[I have accompanying photos that will be added at a later date.]
This care guide is for true fiddler crabs (Uca sp.) only. In fiddler crab species, males have one very large claw. Many other crab species are sold in pet shops including red clawed crabs (Perisesarma sp.) and vampire crabs (Geosesarma sp.) . These species have equal sized claws and have their own unique requirements that may be completely different than Fiddler Crabs.
Fiddler Crabs make interesting pets and can live comfortably in smaller spaces than hermit crabs, making them an attractive option for empty tanks once hermits have been upgraded. They do have their own care needs and while they are not very difficult, getting them set up properly is key. You’ll notice a lot of similarity with hermit crabs, but since they are mostly aquatic animals there are some other aspects of their care that may be unfamiliar to someone who hasn’t been seriously into fish keeping.
FIDDLER CRABS ARE ESCAPE ARTISTS
No matter how well you lock down, tape to death, or wrap the tank in plastic wrap, the demons will get out at some point. They can climb silicone like hermit crabs and can fit through small spaces like spiders. I’ve had dozens of escapees over the years and I’d say a good 80% of those were completely unexplainable. The rest of the time it looked like they worked on loosening the tape just far enough to slide through. A screen lid is not enough to hold them in. An aquarium lid of any kind is not enough to hold them in. You have to cover every air slat, every tube, every cord with tape or another substance that will keep them from squeezing out of any slit that is as small as their carapace. They can’t make it through tiny round holes for airline tubing thankfully, but they can squeeze through gaps that you wouldn’t think possible. And you need to be checking almost daily for loose areas where they may be working their way out over time. I would venture a guess that the biggest cause of death of established fiddlers is them getting loose and getting lost. There is information below on what to do if you think one may be missing. If you have cats or dogs, they will often be the warning signal that a crab is loose, and you need to move fast because harmless curiosity is enough to kill a little crab within a few seconds. You can pick fiddlers up bare handed to put them back in the tank, and I have never been pinched doing so.
A warning about anaerobic bacteria
Sloped tanks (where the substrate is piled on one side to create land) do not work long-term because of anaerobic bacteria. The same problem with bad bacteria growth that happens to a hermit crab enclosure when the substrate floods (bacterial bloom) will happen with a sloped substrate fiddler crab tank. Anaerobic bacteria (bacteria that grow where there is a lack of oxygen) thrive in dark wet warm tight places – like in the middle of a submerged sand or gravel ramp where there is no air or moving water. It may take a month or two, but at some point you’ll go to clean the tank, bump the ramp, and get a faceful of rotten egg smell and see grey or black substrate. It’s even more dangerous in a fiddler crab tank than in a hermit crabitat, the reason being that in a crabitat the bacteria is suspended in the sand, dies upon exposure to air, and is easy to remove by simply scooping out the affected sand. In a fiddler crab tank where water is present, the bacteria and the waste they create get mixed into the water instantly. Anaerobic bacteria can create a mess of toxins from their digestive process, including sulfer, methane, and acids. All of these byproducts are harmful to living creatures - gases are often heavier than air and will build up inside the enclosure, and solids (and sometimes even gases) will mix with water to create very toxic chemical compounds. While substrate bacterial blooms in aquariums are known to be dangerous, they are not as deadly as those in sloped tanks due to the volume of water to affected sand which allows the toxins to dilute further - in a sloped tank there is usually about equal parts substrate to water, and the bacterial blooms thar occur can be substantial. The only thing that can be done when there is a bacterial bloom in a large amount of substrate is to remove all living things to a clean area, throw away all of the substrate, and perform a massive water change. (Affected sand could be washed out, but you may need to rinse it under running water for a half hour or more depending on how much there is.) Sloped tanks can be safely used, but it requires mixing up all of the substrate twice a week to expose all parts of it to oxygenated air or water, and then recreating the slope, which is a lot of work!
