Names: Red Clawed Crab, Mangrove Crab, Red Crab Scientific name:(peri)sesarma bidens Temperature preference: 70-85F Hardness: 20DgH [reference]
pH: 7.5 - 8.2
Omnivorous (primarily decaying plant matter in the wild) Size: Up to 4cm Origin: Thailand, Hong Kong, Singapore Lifespan: 3-4 years
ago I purchased a red clawed crab for my freshwater community tank, but was
unable to find much information on how to care for it. Here is what I have
learned, largely thanks to the efforts of other hobbyists who have written
to me. Since I began this page some years ago thankfully much more information
about this species has become available. I will summarize what I know here,
but please check the links below for even more information.
(Apologies in advance if I have not credited a source!)
after I started this page, a reader found water conditions in in a book and
sent them to me. I have since updated the table above with new information
from other keepers which I believe to be more accurate. In particular, the
temperature range was expanded and the preferred pH changed from neutral to
slightly high (for freshwater anyway). Note that Jeremy
Gay and others claim them to be intolerant of high ammonia, nitrite and
nitrate. It's certainly a good idea to keep your water clean no matter what
you keep though!
on water salinity for this species vary. Frank
Greco writes that while they range from fresh to brackish water in nature,
in captivity it is best to keep them in 5-15 ppt salinity. That is 1/6 to
1/2 the salinity of seawater. My own experience agrees that keeping them in
100% fresh water is not tenable in the long term.
elements in sea salt are beneficial to these creatures. Consequently, natural
or synthetic sea salt should be used in their habitat as opposed to plain
salt. Interestingly, Frank G. also writes that in his experience iodine supplements
help reduce mortality during molts. Try a reef aquarium iodine/iodide supplement
if you are having problems with molting, but be careful not to overdose.
is dangerous to all inverts, but the levels present in some aquarium plant
fertilizers should not be harmful. It would be wise to avoid copper-based
medications for any tank with inverts, though.
Kind of Salt?
best choice for your crabs' health is salt designed for salt
water aquariums. Sea salt has a lot of trace elements that
plain salt lacks.
an emergency, you can probably get away with plain salt, but switch
back to the good stuff as soon as you can. If you need to get
salt at the grocery store, make sure it does not have an anti-caking
agent such as prussic acid. Diamond
Crystal kosher salt is good, and so is Morton's
canning and pickling salt. Other
types may work too--just make sure they are 100% pure salt with
no additives. And don't use plain salt longer than you have to!
crabs eat pretty much anything that settles to the bottom of the tank. I feed
Hikari loach wafers, Wardley shrimp pellets, and dried tubifex worms. In my
own experience they do not care for veggies, but many readers have told me
that their crabs have torn up some plants. All I know for sure is that my
Java fern was left alone, but your mileage may vary. You may also wish to
try algae wafers, zucchini, boiled spinach and peas for your RCCs.
like fiddlers, need access to the air to survive. RCCs are like this. You
may get lucky and keep them completely submerged for a time, but it isn't
healthy in the long term. Give them an easy way to get out of the water, and
some terrain to explore. Just make sure that there is no way for the critter
to escape the tank or to get close to the hot lights under the hood. (Yes,
this is a reversal of my previous posting, based on experience and communication
with other keepers.)
these crabs can climb nearly any cable or hose, plus they appear to be able
to swim upwards at least 6". Be careful how you arrange your habitat,
and use a tight-fitting hood.
is fast and strong for its size. Be very careful when netting it; it will
try to climb the mesh and escape. In fact, when I bought my first RCC, the
first one they scooped up climbed out of the net, jumped onto the floor, and
ran under the rack of tanks. They probably never got it back.
need to catch a crab, I use a paper or plastic cup. It's pretty easy to corner
it and scoop it up, and it can't climb up the slick sides. Hang up the net.
red claw crab ever does escape, it can live for a some hours without water.
I hope never to test just how long they can make it, but some readers have
found escapees alive the next day. Try putting a dish of water down by the
aquarium, or under couches and other hiding places.
crabs seem to molt every few months. I have seen it happen as often as every
2 months, or as long as 6 months.The old exoskeleton looks just like the crab,
only transparent. The more they molt, the faster they are growing, and presumably
the healthier they are. If your crab has lost a limb or claw, he may regrow
it at his next molt!
the molt you may see your crab turn darker or lighter for a few days. Many
people advise leaving the empty shell in the aquarium for the crabs to eat.
I've never seen my guys doing that but it certainly can't hurt.
is a stressful time for the crab. If you have problems with death during molting,
consider trying an iodine supplement as discussed above.
live in large colonies in damp holes. Males are territorial, and they will
fight among themselves—often to the death. Due to this, I would only
ever keep a maximum of one male crab in a small tank, or maybe two in a large
other critters seem pretty safe from RCCs. The RCC may chase away fish that
get too close but neither fish nor crab have ever been injured in my experience—though
other keepers have told me of small fish being caught and eaten.
Other items of interest: Jason K. tells me that their RCCs get along with
their two blue lobsters. My Singapore
Shrimp have never been bothered by my RCC either.
have larger claws than the females. The photos on this
site will show the difference. The shape of the under-shell is also distinctive.
Males have a long, pointy
element. The same part on females is wider and more rounded. (Thanks to Taryn
R. and Misty J. for this tip, and to Taryn for the photo.)
I started this page, this species has been bred in captivity! See Jukka
J.'s site for all the details. All I will say here is that females carry
red eggs under their body. It may look like their shell is opening. (This
is easy to see in the pic on S.
I had an ich outbreak among the fish in the crab's tank. I learned that sesarma
bidens seems to tolerate malachite green based medications like "RidIch"
quite well. The crab showed no signs of stress while the tank was being treated.
(Side note: Since then I have learned a better way to treat ich, the "heat
+ salt" method.)
have written and asked, "my crab was fine yesterday, but today he is
hardly moving... what is wrong?" They may go stiff and flip over on their
back. Sometimes the crab will drop legs and claws. While I do not know exactly
what causes the "creeping death" I know how it usually ends: badly.
Make sure your crabs are in a proper habitat: clean brackish water, access
to the air, and the temperature not above 85F. A proper habitat is the best
way to reduce stress on the animal.
for "sesarma bidens" and you will find an increasing
number of hits, far more than there were when I first posted this
page in 2000. Here are sites which I found especially interesting.
profile (German) - Note that I list different temp and pH
ranges here based on reader correspondence.
people have written to me with stories of very aggressive crabs. I belive
that like other tank creatures there can be large variations in temperament
for this species. For example, some male bettas can be housed with other short-finned
tropical fish, whereas some will attack nearly any fish around them. If you
get a red claw crab, observe it very closely for the first few days and be
prepared to take it back to the store if it proves to be dangerous to your
to everyone who has written me with information about their RCCs: Hank, Yas,
Randy, Karl, Peter, Albert, Loranne, Misty, Taryn, Jason, Jukka, and especially
Frank who came through with some great updates. (Check out his cool
in 2000: First version posted.
12/28/03: Major re-edit, updated with information from reader email, and generally
brought up to date.
02/16/05: Major update. Added much new information from those who have written
to me. Added links. (Yes, I use nested tables. Deal with it.)
"You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat."