The Care and Maintenance of Fiddler Crabs

Bob, male Uca Pugnax fiddler crab

First I shall introduce the crabs in the photos I will be using in this post.  Bob is the male fiddler crab and Claudia is the female.  Bob has one very large claw, that is how the males are identified as being male.  The females have two small claws, like Claudia.

Claudia, female Uca Pugnax fiddler crab

What They Are and Where They Reside in the Wild

Bob and Claudia are part of the Uca Pugnax species of fiddler crab.  They are often called by the common name of marsh fiddler crab because they typically reside in tidal pools and brackish marshes along ocean coasts.  Other fiddler crabs often kept as pets are part of the Uca Pugilator species. The care of both species is similar.

Fiddler crabs are considered marine animals.  They are crustaceans and listed in the phylum of arthropoda with the other arthropods insects and arachnids.  Fiddlers must be kept in salt water conditions but can tolerate very low salinity for a SHORT period of time.  Brackish water is ideal.  Brackish means slightly salty, which means the salinity can range between 0.5 ppt (parts per thousand) to 29/30 ppt (parts per thousand).  If they are left in fresh water, with a salinity below 6 ppt, they will die a slow death over a couple of months. 

In the wild, males and females spend a deal of time on land and an equal amount of time in the water.  Because they are crustaceans, they can dry out easily if they do not have access to high humidity, damp ground, or water.  Males and females of most species enjoy burrowing, with some exceptions of course.  Bob does not care for burrowing but Claudia digs herself a new burrow every few weeks.  The preferred land substrates for fiddler crabs are sand and mud, so they can easily dig and eat.  Land and water areas provide food as well.  Fiddlers use their small claw (or both claws on the female) to pick up pieces of sand and dirt to put in their mouths.  They pass the particle through their mandibles a few times to eat off the bacteria and other microbes on the particle.  Uca Pugnax crabs appear to enjoy eating from the sand in the water (probably because it harbours more microbes), however, they will eat things out of dry sand from time to time.

At high tide on the beaches and marshes, the crabs will reside in their burrows or will even return to the ocean.  At low tide they emerge from the burrows and the water.  Marsh crabs differ from sand crabs (uca pugilator) in these behaviours, but at this point, it is not necessary to go into detail about the small differences.

Every fiddler crab you see in a pet/aquatic store or Wal-mart has been kidnapped from its native beach or marsh.  Fiddlers cannot be bred successfully in captivity and this why they are captured from the wild.  Fiddler crab larvae require deep ocean conditions which cannot be replicated by human means, which means you will not ever have a true "baby" fiddler crab.

Aquarium Set Up, Food, and Care

So you went to the pet/aquarium store (or wal-mart) and have brought home a fiddler crab, you now probably have questions regarding the proper care.  Most pet stores (and some aquarium stores unfortunately, and wal-mart too) list fiddler crabs as fresh water crabs.  That is horribly erroneous and if kept in fresh water,  your crabs will die.  Before i describe the correct aquarium tank set up, i'm going to list what you need to acquire from the pet store first:

• Instant Ocean brand marine aquarium salt (other brands just aren't as good), preferably a box with enough salt for a 10 gallon fully marine aquarium.  Do not worry, you will not use the entire box all at once.  In fact you may not use up the entire box for quite some time.
• 10 Gallon Aquarium Tank, it can be a less expensive brand and they usually cost about $15 at Pet Smart.  (Go for a 20 gallon or larger if you have multiple crabs or there will be hostility in the tank)
• Submersible Filter, preferably one that lists that it does biological, mechanical, and chemical (charcoal) filtering.  I recommend Duetto filters, they do good work and aren't very expensive.
• Plastic Aquarium Tubing of Varying Sizes because plastic aquarium tubing has many many uses around the house even if you won't need it for your filter's air supply.
• Freeze Dried Brine or Baby Shrimps, Tropical Fish Flakes, Freeze Dried Plankton, Seaweed, Algae Disks, and Freeze Dried Blood Worms are what the crabs need to eat to remain healthy, they cannot survive on microbes alone (and often times there aren't even enough microbes when you start a new tank)
• Play, Aquarium, or Bio Sand because crabs need sand, not pebbles or gravel.  Pebbles and gravel can cause injuries and they also cannot pick them up or pick things off of them to eat very well.
• Dechlorinator for Aquariums chlorine is a stress inducing and killer of fish and water invertebrates and there is chlorine and chloromine in all tap water.
• Tank Screen for the top of the tank.  Crabs are excellent climbers and master escape artists so a screen is necessary.
• Heat or Sunlight without Heat Lamp.  Lamps are necessary for your crabs, even a standard plant light can give enough heat when set on the screen to keep the tank warm and bright.  Crabs enjoy light and heat so if you can find a lamp that emits a small amount of heat with decent sunlight mimicking light, then the crabs will be happy.
• Stick on Temperature Gauge, so you can make sure the crab's water/land is between 75 and 85 degrees Fahrenheit.  These are usually very cheap and you can get two, one for land and one for water.  They stick on the outside of the tank quite nicely.
• Optional Aquarium Decorations and Plants.  Fiddlers spend time hiding behind things because they are lower down on the food chain and are easily frightened especially during molting (i will explain molting later on).  A few yard rocks (nothing containing metal, concrete, or asphalt) can be arranged in the aquarium so they can hide.  You can also purchase little decorations with holes in them so they can hide in those.  You can even take pieces of PVC piping (rinse it well) and put it in the land sand so your crabs have ready  made burrows.  Plants are optional, crabs enjoy climbing them and if you can find real brackish water plants, they will even nibble them every now and then.  If you have a tank set up that doesn't have land, i strongly recommend floating "driftwood" like aquarium decorations and tall fake aquarium plants because crabs must leave the water for certain amounts of time or they die.
• Hydrometer, this measures the parts per thousand of salt in the water as well as the specific gravity.