Fiddlers are brackish animals – not quite freshwater but not quite saltwater. They are found in the largest numbers near estuaries, which is where rivers and streams meet the sea. They can tolerate full fresh and full marine conditions for short periods, but they cannot adapt to life permanently in either and their lifespan will be cut drastically short. The recommended level of salinity is a Specific Gravity (SG) of 1.005 to 1.010. This is more salty then fresh (1.000), but a whole lot closer to fresh then full marine (roughly 1.024). Since they are tidal animals they are used to some fluctuation in the amount of salt, so it does not have to be the exactly same all the time and the levels can vary between water changes. It’s a good idea to buy a hydrometer (with a D not a G, sold with saltwater fish supplies) so you know what the salinity of the water is, which will make it easier to keep the levels in their comfort zone. There is absolutely no way to guess and know for sure what the amount of salt is without a hydrometer, although you will have a rough idea by how much salt is being added. A starting point is 1.5 level Tablespoons per gallon to get you to around 1.080, but that can vary a lot based on the salt being used, if it’s gotten wet, etc. Always remember that salt does not evaporate, but water does. Any time there is evaporation in a brackish tank, the salinity increases. If you do a water change with salted water and do not account for that increase, you an end up with too much salt in the tank. Always top off evaporation with fresh water, and make sure to either top off before a water change so you know your starting point, or use your hydrometer so you know what to adjust for. What I did for water changes was measure the SG of the tank, and of the buckets before I added them so I could adjust the salt levels up or down, and then did a final measure of the tank again before it was completely filled so that the last bucket was stronger or weaker to get it to where it needed to be.
The Nitrogen Cycle
Fiddler crabs require a filtered cycled tank in order to have constantly clean and oxygenated water. If you’ve had fish before and know what cycling is, then you’re good to go! In nature, a pond or stream or river or ocean is a huge interconnected system of living organisms including animals and bacteria. Not all bacteria is bad – some species have evolved to fill a niche that does good. All living things create waste, and in a body of water this waste (ammonia) would continue to build up to toxic levels since it can’t evaporate or get washed away to somewhere else. Certain bacteria have evolved that eat the ammonia that is produced, cleaning it up. But that bacteria creates it’s own waste, which is nitrite (with an i) – and nitrite is also pretty seriously toxic to aquatic animals. However, there is a second type of bacteria that eats the waste product (nitrite) of the first, and their waste product is nitrate (with an a). Nitrate is relatively harmless even at higher concentrations, and it’s also what plants absorb as a naturally occurring fertilizer. In any type of aquarium, you have to create the same system of ammonia eating bacteria and nitrite eating bacteria in order to break down the animal waste and keep the water safe. Without the bacteria in place, the water can turn deadly with high ammonia levels in as little as a few hours. The great news is that the beneficial bacteria that are needed are present everywhere and the cycle will occur any time there is a running filter and a food source (ammonia), but it can take a month or more before enough bacteria grow to where they can eat all the waste in the water as more is being added by the tank inhabitants. Cycling a brackish tank is the same as cycling a freshwater tank. There are great aquarium guides out there that explain cycling in detail and offer advice on how to accomplish it without putting your pets at risk. These are a few: http://www.wetwebmedia.com/fwsubwebinde ... ycling.htm and http://www.cichlid-forum.com/articles/f ... ycling.php and http://www.fishlore.com/NitrogenCycle.htm
The filter can be anything used for an aquarium, however something built for turtles or an in-tank filter will probably work better. Basic hang on the back (HOB) are made to be used where the water level is to the top rim of the tank – fiddlers crabs need to be able to get completely out of the water which means that the water level must be at least 2 to 3 inches below that, and these types of filters may not be able to pull water against the force of gravity that far. Turtle filters are made to be used in half-filled tanks, making them an appropriate choice. In-tank filters will also work when the water level is just a few inches deep, although they need to be kept far enough from the substrate to keep from sucking it up and causing damage to internal propeller and motor. There are also really nice (expensive) fancy filters that come disguised in water falls that can also offer land area for the fiddlers. I personally used an outdoor pump made for fountains, and had it rigged to flow into an old and broken HOB filter. If there is a way into the filter, expect at least one fiddler to crawl inside and live there pretty much permanently, since they can sit and be fed continuously with little work. It’s a good idea to make sure there are no moving parts where the crab can reach.
