It’s getting more and more common for people to add creatures other than fish into their aquariums, and fiddler crabs are one of the most popular invertebrates you can buy.
They are easy to care for, which makes them great for those keeping crabs for the first time. They won’t take up much space either because of their small size.
Their claws demand your attention. Males have one larger claw which sets them apart from other species. This unique look creates a different attraction for your tank.
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Fiddler Crab Facts & Overview
|Color:||Various, but most are brown/orange|
|Minimum Tank Size:||10 gallons|
|Tank Setup:||Shallow water with areas above the surface|
|Compatibility:||Species only tank or carefully selected community|
Fiddler crabs, sometimes known as calling crabs, are a large group of crustaceans which make up the Ocypodidae family along with ghost crabs. There are roughly 100 closely related species belonging to the genus Uca (Fiddler Crabs). This article will cover how to care for all types of fiddler crabs.
They are found throughout the world, ranging across both sides of the Atlantic, the Eastern Pacific, and the Indo-Pacific.
In these areas they enjoy mangroves, salt marshes, and sandy/muddy beaches. These are coastal habitats with brackish water.
Unfortunately many pet stores advertise them as a freshwater species, but keeping them in freshwater hurts their health and reduces their life span. In the right conditions they can live for 2-3 years. They are easy to care for and will even survive some basic mistakes.
If you have only kept fish before, then there are a few things you’ll have to get used to, such as their regular molting. There are lots more important differences which we’ll cover throughout this article.
Read Related Article: Vampire Crabs: The Full Care Guide
An important skill for living on the coast is the ability to survive both in and out of water. These crabs are semi-terrestrial meaning that their time is split between sitting on land and being submerged in water.
They have both gills and a primitive lung, which lets them breathe whilst in water and air.
Another interesting behavior you will notice is their claws. This could be used for courting females, digging burrows as shelter or fighting with other males over territory.
Individuals can often be seen raising and lowering their claws as if they’re waving, this is their way of communicating to each other. The signal they give changes depending on how far away the target is.
They’re generally peaceful creatures; competing males is the only time aggression is shown.
Every eight weeks they will molt, leaving their old exoskeleton. If they have lost a limb, a new one will have developed by the time they molt.
Fiddler Crab Facts
- They’re known as “detritivores” because they sift out dead matter from the substrate to eat.
- Some species use “dishonest signaling” to intimidate males. The Uca mjoebergi often grows a weaker claw if it loses one, but continues signaling with it to intimidate stronger crabs.
- Males can’t use their major claw to feed, so females are much more efficient at feeding because they can use both claws.
- They are in the order “Decapoda”, which means having ten feet.
- Fiddler crabs can change their color in response to social interactions and changing environmental conditions.
The first thing that stands out when you see fiddler crabs are their claws. Males have one major claw and one minor claw. The major claw is much bigger and gets prioritized as a tool. Females have two minor claws.
Behind their claws are four more pairs of legs that are used for walking.
The whole crab is covered in a hard coating to protect it from attackers. The main body is protected by a carapace (a hard plate which protects the organs).
At the front of the body are two stalked eyes, a pair of antennae, and a rostrum. The antennae are used to gather information about their surroundings.
Most of the species you can buy are quite small, rarely having a body larger than 2-3 inches.
Color will vary depending on the stock you find, but they are generally not as colorful as fish. Most are some shade of brown or orange all over, but you may find some with more distinctive coloring and features.
The Uca perplexa, for example, has a bright yellow major claw which stands out against the rest of its brown body.
It’s easy to tell males and females apart. Females are smaller and have two small claws, only males have the major claw.
Tank Conditions and Natural Habitat
These crabs normally live on the coastline, a habitat which is known for its changing conditions.
The change in tide, and their natural movement, means that some of their time will be spent underwater and other times they’ll be on land.
Their surroundings would include a sandy substrate and a variety of rocks to hide in between. Plants are also sometimes present.
The brackish water would be warm, slightly alkaline, and well aerated by water movement and waves.
Setting up a tank for fiddler crabs is simple, but it’s not the same as setting up a fish tank.
You should start with a layer of soft, sandy substrate along the bottom of the aquarium. Sand is important because they need to be able to sift through it to feed and burrow. The sand should slope up to a raised area of the tank which sits above the surface of the water; this gives the crabs access to land.
Use rocks to give them somewhere to hide away. Live plants will probably be destroyed so are best avoided.
Don’t fill the tank all the way to the top with water, since air is just as important. You should fill it about a fifth of the way up with water, then your sandy slope can lead to an area above the surface.
The brackish water has a low salinity, but you still can’t use freshwater. Keep salinity between 1.01 and 1.08.
Use a heater to keep the temperature between 75-86°F. The pH should be 8.0-8.3 and hardness around 15 KH.
As they are naturally used to high oxygen levels, you need to aerate the tank. You can use either an air pump or strong filter outlet.
What Size Aquarium Do They Need?
Fiddler crabs need at least a 10-gallon aquarium (this can comfortably fit 1-4). Give each additional crab 3-5 gallons. If they are too cramped, the water conditions will get worse and it may cause disease.
