Favid species with sweeper tentacle

Sweeper Tentacle Epiphany


I’m not sure how you feel about it, but whenever I imagine the ‘ideal’ reef aquarium, I conjure a vision in my mind of a bountiful, reef tank chock full of corals. The hypothetical aquarium is an aquatic cornucopia with SPS, LPS, leather coral, mushrooms, and more all living together, filling nearly every square inch of rock work, cascading in all directions. That vision has been ingrained in my mind since the first day I decided “I have to have a reef tank.” Do you share my vision?

In fact, I have seen pictures of a few tanks that look like that. Almost every ‘tank of the week’ that you will see on other aquarium websites looks like that. I want my tank to look like that. But it doesn’t.

A trip to the local public aquarium

The reason I’m writing about this now is that I had a bit of an epiphany this weekend related to my previous image of what the ideal tank should look like. I took my kids to the Adventure Aquarium, which is a pretty cool public aquarium in Camden, NJ. I brought my camera with me and took as many pictures as I could before my kids would drag me onward to towards the next exhibit.  They don’t nearly have the same nerd-power that I do, but they are still young…I’ll work on that.

The kids were really interested in this display that allowed them to crawl under and then pop their heads up into a ‘bubble’ that allowed them to look through the tanks from the inside-out.

sweeper tentacle epiphany

Of course, while they were doing that, it was my opportunity to take pictures of an adjacent coral display. There were electric-looking brain corals, hairy mushrooms the size of dinner plates, and then I noticed this humble-looking favid species, tucked in between. I wasn’t blown away by the colors, but I distinctly remember thinking to myself, that it was so ordinary that it looked peaceful…tranquil. So, I checked back on the girls playing ‘prairie dog’ popping up and down along the display of aquaria, and then finally, ‘click’ I snapped the photograph.

Favia coral sweeper tentacles

When I got home, I pulled out my SD card and downloaded the images. I flipped through, sometimes impressed with the sharpness of the photos, other times disappointed at the grainy, blurry images. When I got to the picture of the ‘peaceful’ favid, I was shocked to see that the image wasn’t peaceful at all. Extending precariously from a polyp near the base, I noticed the long, thin profile of a few menacing coral sweeper tentacles.

Favid species with sweeper tentacle
Favia coral Sweeper tentacle not causing any harm 

This wasn’t the first time I had ever seen coral sweeper tentacles, but there was something jarring to me about the fact that this coral was fighting for its life, while I was having the time of my life, and I hadn’t even noticed.

What is a coral sweeper tentacle?

A coral sweeper tentacle is a specially adapted appendage that certain coral species have (especially Large Polyp Stony Corals), that is generally several times longer than the longest feeding tentacle and packs a powerful nematocyst stinging punch at the end.

We call them “Sweepers” because the corals seem to send them out, sweeping back and forth just looking for something to sting.

Corals use sweeper tentacles to damage or kill nearby rivals to secure their own place on the reef and prevent being out-competed by a neighbor.

A stress-relieving hobby for us but the coral sweeper tentacles were signs of a potentially stressful situation for them

The realization I had come to is this: while this hobby may be a stress-relieving, peaceful activity for us—every day is still a life and death battle for invertebrates in our care.  The corals don’t know they are ‘supposed to’ be tranquil. They simply carry out the actions coded for in their genes. Sometimes, that program says… “begin a war with the coral next to you. Take it out at all costs.”

Please forgive the anthropomorphic liberties I took there, but I think you get the point. As much as we may want them to be, our tanks may not be the placid paradise we want them to be for our corals. And while the natural coral reef environment may look like an underwater city with coral ‘skyscrapers’ racing to the surface and occupying every possible millimeter of the substrate, the reality is that what looks like a seemingly placid underwater scene could be characterized as a violent battle (in some cases) fought on a coral-by-coral basis.

What lessons can be gleaned from this?

So maybe the ideal tank shouldn’t be crammed to the brim with different colonies. Maybe the ‘tank of the week’ shouldn’t be so loaded that any minor blip in water parameters is likely to cause a catastrophic cascade of events that causes a significant die-off.

These coral are in our stewardship, but they are battling for their lives every day. Can we agree, as hobbyists, to try to take that into consideration when we plan out our tanks?

I hope this doesn’t come across as too preachy. But that simple little sweeper tentacle really gave me pause, and I was wondering if anyone else out there felt the same way.

Remember to give your corals enough space

In our home reef tanks, the problems pop up when we over-crowd. It can be so hard for many of us to leave that space between two frags–but we have to allow for them to grow out and fill in the available space.


Am I being too sappy, or do we owe it to the invertebrates in our tank to provide enough space for them to coexist without provoking them to initiate war on their coral neighbors?

The next time you find yourself daydreaming about what new frag you want to glue into that 1 x 1 inch bit of space between your frogspawn and your zoanthid polyps—try to remember this article—and that one little coral sweeper tentacle in the image above. And let’s band together and vote for the tank of the week…that gives each colony enough space to grow without being attacked by the other corals around it.






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