This is the stuff that is placed inside the filter and offers surface area for the good bacteria to grow on. The water that is moved through the filter brings the ammonia and nitrite that the bacteria feed on. The most important thing to remember is to NEVER change this media! If you throw the media away, you’re also throwing away all the bacteria that has grown up to keep the water safe, and you’ll have to start all over again with the Nitrate Cycle. Fiddlers create a lot of solid waste, and this waste does need to be removed from the media regularly to prevent too much of a buildup. Because of this, using a sturdy media that will last for years is recommended. Aquarium filter foam sheets and blocks, sold for some brands of Hang on the Back filters (but n be used in any type of filter) work excellently and will last for decades. Filter floss is polyester batting and it can also be used, although it is much harder to clean and may need to be replaced sooner. To clean the filter media, submerge it in a bucket of tank water that was removed during a water change (discussed below), and squeeze and slosh it around to remove as much debris as possible, and then place it back in the filter. Never do anything that may kill the bacterial colonies (or your fiddlers). The list of things NOT to do to the filter media include running it under tap water, boiling it, microwaving it, or cleaning it with soap or bleach. The only place filter media should be is in the filter or in a bucket of tank water. It also should never be allowed to dry out, as this will also kill the bacteria.
Water changes and vacuuming (using an aquarium syphon) to remove solid waste from the substrate should be performed fairly regularly. The smaller the tank the more often this needs to occur. Fiddlers don’t seem to be terribly fussy about nitrate levels and forgetting or putting off water changes for a little while won’t have the same effect on them as it does with fish. Once the tank is cycled, water changes once a week would be ideal. You only need to remove a portion of the water – around 25% and replace it with newly mixed brackish water.
Substrate for the bottom of the tank can be anything that can be used in an aquarium. I highly recommend a gritty sand substrate like washed playsand. Fiddler crabs will filter out small particles of food from sand, and they naturally live on silty muddy substrate in the wild. However, sand that is too fine (like the fancy coloured or natural sands sold for aquariums) will run into trouble with anaerobic pockets no matter how shallow of a layer if not provided with constant upkeep. I have never had this trouble with washed playsand. Preparing playsand for use underwater means rinsing it with running water in small batches for several minutes in a bucket, using your hands to stir up small loose particles that can run over the edge, until the water runs clean and the clouding subsides within a minute when it is submerged and swirled around. A garden hose is the only safe way to do this, as you could clog the pipes in the house by doing it inside. (Although I have done it twice in the deep sink in the basement without any troubles.) A sand substrate should never be more then a 1/2 inch deep (first joint on your pointer finger) to prevent anaerobic pockets from forming. Even then, it’s a good idea to run your fingers or a chop stick through the sand bed weekly to mix up the substrate and expose everything to oxygen to prevent anaerobic spots from forming. Be cautious when using aquarium gravel and sand – even the natural colored ones are often coated in clear epoxy, and of course the colored ones are coated in paint. These coatings will flake off over time as the fiddlers pick at them.
Ways Out of the Water
Floating plants and/or a land platform of some kind is a must! Fiddlers will spend most of the time underwater, but they do need to be able to remove themselves completely from the water as they wish. This can be achieved by creating a platform using floating decoration or basking logs/docks that are sold for turtles, frogs, or newts. Using thick floating plastic plants (also usually sold for turtles) is another great way to be able to give them places where they can emerge from the water. In a pinch, ZooMed ReptiRocks will float. Fiddlers love being submerged to where just their eye stalks are out of the water, so floating plants can serve double duty for them. Please watch carefully if giving them a sectioned off land area with a ramp – flooding is exceptionally common in these types of tanks, and the same bacterial problems with a sloped tank can happen! If they can be provided a solid land are with several inches of moistened sand and/or cocofiber substrate, the males will create an underground tunnel. In the wild, these tunnels are created below the high tide line and are used to tempt females into joining the male inside to mate when the water rises. Flood-proof home-made ramps and platforms can also be made somewhat easily, please see my posts on the HCA located here for ideas and instructions (although I do NOT recommend using anything with substrate anymore – it kept flooding for no reason!) : viewtopic.php?f=55&t=80873&start=45#p756972
Temperature is dependent upon species, as some can be found as far North as New England and as far South as the tip of Florida. For my own tank, I chosw to keep it between 72 and 74 in winter. It raised to around 80 in the summer naturally, with the heater set to 72 in case of cold weather. The first two winters (2011 & 2012) I attempted to keep the tank warmer at around 78 to 82 degrees, similar to the hermit crabs. However, both of those winters I had massive die-of of nearly all of my fiddlers as soon as the cold weather came around. After lowering the temperature on the heater I did not have another in-tank death until that last tragic accident.
