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:
- 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.)
- 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.
- 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:
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.
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.
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.
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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