Some hobbyists experience unexplained fish losses.  New & established fish are jumping tank (known as "Kamikaze Fish").  Dead fish bodies with large vertical slashes being found on the tank floor.  Then there's the fish that just disappear, never to be seen again.  What could be causing this?!

We all buy live rock & corals, whether tank raised or wild specimens, every piece we buy has unseen hitchhikers.  Most are good guys...but one particularly bad one has been a culprit of death & destruction in many hobbyist tanks....hitchhiker crabs.  Tough as nuts, able to survive in dry shipped live rock & rarely seen upon purchase, these little (sometimes big) hitchhikers arrive in our tanks usually as harmless scavengers.  Over time though, these once docile creatures grow, & appetites change, into meat loving, fish eating monsters!

Signs of Crab Infestation:
Dead fish on the floor.  Fish will jump tank when being attacked, especially at night.  (See my Self Help article: "Kamikaze" Fish) Some unlucky fish (especially new ones who don't know the bad guy is there) will be caught & eaten never to be seen again and sometimes they won't even be eaten but rather mortally wounded (often noted by vertical slashes).  Another sign of a possible "bad guy" in the tank is to see your fish hover in an upper corner of the tank...or may not swim about freely.  We now suspect but what can we do?

Pictured right is a type of "Gorilla" crab noted by the hairy body & legs.

Prevention:
Inspect every coral & rock purchased.  Look into & poke at every hole & crevice.  Even under coral tissue where the coral is attached to the rock.  Sometimes there is a little space & just enough for a small crab to hide.

Intervention:
Think you may already have one?  Then you'll have to go "crabbing".  Crabs are extremely elusive.  You'll almost never see them with the lights on.  Here are a couple of things you can do to find them:

    Try to figure out where it might be staying.  Think of where your dead fish used to sleep.  Where did your new fish go upon being introduced to the tank?  Focus your attention on that area.

    About an hour after all your tanks lights are off (having room lights off is good too) take a flashlight (wrapping the lens with red Saran Wrap decreases the chances of scaring off the critter) point it at the area of suspicion & turn it on.  Do this for a couple of nights.  If you don't see anything...

    Bait the tank.  Do not feed at all for a couple of days (rest assured your beloved ones won't starve!)  Counter sink a "clean" glass jar in your sand baited with a weighted down piece of marine fish or shrimp works well.  The jar should be 3 inches deep or more.  Crabs fall in & can't crawl out.  This is one baiting method but there are other ways to trap.  Do a little research if the "jar method" doesn't work for you.

    If all else fails you may have to bite the bullet & pull out your rocks in the suspect section & inspect every hole in each of them.

Remember, there are many different crab species out there in the ocean that can potentially hitchhike into our tanks.  Each have fantastic camouflage & are hard to detect in dark holes/places.  Take your time & look closely at all new purchases.  Use good light & pay special attention to slight movement.  Healthy corals can handle being out of water for a couple minutes.  Poke into holes with a screwdriver or other poker.  It's much easier to prevent introduction of these pests than to get them out after...once they've caused death.

Final note.  Most crabs arrive in live rock used to build our initial reef structure.  There is a HUGE difference where you purchase your rock!  Though I'm all for aquaculture I have found this rock is almost always inundated with bad crabs & even Mantis Shrimp.  Be careful!  Purchase your rock from a reputable source like AquaCorals where the crab incident rate is almost non-existent.  That cheap rock you bought just got expensive in fish deaths.  With AquaCorals, your savings will be counted in precious livestock that stays alive.