Diet should be as varied as you can make it. Personally, I always fed commercial fish foods and freeze-dried proteins. Mine got a huge selection of different items – I bought things on super sale, decanted a small amount to a smaller container, and deep froze the rest so it stayed fresh indefinitely. They rarely got the same thing for dinner more than once every 3 weeks. They are scavengers, so buying food specifically for crabs is unnecessary, though they really do enjoy it. In rotation for my guys: Hikari betta pellets, freshwater micro pellets, freshwater micro food, guppy food, carnivore pellets (a favorite), crab pellets (also a favorite), algae wafers, marine S pellets. Omega One betta pellets, algae wafers, goldfish flakes. Some off brand of crab granules, freshwater fish flakes and goldfish flakes. Wardley freshwater flakes and goldfish crumbles. Freeze-dried krill, shrimp, crickets, grasshoppers. I also had four kinds of laver (seaweed) from an ethnic grocery that they got occasionally. Items that sink are easier for them to eat, since unless you have floating plants they can’t reach the surface to catch floating food. They don’t eat a lot so be careful not to overfeed at any one time, you can always come back and give them more later. I’ve tried giving things like fresh or dried fruit and veggies but that tends to rot and foul the water. A natural source of calcium like a chunk of coral is a nice thing to offer (and gives them something to climb on), but it doesn’t seem to be a necessity as they seem to be able to get what they need from the salt mix and a commercial diet. Larger items like algae wafers they will learn to take from your hands, but be prepared for them to grab onto your fingers (not pinching) and either not let go or try to climb up your hand!
Decorations and Hiding Areas
Decorations can be anything used for fresh or saltwater tanks, including driftwood and coral. (Not live coral, you want the dried white stuff that is used to build up reef structures.) Fiddlers should be provided with lots of hiding spots in the form of aquarium ornaments, or natural items like shells. They get startled by movement easily – remember that pretty much everything where they come from wants to eat them – so they will be calmer and less stressed in an environment where they can quickly hide. A fiddler crab tank is a great place for those unwearable hermit crab shells that we all have stuffed somewhere! Plastic aquarium plants can also be used, and they will climb and hang out on just about anything you put in with them, both underwater and above. Fiddlers crabs are excellent climbers, so if you can take advantage of the space at the top of the tank, it will be used. Unfortunately there aren’t very many live plant options for a fiddler crab tank due to the strength of salinity they require. Since fids do best in the sweet spot between SG 1.005 and SG 1.010, it’s a little too salty for most brackish plants but not salty enough for anything marine: http://www.aquascapingworld.com/threads ... -ups.4751/
Life Span and Maximum Size
There are stories of up to 8 years, although 4 to 6 seem to be pretty amazing. They are going to be adults when you get them, so there is no knowing how old they already are. Smaller are younger, and bigger are older. Full adults have a leg span of 3 to 4 inches (the width of your palm), with some big males being recorded at 5 inches. A full grown male’s claw will be close to that size as well. They stay slender, so their size isn’t shocking unless you go to move them and get a sense of how big they are or you have a known size reference in the tank.
Fiddler crabs molt every few months when they are younger, to once or twice a year as they get older. Molting is a quick process for them, just like with shrimp. They pop out of their exoskeletons from an opening inside the fold of their tail and they can immediately move around and eat and interact with others. They will be soft for a few hours to a day afterwards and can be injured easily, so don’t work on the tank or do anything where they may be bumped. They are at no risk from the other fiddlers in the tank and should not be separated. Leave the exoskeleton in the tank for the molted crab and the others to eat as it’s a good source of calcium and minerals. You may find whole sections of the exoskeleton that have not been eaten at a later time, including entire leg segments, carapaces, and long pieces of the males’ claw. A freshly shed exoskeleton looks exactly like a limp crab, only paler and often slightly transparent.
Regeneration of Lost Limbs and Claws
Just like hermit crabs, they repair limbs on their next molt. Unlike hermit crabs, the new limbs usually look completely normal in one molt, although a really big claw on a male may take an additional molt to grow to the proper size.
Stocking aka How Many
When it comes to how many fiddlers a tank can support, there are a few things that need to be considered. One is how much above water space there is, as all the fiddlers should be able to emerge from the water at the same time without touching each other. Another is how many gallons of water is in use in the tank, as there should be at least 1 to 2 gallons per crab. If the ‘empty space’ in the water is filled with plants or other climbing items then it can be as low as 1 gallon per crab since they will be able to spread themselves both upwards and outwards. Filtration is another very important aspect, as water quality must be maintained at all times and the bacterial colonies should not be overwhelmed with too much waste. The larger the filter and the more water it moves per hour (GPH), the more crabs the tank can support without trouble. It’s best to start slow with a few crabs and watch the ammonia, nitrite and nitrate levels in the tank as more are added.