Aquarium Set Up

Bob's Tank Set Up

 Now that you have the necessary items we can move onto the tank set up.  Your tank, if it is not fully aquatic, should look like Bob's tank there in the picture.  To get the tank set up properly, we first added 2 gallons of de-chlorinated salt water.  The easiest way to de-chlorinate and mix salt with the water is to use a plastic milk jug (rinsed with hot water, but just rinsed, do NOT use soap!).  You add warm tap water to the jug, put a few drops (or what the instructions on your dechlorinator said to use) of dechlorinator in the water, put the lid on, shake, then take the lid off, use a funnel to add the marine salt.  For 2 gallons of water i recommend 2-3 tablespoons of the Instant Ocean marine salt.  Shake the new salt water mixture a few times until the salt is dissolved.  Now poor that into your tank.  

Next, take your sand (it took about 20 lbs of aquarium, play, and bio sand to get Bob's tank to look like this) and add it to the water.  It will look terrible, brown and cloudy but that's ok.  Pile most of the sand to one side of the tank and keep adding sand until you've got the desired height of land.  Notice how as you pile sand to one side the water gets deeper.  If you find you have too much water, take one of those plastic aquarium tubes, place one end in the water while hold your finger over the other end.  Place a bucket near the tank, put the end of the tube your holding in the tank and let your finger off of the hole.  The water should flow into the bucket.  If it does not, keep the one end of the tube in the water and suck on the other end until the water is just about near your mouth (don't drink the salt water, it will make you vomit).  Place your finger over the end of the tube that was in your mouth quickly and then place that end in the bucket.  It sounds complicated but it's a lesson that is quickly learned once you do it a few times.

Once you've got a good sand to water ratio going, find yourself some decent sized rocks (no metal, no concrete, no asphalt) and place them in the water at the base of the sand hill and push them into the sand a bit.  This will help keep the sand hill from eroding into the water area.  

The water will be dark brown or murky, this is alright just wait a few hours or a day and the sediment will settle.  Once the sediment has settled, place your filter in the tank according to the instructions that came with the filter.  Add the other decorations you want at this time as well.

Different View of Bob's Tank

 Before adding the crabs make yourself familiar with the nitrogen process/cycle ( http://www.fishlore.com/NitrogenCycle.htm ).  Crabs are hardy creatures but ammonia levels can get to them and make them sick and eventually cause death.  If you have the crabs already and they must be housed in the tank, feed them sparingly but daily at first and put most (not all) of their food on the land when you feed them.  Ammonia and PH test strips are essential for maintenance and health of the crabs.  Also at this time you should use your hydrometer to check the salinity of the water.  My crabs have done well with a salinity of 20-22 PPT with a specific gravity of 1.014-1.016.  If yours is lower, that's okay, as long as it is above 1.008 specific gravity.  It's okay too if it is a little higher (as long as you haven't produced brine, which is 50 PPT or more).  If it's too low, when you need to add water, just add more salt to that batch of water.  If it's too high aka it has become brine, you will need to remove the water and use considerably less salt when you make a new batch of water.  Be sure to place your lamp on the tank to heat the water.  If the water gets too cold your crab will move very slowly and may not wish to enter the water at all.  If the water (or land) gets beyond 85 degrees Fahrenheit, your crab can end up cooked.  Ideal temperature is between 75 and 85 degrees Fahrenheit.

