I recently took home a nice Zoanthid (zoo) rock, which was doing beautifully in the store where I got it, which shall remain nameless. After a few days in my tank, I began to notice there were small patches where the polyps were closed up. These patches were more like stripes, where there would be a path of closed polyps. I started looking for the culprit(s), and with a keen eye, found plenty: Zoanthid-Eating Nudibranchs.


These guys feed on the tentacles of zoos, which causes the zoo to close up, forcing a pest to move onto the next polyp. So I began using a piece of airline tubing (used for aquarium air pumps) to siphon out these little guys. I had to do it over time, as I found them. But over the few weeks that I casually removed these nuisances, I noticed something interesting. The color of the first ones I had found were green, similar to the green of the zoo rock I bought. Once I began to notice them on my orange zoo rock above, they were now exhibiting a very similar orange.

These nudibranchs are quite extraordinary, despite being an aquarium pest. Similar to other nudibranchs, they have the ability to utilize live pieces of their food, which is in this particular case, Zoanthid tentacles. Many photosynthetic invertebrates get their colors from algae cells that live within the living tissue, which are called Zooxanthellae. As these nudibranchs feed on the tentacles of zoo polyps, they are able to use the color-causing algae within their own bodies, often times becoming nearly undetectable when living among a colony of polyp.

Here’s an example of the color change I saw in my own tank:

(Original infested rock and the first nudibranchs caught)

Here’s my orange zoo rock I have had for years. Notice the color difference of the nudibranchs that I pulled off this particular rock. This was only a few weeks after the first zoo-eating nudibranchs introduced to my tank, and this was still not the most orange one I removed from the tank.

So, here’s what to look out for if you’re trying to avoid these guys. If you’re buying a zoo rock from a store, look at the other zoo rocks as well. You may see obvious stripes where something disturbed a section of polyps, which would be a good place to look, very closely. These guys are about the size of a grain of rice (or smaller!), and can blend in very well. Of course, it is possible to get these pests on a rock that seems unaffected, so you may notice changes on that rock, or even on other zoo colonies you already have over time. But another solution to avoid any pests being introduced would be the use of a coral dip before adding any new rocks or corals. There are a few companies that make concentrated blends designed to kill pests like flatworms, bristle worms, pest nudibranchs, and even disinfect damaged or recently fragged corals. Just be sure to follow the instructions so you don’t kill your coral!

But if you find you do have these guys in your tank, don’t fret. Once you find your first culprit, you will be better at seeing others and noticing changes in your zoanthids. Starting a siphon with an airline tubing is the best way I have found to remove them from an established tank. It doesn’t take too much water out of your system, and by putting your finger over the end of the siphon, you can avoid sucking out any more water than you need to. Simply open the siphon again when you find your next one.

After removing what I thought was all of these nudibranchs from my tank, about a week later, my orange zoo rock was having a lot of difficulty opening. Upon first glance, I saw nothing special. After a much closer look, I found that there had been an explosion of orange baby nudibranchs. These were the size of a single zoo tentacle, at MOST. They were usually found at the opening of a closed zoo, often looking like just a piece of tentacle hanging out. They were easy enough to remove with the tubing, but finding them was the tricky part. I’m telling you this to put it in the back of your mind, in case you experience something similar. When you don’t know what you’re looking for, troubleshooting issues like this can be a pain, especially when one of nature’s miracles is designed to hide them so well.

Hopefully, this is never an issue for you, but now you know what to look out for, how they may trick you, and how to remove them. Please post any follow-up questions if you have any, and we’ll be sure to get them answered for you.

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