One Male, Multiple Females
Only keep one male per tank, they are territorial and those claws are not just for display. Every time I tried multiple males in the same tank, it became a Highlander movie and only one remained within a month or so. No matter how small they are, or how close they are in size, one will be the victor and the other will end up dead with a puncture hole through the top of it’s shell. I had a 29 gallon tank that was 2 feet long and there was not enough space for males to share. I imagine that if you have a four foot tank with multiple land areas in different places for them to lay claim to and display on, and of course have enough females to go around, that it could potentially work with two or more males. Females are completely benign to each other and to the males and can live perfectly happily being rather cramped. I really recommend having a fairly large harem both for their comfort and to keep the male happy – the ladies like to stick together in numbers for safety, and having both genders will cause them to act more naturally. (Plus, a displaying male is wonderful to watch.)
Sexing Fiddler Crabs
The easiest way to do this if they have both claws. Males have one big claw that will range from twice the size of the other claw to more then the size of their bodies. Females have two equal sized small claws. Males have been found in captivity with two large claws before, but it’s a rare mutation. A fiddler’s sex organs are hidden beneath the flap of their tail (yes they have tails, but it makes up the underside of their body) and the organs are only exposed during mating or when the females are carrying eggs. However, the tail flap indicates their gender, as the female’s is very wide and the male’s is slender. This can be seen from the front when they are standing.
Handedness of Males
While a male will always have one claw that is larger, it is completely random if it will be the right claw or the left. Fiddlers populations are split almost perfectly 50/50 between lefties and righties. What is even more fascinating is that if they do somehow loose their large claw, it’s the remaining smaller claw that will turn into the large claw on the next molt. That way they can get it back to size quicker and be able to win over the ladies again sooner.
If conditions are right and you have a mature male that is larger than your females – and they are the same species – fiddlers will mate in captivity. When females carry eggs it’s known as being “in berry”, a term taken from shrimp breeders. They can develop eggs at any part of the year and most females will carry between 2 to 4 clutches per year. The eggs start off a very dark brown and will lighten as they age. Carry time is around 3 to 4 weeks, with females in berry being most common during spring and summer (roughly February to September). Just like hermit crabs, the young are considered to be nearly impossible to raise in captivity. They live as marine zoea for several weeks, which makes them hard to feed and keep in clean enough water since normal filtration methods will kill them. After a period of time, the zoea fall to the bottom and roll around finding food, and then hunker into debris to continue molting until they are large enough to survive venturing onto land. It’s common to have females in berry if you have good conditions, but it’s next to impossible to get viable young. There is no need to separate the genders to prevent them from breeding. You can see images of their life cycle here:
Color Changing Crabs
Fidders can change the color and pattern on the carapace at will. In the wild it’s been noted that they will change colors for a variety of reasons, from coming into contact with another crab, being near a crab of the opposite sex, when they find food, when they don’t find food, when they fight, when the tides change or the weather changes. In captivity when conditions are stable it usually doesn’t happen as often, but you will not be able to tell them apart in the future just because one is pink and one is brown right now – in another hour they might the same shade or might have swapped colors. There is usually a major color change within the first few days of coming home as they relax and settle in. They are often darker in the store and show less pattern on their back. Females have a much wider range of variability then males. An example of the color changes a fiddler crab can show (with Australian crabs) is located here: http://www.plosone.org/article/info%3Ad ... ne.0001629
These are not calm little animals that you can interact physically with. They are super quick, and will fly out of your hands or wherever else you have them. If you need to transport them, do it in a cup deep enough where they cannot reach the top or get traction on the sides. You can offer them treats by hand which they can learn to expect when the lid opens and some will come and wait for it, but otherwise enjoy them where they are!
Not Dangerous to Humans
I have never been pinched by one of my fiddler crabs, although they remind me of spiders so I was always on guard around them and I have never given them the chance. I have picked them up several times with my bare hands to move them (when they are new, or when they have escaped, or for photos) including the males without issue. The big males made me nervous and I was always wary of not letting them get that claw around my finger, but I never had one come after me or seemingly even try to cause harm to me. I also hand fed crabs at the surface often. Females will pick at your fingers if you offer them food or get your hand too near, but it’s a little cleaning pinch that tickles.
