Mini Crabs as Pets

by Keri K. on January 26, 2011

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Male Uca annulipes, saying Hi

Male fiddler, Uca annulipes, saying Hi

My office space is also home to a tank of crabs. The more common species found at pet stores are Fiddler Crabs (Uca sp.) and Red Claw Crabs (Sesarma bidens). They’re also often mislabeled, so do some research before you take one home! These particular species stay small, about 2″ across, making them ideal for the home terrarium.

Much like land hermit crabs, these little guys are often misunderstood by the pet trade, leading to very short lives in captivity. Crabs are not simply neat additions to a freshwater aquarium, but specialized crustaceans that require a little different habitat and care to thrive and be their most intriguing. Here’s why:

  1. These crabs live in brackish water — that is, where salty ocean water and fresh river water mix. They can’t survive indefinitely in only fresh water. (More on brackish water below.)
  2. They’re a semi-terrestrial species, NOT aquatic. In the wild, they spend time on land, digging burrows and looking for food. They can’t survive totally submerged, and require adequate land area in addition to water.
  3. They’re extremely agile climbers and can fit out a very small space. They’ll easily escape through the openings that most fish aquariums have for the filter, airline tubes, cords, etc.

Because of all this, crabs do best and are most interesting in their own habitat.
Here’s what makes up mine:

Crab tank, left, and my paludarium in the background.

Crab terrarium, right, and my paludarium in the background.

Aquarium – Mine is a 20-gallon long. The general rule is no more than 4 crabs in a 10-gallon tank. Of course, more space is always better for your animals, and gives you more options for filtration and landscaping.

LidFresh Air Screen Cover. If you need to keep more heat and humidity in, a glass top can also work.

Substrate/Land area – Fine sand is an ideal substrate, as they enjoy burrowing. I also have a slope of smooth gravel emerging from the water at one end. I’ve given them the hiding places they need with small pieces of live rock donated from co-worker’s marine tanks. I really like the natural look of the live rock, but any sort of shelter, like driftwood and plants, will do. The crabs are more confident with lots of hiding places, and are more willing to sit out in the open when they know they have safe retreats. It also helps avoid territorial disputes when there’s lots of territory to go around.

Filtration – A small terrarium filter will keep your habitat clean and aerated. My Decorative ReptoFilter looks natural and creates a pleasant trickling waterfall.

Female fiddler crab, Uca XXX.

Female fiddler, Uca flammula, doing some landscaping

Heat – Crabs require tropical temperatures — between 72 and 80 degrees F. This can usually be maintained with a small aquarium heater in the water. That will also help to keep the humidity up where it should be, about 80%.

Light – Special lighting isn’t thought to be necessary, but I have a compact fluorescent UVB bulb over my tank for better viewing.

Brackish Water – “Brackish” is considered water that has a specific gravity of 1.005 to 1.010. It might not seem like it, but this is actually a fairly wide range of saltiness, and crabs are fairly tolerant of different specific gravities. I fill a 5-gallon bucket with dechlorinated RO water, then mix in a few tablespoons of aquarium marine salt (not table salt). A swing-arm hydrometer is an inexpensive piece of equipment that makes testing specific gravity easy. Remember that when water evaporates from your habitat, the salt does not. If you’re just “topping off” your tank, add dechlorinated fresh water. Do a partial water change with brackish water every few weeks — more often if you don’t have a filter.

Male red claw crab, Sesarma bidens.

Male red claw crab, looking for dinner

Food – These crabs are omnivorous, and sift the mud with their small claws for detritus. I offer mine foods like frozen bloodworms and mysis shrimp, spirulina and seaweed, and various fish foods. Mine have an obvious preference for the meaty frozen foods! There’s no need for a dish — instead, I sink bits of food along the waterline and in front of their favorite hiding spots. They’ll usually appear to do some snacking, or hustle a clawful back to their burrow. Anything that isn’t gone by the next day I siphon out with a turkey baster (a great tool for quick clean-up in small aquariums).

