Hitchhikers and Fun Things We Find in our Tanks

by Jon Norris 

Author

Jon is a Marine Biologist and Professor. He began his saltwater experience in the early 90's, breeding clownfish. After High School, he went on to receive multiple Bachelors of Science in Biology and Education from California State University Northridge. He then went on to get his Masters in Biological Sciences and majored in Marine Sciences from Arizona State University. He was teaching Biology when he decided to go back and get another Masters in Biological Sciences and now finishing up his Doctorate in Biomimicry; a new branch of Biological Studies. He has kept reef tanks and even had his own business aquaculturing corals for the hobby and aquariums. He is now living in South Florida working for Jellyfish Art as the Director of Education and Biologist, where he helps to raise moon jellyfish and help students learn even more about the ocean and conservation through the lens of Jellyfish.

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We have all had the “dip before you trip” metaphor thrown around too often. We are told by many sources online to dip any corals we receive to eliminate pests. Most of the time, pests come in the form of hitchhiking nudibranchs that can mow down zoas and other soft colonies. Then you have the flatworm family that is so broad, it is so hard to signal out a specific species unless you have a microscope. However, there are the larger hitchhiking pests we can see without the use of a scientific lab. We have the beautiful aiptaisa and the gorgeous majano anemones that will sting your new found friends to no end. Now let’s talk about an interesting group of hitchhikers in the crab family, and one nasty one in particular; the Gall Crab.

Gall Crabs are pretty unknown to many an untrained eye and reefer, as they are not as abundant in reef systems like other hitchhiking crabs such as gorillas, emeralds, or decorator crabs. The problem with these crabs is they are parasitic to corals; hard corals especially like the Trachyphyllia family, (Brain Coral), and other members in the Faviidae family such as Platygyra daedalea.
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                Gall Crab peeking out its burrow. Photo Courtesy of William M. Bowen Jr.

So what makes this little bugger so harmful? After close observation in Mr. Bowen’s tank, he was only able to see at night and the other tank mates seemed to be unaware of this little pest’s presence. So, they can hide rather well and with no known predators, it is hard to know that they are even there. In William’s tank, he noticed a small hole in his brain coral and didn’t notice anything odd until one night he shined a light into the hole and caught a glimpse of a scurrying red banded demon child. I was called in to get an ID, and while it has been awhile since I had seen one I had a feeling it was an old nemesis of mine, aka the Gall Crab.

Crab with damage to coral along with flesh bits form the burrowing. 

​The Gall Crab does not look like a crab at all, more like a parasitic Isopod. Have some creepy eyes and an odd shaped claw. There is little research available on this little critter and it was first thought these were commensal crabs and benefited the coral by bringing in nutrients and feeding the algae storage for food.  These crabs were also thought to be filter feeders and harmless but we now know that to be untrue. In an article written by Roy K Kropp, studies shown that these crabs are indeed parasitic and do cause damage to corals.

Notice to the right the hole made by this crab. Photo Courtesy of William M Bowen Jr.

​So the million dollar question is….How do I remove this if I find one in my tank? Many treatments were used in attempts to remove this pest. Dips, peroxide, iodine and freshwater were all tried with no success. Manual removal seems to work the best. Mr. Bowen used a pair of tweezers to remove this little bugger. Here are his accounts, “Clear body with a black head and an orange strip on claw that glows under blue actinic lighting. After removed anal end was still stick to a piece of the coral’s skeleton and seemed to have red vein-like strings running through it. The anal end was soft while the body was semi hard but head was unable to break open. One leg seemed to have three little finger-like ends and other legs came to a point. When stabbed in the center it would activate a nerve where the claw would open a little. Removal tried a straight iodine squirt which apparently did nothing upon removal with tweezers, white mucus came out when I found the crab and probably injured it. I'm almost 100% sure wasn't the coral mucus. I rinsed hole after removal to find a lot of white material which probably was skeleton matter. On first inspection on microscope crab was surrounded in semi clear mucus as well” (Bowen M Jr.).

Crab out of the coral

If you happen to stumble upon one of these alien crabs, manual removal is best. On the positive side, the corals recover quickly and begin repairing the damaged area within a few hours of removal. As you can see in this picture below, this particular coral which was damaged is now recovering nicely. 
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We can never be assured our methods of removing hitchhikers from our tanks are successful but if there is a will, there is a way. Patience, a little research and some diligence can solve any issue; just have to be prepared to see the unexpected.

​Happy Reefing!