Take a deep breath and double check the tank. They can squeeze themselves into really tiny places, so carefully move and check in, on, and under everything before panicking. They can also dig and bury, but they normally leave tunnels to the surface. If you cannot find the crab in the tank, you need to start searching the room. If you have dogs or cats or other pets that are loose in the house confine them to an area far away from where the fiddler crab tank is, because a loose fiddler is a great playtoy. Unfortunately my cats usually found the escapees first, and it often didn’t end well. Fiddlers dry out quickly and most times won’t travel far. They tend to end up inside or under the aquarium stand, or underneath furniture nearby. They aren’t great at climbing hard surfaces, and if they are by themselves they don’t like open spaces. They hug walls and other solid items looking for a place to hide until it’s dark. Put stinky bait out like fish or other seafoods to draw them in, and leave out small bowls of tank water wherever you can so they can stay hydrated. They may only last a few hours to a day outside the tank, so time is of the essence. Remember that they are very tiny and can hide in ridiculously small areas, so be careful not to crush them while searching. Most of the time all you’ll see is legs sticking out of somewhere, and yes, they look like giant black spiders when outside of the tank. Insanely so. I always had to steel my nerves to pick them up!
Harvested from the Wild, in the US
All fiddler crabs are wild caught, which is why there is often heavy losses of new crabs. However, unlike hermit crabs, fiddler crabs are well studied and the populations are being monitored. True fiddler crabs (one large claw) are mostly harvested here in the US and are not imported. There are dozens of species along all the entire coastline of the United States. I have not found any guides to figure out species for females – they apparently look the same! Males however can be identified by their claw, but one does need to have a rough idea where the crab came from. It’s likely that the crabs are being shipped in from whatever place is closest to where you live, so Pacific fiddlers for Western crabbers and Atlantic fiddlers for Eastern crabbers.
Picking a Healthy Fiddler Crab
You want one that is active and climbing. They are very spider-like in their behaviour and will be extremely fast and will constantly try to escape from wherever they are being confined once bothered by a human, so long as they are healthy. The best conditions would be where they are being kept in brackish water, although I haven’t heard of any places doing that. The more time they spend in freshwater the harder time they have bouncing back, so find out when delivery day is and get them as soon as possible. Try not to buy males that have been housed with each other, as they may have already injured each other. They should support their weight entirely on their legs and their eyes should be bright and upright. (Mind that they’ll be moving around and will hunker down and clean their eyes often, but an unhealthy crab will have trouble lifting their eyes and bodies at all.) I have purchased crabs that had mold growing on their limbs from being in stagnant, food-fouled water and also crabs who were missing various limbs who have completely recovered. It is very common for males to drop their large claw and it will grow back on the next molt, so don’t be alarmed if a limb is missing
Sudden Deaths of New Crabs
They suffer from their own form of Post Purchase Syndrome. Since they are wild caught, they have to adapt to life in captivity the same as hermit crabs. You also can’t know what they may have been subjected to during transport. They are very delicate animals physically, and it doesn’t take much to harm them. The people who collect them, ship them, and sell them use freshwater since it’s cheaper and less of a hassle – the more time they spend in the freshwater, the worse they will be. Like hermits, if they make it through their first molt in captivity they seem to be fine and will live a normal life so long as the conditions are kept perfect and they don’t escape.
The first two years I had fiddlers were a challenge. Heating the tank too high caused a massive die off two winters in a row. I also lost all of the Uca pugilators within a few months of purchase, and there were over a dozen. Since that massive die-off in December 2012, I hadn’t purchased more of that species, and I didn’t loose any crabs in-tank. All of the deaths since then were due to them escaping the tank and either getting lost or being found by our cats. Deaths can certainly be random since we don’t know a lot about diseases that may affect them, but most deaths seem to be directly related to their environment.
Determining If a Crab Has Died
The smell is unmistakable if they’ve passed – take a dead hermit and mix in some dead shrimp. They tend to melt if they’ve been dead a while, but they will start to smell quickly. Fresh molts will look exactly like a living crab, only lighter and hollow. Unlike hermit crabs, they don’t slow down before or after molting, and there is only a very short period of a few hours where they may look like they are dying before a molt. If you have one get loose in the house you may not smell if something happens if the environment is dry, as they will desiccate and mummify quickly.