Crabs grow by molting, during which they discard their entire exoskeleton. If you fear you have a dead crab, check it carefully — it may just be an empty shell! It looks exactly like the crab that left it behind, jointed legs and all. Leave the exoskeleton in the aquarium for a few days, as it will often be eaten for its calcium.

Once you have a proper environment setup, mini crabs are relatively low-maintenance pets that are fun to observe. Co-workers that stop by always take a moment to check out my tank and see what the little guys and gals are up to today.

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Keri is a lead catalog designer for Drs. Foster and Smith and holds a Bachelor of Fine Arts degree from UW-Stout. She shares a small home with her husband, two Chinese Crested dogs, two cats, two ferrets, several reptiles and amphibians, and 30-some gallons of freshwater planted aquariums. See more articles by Keri K.

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{ 33 comments… read them below or add one }

Keith January 26, 2011 at 2:29 pm

Hey, it’s my old crab friends! Do the females come out more often now?

Keri K. January 27, 2011 at 2:25 pm

Yup, there’s almost always somebody on the move or at least sitting out in front of a burrow. At feeding time, I’ve seen everybody at once! Otherwise, the biggest red claw male is usually swaggering around like he owns the place.

Ranfe Ortiz March 24, 2011 at 4:12 pm

Hey u have some of the best advice! I wanted to know how u keep ur tank so clear? I have the same heater but with the humidity the glass get all cloudy and u can’t see as well as I liked to!…

Keri K. March 29, 2011 at 8:17 am

Hi, Ranfe. The lid of my tank is open mesh, so humidity hasn’t been building up inside — the air in our building is usually pretty dry.

I have a bigger issue with salt creep making the glass cloudy. The waterfall filter is great, but it does tend to splash, and that slowly builds up salt. I have to wipe it away every few days.

Another challenge I’ve been having is red algae. It tends to bloom over my nice white sand and on the glass under the waterline. And I don’t trust an algaecide liquid with my precious crustaceans! Instead I use the turkey baster every couple days to siphon up debris that collects on the sand or rocks. It’s like a mini water change and has been helping keep the algae growth down.

tc May 7, 2011 at 9:41 pm

i had one but it died i wonder what they eat.

johnathon dunn July 13, 2011 at 6:24 am

i dont even know if you check this website you made anymore but im going to see if you do. i bought a male yesterday and bought the marine salt and hydrometer, but i have 2 questions.
1.do i just change it immediatly to the salinity i want? or should it be a slow change?
2.i have a male red claw right now in a 10 gallon and im going to get a female soon and then move them to a 20 gallon long with a male and female fiddler is this ok or will the different species fight? could i add more females of each species and if so how many?
thanks Piere(mr.red claw) is very happy i found this website

Keri K. July 13, 2011 at 1:25 pm

Hi Johnathon!

I have used a very simple acclimation process with new crab additions. I put the crabs in a separate container, then added brackish water from my terrarium a little bit at a time over the course of an hour or two, much like you would for new fish. It is less stressful than just dropping them into an environment with a very different salinity.

I currently have a red claw and fiddlers living together, but there’s no guarantee it will work for all crabs. Red claws are said to be more territorial, and I have found that to be true — I now have one large red claw male living alone because he was extremely aggressive to the others! So watch them carefully and give them LOTS of hiding places to get away from each other.

Also remember that male red claw crabs have the same size claws, so they’re a little harder to sex than male fiddler crabs (who have one large and one small claw). However, male reds will have bigger, heavier-looking claws than the female reds will. So far, my male fiddlers have done a lot of claw-waving but no actual fighting, and the females seem peaceful as well. Adding a few more female fiddlers is probably your safest bet.

Happy crabbing!

johnathon dunn July 13, 2011 at 3:27 pm

could i just keep it at 1 male to the tank and the rest female (even the fiddlers)? would that give the fiddlers a better chance?

Jake September 3, 2011 at 8:01 pm

i need some help on keeping my red clawed crab and my adult male fiddler from fighting because evrytime i put them together they fight

Keri K. September 7, 2011 at 8:13 am

Johnathon, having one male will probably decrease the chances of aggression in the tank. But they do have personalities and you should observe them closely for a few days no matter what!