You can add some fish in with them to make a community tank, but your options are limited. Most freshwater fish won’t survive in brackish waters, and usually they will either be attacked by the crabs, or larger fish will attack the crabs.
Some people have had success with Mollies because they can thrive in low salinity environments but they may be too big unless you choose to use a larger tank of 30 gallons. Some other options include Platies, Swordtails, Guppies, Bumblebee Gobies.
Any fish you include needs to be fast enough to get away from a rare attack of a claw. It is safest to keep crabs on their own if you’re not prepared to lose fish.
You can introduce some other invertebrates at a lower risk. Shrimp (like Amano and Ghost varieties) should stay out of the way. Snails will too, Nerite snails are a popular choice for these brackish waters.
Keeping Fiddler Crabs Together
You can keep them together providing they have enough space. A group should only contain one male though, otherwise they will fight. Keep at least a pair because they live in large groups in the wild and will get lonely on their own.
When feeding in the wild, their claws are used to bring substrate to the mouth. They will then sift through the sand and eat anything nutritious, such as algae and fungus. The rest of the substrate gets deposited in little balls.
They wouldn’t stray far from their burrow, and only feed within a certain radius and then head back home.
In captivity they will happily feed on foods that you add to the tank, most of which can be easily found in pet stores. Since they are omnivores you have a few choices.
Sinking foods aimed at scavengers (like shrimp pellets) will be collected as they explore the aquarium. Frozen foods are probably the most common part of their diet, things like frozen plankton and shrimp are nutritious and full of protein.
They will accept green vegetation too. Seaweeds, zucchini, and lettuce offer nutrients that prepared foods can lack.
Calcium is very important to include in a fiddler crab’s diet, they need it to develop new exoskeletons before molting. Once they molt, you should leave their exoskeleton in the tank for a week or so, as they might eat parts of it for the calcium. Remove the exoskeleton if it’s not being eaten.
Feed them once a day, giving a small mix of any of the foods above. This could be a couple of frozen plankton and a few shrimp pellets. Mix up the foods every couple of days, perhaps switching to bloodworms and seaweed.
As a rule, if your tank develops the smell of ammonia, you’re feeding them too much.
Most health problems are caused by poor water conditions, so you need to keep your tank clean. Poor nutrition makes it harder for the crabs to combat disease, so designing a suitable diet that provides a range of nutrients is a good way to prevent health issues.
Shell disease is one of the most common culprits which impacts their health. Bacteria, viruses, and fungi can cause colored lesions on the exoskeleton which can lead to internal damage too.
An individual should be able to cure themselves through molting.
Disease in captivity is rare as fiddler crabs are hardy. Most damage in the wild comes from fighting, but this shouldn’t be a problem with only one male in your tank.
Regular molts every eight weeks or so are natural and important. They do this to keep themselves healthy by removing ectoparasites or reforming lost limbs. After a molt you will see the old exoskeleton sitting in your tank, don’t be fooled into thinking that they have died!
After a molt they are particularly vulnerable because it takes time for their new exoskeleton to harden. You should keep a close eye on them until the exoskeleton hardens.
They don’t mate like fish. Unfortunately, it’s not possible to breed your crabs in captivity because of how the larvae develop.
These larvae are planktonic, which means they spend a couple of weeks living in the ocean’s water column before heading back to shore.
There’s no way to accommodate this planktonic stage in your aquarium.
You might see your fiddler crabs attempt spawning though. A male will court a female by using their claws to perform a series of signals.
The female tends to selectively pick an ideal male. Those with bigger claws are preferred because they can dig a larger burrow for the laying of eggs. You may see female carrying eggs in your tank, but just because you see eggs doesn’t mean that they will develop into adult crabs.
How To Buy
The first thing to note when buying a fiddler crab is that a lot of stores will list them as being freshwater. This is not true and keeping them in freshwater for an extended period of time will cause health problems and dramatically reduce their life span.
If a store is keeping them in freshwater, either find a different store that’s keeping them in the right conditions or wait for a new shipment to come in (this will minimize the amount of time they spend in freshwater).
To ensure that you pick the healthiest individuals, check that they have all their limbs attached and that their color isn’t faded.
Not all pet stores will sell these crabs because they’re not as commonly kept as fish, but a quick search online should reveal a supplier near you.
Expect to pay anywhere from $4 to $10 for a single crab. Prices vary with size, age, and store.
Are Fiddler Crabs Suitable for Your Aquarium?
It’s very unlikely that you will be able to add these animals to a pre-existing tank. They have some unique requirements that means they will likely need a new tank.
Setting up a 10-gallon tank is not too difficult, and you can use a larger tank if you have extra space. If you can keep the brackish water under the right conditions then you shouldn’t have many problems.
You must be careful if you’re keeping fish with your crabs. It’s not impossible to do but there will always be a risk here.
These crabs are a great option for someone looking for a change from fishkeeping. They create a completely different look and display some interesting behaviors.
Have you tried keeping fish with your fiddler crabs? Let us know about your setups in the comments section below…