There are 102 species of fiddler crab in the world, and more than a dozen within the United States. I’m sure there are ways to ID which species are being used in the pet trade, and I’m sure that scientists who work with them daily can do it, but for the average person trying to guess by using pictures it’s extremely difficult unless the crab looks exactly like a photo. Knowing the location of where they are found is a major way of narrowing down the possibilities, and that information is not include when they are sold. Images online are often falsely identified by well meaning people, so unless they are from an academic source they cannot be trusted. There are no ID guides for female crabs, but there are ones for male crabs. These a few: http://www.usm.edu/gcrl/public/gulf.cre ... xonomy.php and http://bugguide.net/node/view/354364/bgpage also try http://www.fiddlercrab.info/uca_species.html
Mixing Fiddlers and Hermit Crabs
Many crabbers have tried this, but not many have reported back after a long period of time to say if it worked. The ones who have had losses of their fids. While they do need similar conditions, fiddlers specifically need a very large and filtered brackish water area in order to molt and spend time in. This is not something that many crab tanks can fit, and water quality becomes an issue thanks to the crabs dragging who knows what into the water on a regular basis. Hermit crabs will also prey upon fiddler crabs if they find them in a weakened or molting state. Fiddlers are also able to escape from much smaller holes then hermit crabs so even if you had a large enough enclosure to mix the two with the extra tank for brackish water (say a six foot long tank), it would need be to locked down tight to prevent them from getting out which would make daily care for the hermits that much harder.
Keeping Fish with Fiddlers
If you want to add fish to the fiddler tank but haven’t kept fish before, then a mixed species brackish tank may not be the best starting point. It’s a breeze if you’re going from fish to crabs, but the other way around is much harder and there is much more to learn and fish are a lot less forgiving if mistakes are made. Water quality becomes paramount and it is very easy to overwhelm your bacterial filter with too much waste when adding fish on top of crabs. Please also keep in mind that the fiddler crabs do catch and eat fish, so anything smaller or the same size as then them should be considered dinner and should not be something you feel strongly attached to. Larger species of fish will make dinner of the fids, including any size of puffer, even the tiny ones! The type of fish that are good choices are based on how much water is present, and the size of the tank. A 10 gallon tank is really too small to mix fish with fiddlers since there isn’t enough water for the waste to be managed properly. A 20 gallon tank that is only a third filled with water is again really not a suitable home for fish either. Please make sure that the fish you’ve chosen to keep are given adequate space to thrive. Again, don’t go by pet store recommendations! They want to make money and I’ve personally seen them selling animals that get much larger then they claim. I very much recommend joining a good fish forum (I like http://www.fishlore.com/fishforum/brackish-tanks-forum/ or http://www.fishforums.net/index.php?/fo ... ish-forum/ as they’re usually kind to newbies and are very knowledgeable) and asking their brackish keepers what they recommend as a good choice for the space and amount of water you have. Another great resource is Neale Monk’s archives at http://brackishfaq.webspace.virginmedia ... shfaq.html as he is considered a guru of the brackish world. One more place to check is Badman’s, but it is many years old and some advice and information may have changed since it was written: http://badmanstropicalfish.com/brackish/brackish.html
Keeping snails with Fiddlers
Several species of nerite snails are brackish by nature and do great with fiddlers. Keep in mind that nerites will be triggered to reproduce in brackish water and females will leave egg sacs (little white dots) all over everything. There are contradicting reports on whether the eggs can successfully hatch in brackish water, or if they require a full marine environment. I had two females so I can’t add to the debate, although both were bought at a breeding age and I never had any babies. Snails will produce a lot of waste on their own and should not be considered a clean up crew. They will take care of any algae on the glass, but you will have to clean up their poop along with the crabs! One nerite snail can be kept per 10 gallons of water. There is no way to sex a snail unless you’ve caught them mating or see a female laying eggs.
A scientific-oriented site that lists all known species of fiddler crabs from all over the world. Has range maps, identification drawings, and photos of most species.
A blog from several years ago that contains information about fiddler crab care, and also breeding attempts.
Really in depth paper on all aspects of fiddler crabs including illustrations of fighting, male displays, and descriptions of dozens of species.