Male red and black Fiddler crab (Uca sp.). Only male fiddler crabs have one giant claw.

A fantastic addition to the reef aquarium!

They’re great for reef tanks

These are our favourite invert for a reef aquarium. These small, peaceful, reef safe crabs only get to 50p size. Not only do they look really cool, but, they also do a very useful job of turning over the sand and keeping the sand clean.

Cleans your sand

Fiddler crabs pick through your sand feeding on algae and tiny pieces of uneaten food. They also bury in the sand helping to keep it turned over too.

Helps to reduce nitrates

The sand bed in a reef aquarium is where most of the denitrifying bacteria live. It is the denitrifying bacteria that eat nitrates and phosphates, so keeping the sand bed in good condition is key to reducing nitrates. Keeping nitrates low leads to faster growth and better colouration on your corals, plus less nitrates means less algae, so less work for you in cleaning glass too.

Some facts about fiddler crabs

The male crabs have one normal sized claw and one giant one.
Most fiddler crabs are left handed, but, about 1 in 16 are right handed. Interestingly this is the same ratio as in people, but, the other way round!
The large claw may look quite intimidating, but, it is purely decorative. They cannot even fully close the claw and it is only used for display. Male fiddler crabs wave their claws at each other, and the one with the biggest claw is the winner and gets all the lady crabs. Some things are universal across all species!
Some species of fiddler crabs need access to dry land, in fact some species of fiddler crabs have to have access to air, and can drown in water! The species of fiddler crabs we get can live permanently fully submerged though, and don’t require access to the air.
Fiddler crabs help clean the sand, and a clean sand bed means less nitrates. they are very useful things to have!
Fiddler crabs are very sociable creatures and whilst you can keep them on their own, they are much happier in pairs or groups. They don’t fight either, and you can keep as many as 1 or 2 per litre of tank volume without any problems at all.

This is a female Fiddler crab. Note that only the males have the giant claw. Females have 2 small claws.

FAQ’s

Are they reef safe? - Yes they are 100% reef safe.
Will they attack other animals in the aquarium? - No they are very peaceful.
Can I keep them with what I already have in my tank? - Generally yes. Obviously fish that eat crabs such as; pufferfish, triggers, large wrasses, etc, may eat them, but, they are fine with all the other typical reef fish, inverts, and corals.
How many can I keep in my tank? - Almost as many as you like. Certainly up to 2 per litre is no problem at all.
Do they require access to air or land? - Some fiddler crabs do, but, not these species. These can live happily fully submerged for years.
Is the big claw dangerous? - No, it’s completely harmless and purely decorative, like the tail of a male peacock. They can’t even fully close the claw.
Do you get them in often? - No, they are very hard to get, and we usually have a lot of people waiting for them to come in, so they usually sell out within a few days.
Do you post them out? - Yes we are online mail order marine specialists we deliver to all of the mainland UK. You can get your whole order delivered for just £10.
Can you have males and females can you have? - They are quite happy in any sex ratios, you can have all females, all males, or any mixture of the two without any problems at all.
Do the males fight? - No they just wave claws at each other, they don’t attack each other.
What do they eat? - They usually scavenge their own food, but, if you wish you can feed them sinking pellets or frozen food such as frozen mysis if you like.
Do they have any special conditions? - The tank needs to have sand for them to bury in, other than that they do not require anything specific.
Can I keep them in freshwater? - No, they require heavy brackish or full saltwater.
Should I get some for my tank? - Yes! they are one of the most entertaining and useful creatures you can keep in a reef aquarium.