Maintenance

 Feed your crab daily 1 brine shrimp or 2 plankton and a few fish flakes (flakes in the water only).  Every few days switch up the routine with 3 freeze dried blood worms, a few fish flakes, and a couple tiny pieces of seaweed (seaweed can be given in the water and will promote algae growth on the filter and rocks, which your crabs will eat).  Every once in a while provide a small piece of algae disk in the water.  If you have multiple crabs in one tank, up the feeding accordingly and always be observant of how much your crabs eat in a day.  Overfeeding can make the tank offensive and the water murky and ammonia filled.  Crabs spend a large part of their day "grazing" the sand (both in and out of the water) for food and after the tank is established for a few weeks, there will be microbes in the sand that the crabs will enjoy but you must still provide other food for them.  Crabs enjoy eating rotting things so if a piece of food gets a bit gross on the land, eventually the crab will probably eat it.  If not, take it out.

Water evaporates quickly in the summer from the tank so you may have to do frequent water adds.  Add only a tiny amount of salt to the new batches of water (less than a tablespoon) and if you find that your water is getting too salty, skip adding the salt for a couple adds.  Always dechlorinate any new batch of water even if you don't add salt.  One quick common sense note, NEVER USE TABLE SALT or any salt made for human consumption, our salts contain chemicals that will kill the crabs.  Always use marine aquarium salt.

Crabs (like other arthropods) shed their exoskeletons to grow, this process is called molting.  When your crab is about to molt he or she may become reclusive, hiding frequently or staying in a burrow for most of the day.  This is normal.  Fiddlers will also skip meals before molting but do not stop providing the food because they will eat it after the molt is complete.  Your crab will need the water in the tank to actually perform the molt.  When they molt, they need to be in the water because they absorb it to crack the old shell.  Weeks prior to the molting action, they reabsorb calcium carbonate from their shell and produce enzymes to loosen the shell from the "skin" underneath, this skin then starts producing a new soft shell. The molt itself seems to take only a few hours at the very most (usually quite a bit less time, about 20 minutes).  After the molt, the shell is left aside and it will look like a weird ghostly version of your crab.  But the real version of your crab may be no where to be seen, fear not.  Crabs hide for extended periods after molting.  At this point, until roughly 2 weeks have passed, do not alter their home or remove them from their tank and do not handle them.  After the molt they are extremely vulnerable to injury (which is why they hide) until their new shell hardens.  Fiddlers molt times depends on their age.  Young fiddlers molt more frequently while old fiddlers may not molt at all anymore.  Bob and Claudia both molted in the Spring after i saved them from the wrath of Wal-mart. When we got Bob, he was missing a leg and after his molt he grew a new one.  That's a neat thing about crabs, if they lose a leg or claw they will regrow them with a molt.

Adding Friends to the Tank

Some will argue that fiddler crabs are somewhat social animals.  I beg to differ.  Male fiddlers seem to be rather obnoxious when it comes to territory.  True, they emerge from the ocean in groups, live in groups, but i don't think they really want to be friends with each other.  Our Bob kept waving out of his tank and from various articles i read i assumed he was doing mating waving.  So we searched out a female, Claudia, and introduced her to his tank.  Well, Bob attacked her (not sexually) as soon as he saw her.  He got a look about him at first like "Who the bloody hell is that?" and then he charged her.  She cowered down and he scraped her carapace with his claw a few times then let her run off.  From that point on he would not let her leave the water and had her confined to a small corner behind the filter.  Every time she attempted to leave the corner he would chase her back into it.  I removed Claudia from his tank after viewing 2 days of this activity from Bob.  She now has her own tank and we have come to learn that his waving is strictly territorial.  He still waves violently and obnoxiously at her from his tank while staring into hers.  She ignores him most of the time but at one point a few weeks ago went to the corner and did a few waves of her own to which Bob responded with violent waves of the big claw.

I don't recommend giving these creatures "friends" of other species.  There aren't a lot of brackish animals that fiddlers will get along with.  If you add friends of the same species, put everyone in the tank together so they can duke out territories at the same time.  That way you don't end up with one dominant jerk crab, like Bob.  I also don't recommend mixing crab species as some grow larger and more aggressive than others.  Uca pugilator are generally larger than Uca Pugnax.

Bob Doing a Defensive Wave

At this point I'm going to wrap up this article.  I hope it is of some use to those with fiddler crabs.  If you have any questions about your crabs or tank set up, feel free to ask, I may be able to help you out :-)  If you have any advice (or corrections for my article), please share.