Jake, if you’re already providing lots of hiding places for all the crabs in a large enough enclosure, there’s little you can do to make two males get along. Leaving them together will almost certainly result in missing limbs or death for one or both of your crabs. The best thing you can do is set up a different enclosure for one of the males, like I had to for one of my red claws.

garrett b. September 15, 2011 at 3:36 pm

i had 1 in my 10 gallon fish tank and we found it cralling around in my garandpas bedroom and he thought it was a spider how do u think it could have got out

Jake September 23, 2011 at 6:06 pm

My Fiddler died and i was able to get another male red claw crab they dont seem to bother each other. also i have a female but its under side is turning black what does this mean?

mason December 4, 2011 at 6:56 pm

i thought they were brackish water

mason December 4, 2011 at 6:59 pm

oh wait nevermind lol i didnt read the whole thing

christian24685634 December 17, 2011 at 3:09 pm

i have many red claw crabs and fiddler crabs which fight and is cool lol ,but i keep finding them in the same filter, can you tell me why

GameFreak December 28, 2011 at 10:40 am

Where to buy?

hank hill December 28, 2011 at 8:41 pm

hay this dont tell what to do if it crawls out / mine did :(

crab expert January 22, 2012 at 6:04 pm

keri K, ive been looking at all your answers, and most of them are wrong. I don’t know where you get this information from, but it is faulty

mike pabst January 23, 2012 at 12:46 am

For all of you whose crabs get out you need to secure your lids better like tape over the holes for your air lines and filters tape over the underside of the liud though so none get stuck lol but. check your filters they hide in there like right behind the tube so be careful but I came to this site as a crab lover and I wanna know how big red crabs get ? And what is the biggest fresh water crab I can get ?

Keri K. January 23, 2012 at 11:41 am

Hi Mike. Yes, a tight-fitting lid with no escape holes is a must for keeping Fiddlers. That’s one of the reasons they require their own specialized set-up. Also, it’s difficult to know how large a crab will get without knowing its real species name… I have seen more than one different species of crab called a “red crab” before. However, anything in the Uca family is not going to get an bigger than about two inches.

Hi, Crab Expert. If you have different sources of information for the captive care of brackish water crabs, please share. Unfortunately there hasn’t been a great deal of research done on these little guys, and pet stores often keep them incorrectly.

pet food January 31, 2012 at 6:44 am

The idea of choosing crab as a pet cannot came into my mind. I don’t think that it was possible. But it is cute.

Bonnie Arnold February 10, 2012 at 8:17 pm

I just got two crabs, I thought that they where so cute I have them set up like they should be and the temp is at about 78 degrees. I had no clue about the salt until I was reading this site ( pet store never said anything about it), well my crabs have been eating and moving around just fine and now it looks like one may have died and the other one is not looking to good, what should I do? Any help would be grateful!

Keri K. February 15, 2012 at 8:36 am

Hi Bonnie. Sorry to hear about the loss of your crab. Unfortunately, the poor handling and conditions these little creatures may live through when being shipped and at pet stores is too much stress for them, and they will hide being sick until it’s too late. The best thing you can do for them is provide a proper habitat with dechlorinated, brackish water at the right temperature ASAP. Checking your water parameters and salinity and keeping the habitat clean may help your other crab pull through. Best of luck!

Blacki February 19, 2012 at 6:39 pm

Im thinking about getting a Fiddler Crab, I am 11 and can take care of one (I already full care for two male rats) but my only problem is understanding just how much salt should be in the water! Im thinking about getting a 20gal tank as well if that helps…

loves crab-cackes March 12, 2012 at 10:45 am

I got a male Red clawed crab the other day and he seems content in the one gallon tank I have him in. Is that size ok? I know bigger is better but its all I have at this time. I have all the rest right. Brackish water, heater set at 78 degrees, light, rocks and things to climb on. Will he be ok?

