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Killer Hitchhikers - Bad Crabs
 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 "Kamikaze")
 Some unlucky fish will
be caught & eaten never to be seen again and sometimes they
won't even be eaten but rather mortally wounded (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 here 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 extremely low. That cheap rock
you bought just got expensive in fish deaths. With
AquaCorals, your savings will be counted in precious
livestock that stays alive.
No matter where
you get your rock ... inspect it before tanking it!
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