I've Been Told It's a Jellyfish:

[reef4me2]

Okay, long story short, I've been gradually breaking down my reef for the last several weeks. After I started moving rocks around, I began seeing these tiny creatures that look like jellyfish. As I haven't added anything in recent years, and just began seeing them, I assumed that they were probably larvae of an anemone (there was a small colony of club tipped anemone in my tank) as many cnidarians begin life with similar appearances, but the guy at the LFS (who is quite knowledgable) said that it sounds like jelly fish to him.

They come out at night, and there are literally thousands of them. Their clear bells range from about 1mm to 4mm across, and have clear tentacles of about the same length dangling below, and move in a typical jellyfish motion. They all but disappear during the daytime, but swarm towards the top of the tank at night.

I've had this tank for about five years, and did not see anything like this up until the last couple of weeks. The live rock was all S. Pacific rock. Anyone have any guesses as to what type of jellyfish these could be? I'm just curious, really, seeing as the tank is mostly empty now...

 

[Elysia]

Jelly shaped animals that are clear/white are usually hydroids.

 

[reef4me2]

Well, I seem to have either stumped a lot of people, or no one thinks this is worth their time, but if anyone has heard of certain species of jellyfish being found in reef tanks, I'd love to hear about it....

 

[reef4me2]

Oh, sorry Elysia! It'd been longer since I refreshed the page than I realized! Didn't mean to come across as a, well, you know! LOL! I looked at some pics of 'em earlier, but didn't find any that looked exactly like what I was seeing, there seem to be many different types, though! Thanks!

 

[reef4me2]

So, if they are hydroids, why would they appear all of a sudden? Could they have been in between and under some liverock in their sessile stage for the last several years, and the commotion with breaking down the tank made 'em complete their life cycle and the little free swimming critters rush out to spawn when the lights go off? This is fascinating to me... Who would have thought that I'd be discovering new creatures in my tank five-ish years after adding the liverock!

 

[reef4me2]

I do think pics of Staurocladia oahuensis match what I've been seeing!

 

[Elysia]

From what I have read about hydroids, it seems as though they can go dormant and emerge whenever the situation favor them. It could be that, in breaking down your tank you stirred up some long settled detrius that increased your available nutrients or some such. Or maybe some hitchhiker sesile inverts were killed while you were moving things in the tank, which did a small spike in nutrients? Or maybe just not having some of your higher animals in there permitted some of these smaller guys to get an upper hand? Or, if you have stopped filtering the tank, maybe these hydroids were always there but were being effective filtered away?

I don't know if you'll be able to figure out a single reason for their appearance now. Some of these animals are sort of like macros in that, you can have a rock in your tank for years and all of the sudden something can just sprout out of it, seemingly with no rhyme or reason. When it isn't harming anything, sometimes it is nice to just enjoy the amazement (unlike the amazement when fish go off their food or a fish gets sick despite precautions.)

Just so you know, I have had a few animals show up that I assumed were hydroids but I have since been told they were likely upside down jellyfish medusa. They differed from the "typical" hydroid medusa (at least, the ones I have always seen in tanks) in that they were larger (up to 1/4"), colorful (a purplish-brown rather than clear or white like the hydroid type), would sit on substrate upsidedown or pulse with the tentacles above the bell (unlike the hydroids which I would either see attached to a vertical surface like the tank glass or pulse around the tank sort of randomly.) There are apparently upside down jellyfish from Hawaii and Florida; perhaps elsewhere, I'm not sure. This did make some sense as I have added things to my tank that have come straight from the FL Keys.