Michele Red-Claw March 14, 2012 at 12:59 am

I baught 3 red-clawed crabs, put them into a 20gallon tank. I baught two males and one female. And have already named them. The female is Luvy (she is just so danty), The male with the bright red claws is Fire (since that is what color his claws look like), And Peepers (the dull Bown male I have). Peepers is the ONLY one that climbs into the filter, no matter what I use, he seems to find his way into that filter. And not only that but if you try to take him out he gets mad. I named him Peepers, because if you even bump that filter to get him out he peeps over and trys to get you. It wouldnt be so bad if his claws wasnt so big for how little he is.
What is so bad about these little buggers is they refuse to eat anything but ghost shrimp. which isn’t good when my pet store is sold out. I have even tried everything from the frozen food that the petstore has all the way to the little minnow like fish that the petstore has. It even has the people at the petstore stumped.
Would it be a good idea to just have a small one gallon tank to fill full of their little favorite ghost shrimp? I am just at a loss, I love me little crabs, and their bad habits. I just do not want them unhappy, and NOT eating.

glaze March 29, 2012 at 2:20 pm

I have a red claw crab male and femail and a cat I worried about the crabs

Trey September 10, 2012 at 12:10 am

I always wanted a pet crab and I had my hermit crabs for two and a half years and they’re fine so I thought about getting some pet crabs. I wanted an oregon shore crab but I figured the salt water tank would be harder to maintain and I haven’t found anyone else keeping them as a pet so I guess getting few red claws and/or fiddlers would be just as fun. The only problem is I bet it will be a pain to keep the water aerated, filtered, brakish and at a specific gravity of 1.005 to 1.0010. How much would it cost to purchase devices to measure and maintain these requirements?

Keri K. September 11, 2012 at 9:09 am

Hi, Trey. There are a lot of variations on a basic crab terrarium setup, for just about every budget. I suggest taking a look at both our Reptile ( http://www.drsfostersmith.com/reptile ) and Fish ( http://www.drsfostersmith.com/fish ) supplies to see all the options! For a small tank, an internal filter can be less than $10. I use a five gallon bucket with a powerhead (a small water pump works the same, under $15) to mix my brackish water. And a hydrometer to test salinity is also under $10. Just like for any pet, there’s definitely an investment up front to create a proper home, but a crabitat can be very affordable.

As for some of the older questions… Michele, if you notice your crabs digging through the sand and putting it to their mouths, they’re eating! Finding little round balls of sand on their “beach” means the same thing. Some of my crabs will come out in the open daylight to grab pieces of food, and some only come out when no one’s around to sift for their microscopic meals. Your crab likes the filter because it’s cozy and the food is coming straight to him.

Crab cakes, it’s possible your crab will be happy alone in a small space, as long as the water quality is good. However, the longer I’ve kept my crabs, the more I see that they spend much more time digging burrows on the beach than hanging out under the water. I think that providing them with an area of dry sand to dig in is very important to their long-term happiness.

Blacki, a hydrometer is a cheap, simple instrument that will let you know just how salty your water is. Just dip it in your brackish mix and the floating arm points to the specific gravity!

Michael F February 25, 2013 at 10:52 pm

I keep 4 fiddlers (2 male, 2 female) in an Eclipse 12 with guppies and they are doing really well. My 8th graders love to watch them, especially the males in their territory dance. I use texas holey rock for the alkalinity, dry ground at the top, and hiding places in the water (the females are almost always hiding in there), and I keep my salinity at 1.005 or so. Since October, the crabs have each molted at least once which I take as a sign of good health.

Thanks for posting good information for others to learn about these great animals!

Monica April 18, 2013 at 12:14 pm

Why are my females mating. Both my females tummies or what ever you call that are open
and they are attached, but the head of the female is in the other females reproductive parts.
My male and the female mated yesterday and now my female are stuck together what gives,
What’s going on is she eating my other female. I am worried.

Robert June 19, 2013 at 6:53 pm

I have a crab with one big claw and one little claw in my a tank and his big claw either fell off or my Cichlid attacked him, is this part of the Molting process??

Michelle April 15, 2014 at 10:01 am

Hey I live in Queensland Australia so you guys probably wont know how to help me
but basically im interested in getting a pet crab and have no idea how to attain one and which species would be available to me.

Ive tried looking online to inquire about pet stores selling pet crabs but have had no luck.

any info would be useful. :)

Thanks,

Michelle

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