hydroids...they may have just moved. They are mobile after all.

[Iwantshrimp]

If they don't harm the shrimp I wouldn't mind having some