Crabs come in many shapes and sizes and can be an excellent addition to any aquarium. Pet owners can choose from land or sea-dwelling animals in a variety of colors. They typically range in size from less than in inch to more than 12 inches in diameter. Crabs are known for keeping tanks clean by eating unwanted algae that builds up inside. According to James W. Fatherree of Advanced Aquarist, Mithrax, Sally Lightfoot and arrow crabs are among the most readily available marine species. Each can be helpful in keeping the tank free of algae but may pose some risk to other fish.
Emerald Mithrax Crabs
These green invertebrates are popular options at pet stores that sell fish and aquarium supplies. They grow to about two inches in size and are helpful additions to a marine (saltwater) aquarium. They are known for eating various species of algae, most notably bubble algae, which can form even in the best aquarium conditions. Emeralds generally get along well with fish, but may nip at coral setups. They are usually shy when first introduced, but emerge more often as time passes. Occasionally they may display aggressive behavior toward other tank fish, especially when hungry. Emerald crabs can live as long as 10 years in captivity.
To prevent an emerald crab from becoming aggressive, add only one crab per 20 gallons of tank space and keep it away from smaller snails.
Sally Lightfoot Crabs
These saltwater crabs are easy to care for and come in orange, yellow and tan. Their flat carapaces make it easy for them to hide in crevices. Although compatible in reef environments, these crabs are known to be semi-aggressive toward other invertebrates in a marine aquarium. They prefer to reside in larger aquariums with plenty of rocks for good hiding places. More than one can be kept in a single tank, but you need to provide about 25 gallons per crab. The Sally Lightfoot crab will eat algae in the tank, but if there isn't enough to satisfy its appetite, you can supplement its diet with seaweed. This crab can reach 3 inches in size and live up to seven years in captivity.
When Sally Lightfoot crabs grow larger, they sometimes attack and eat smaller fish.
Arrows are also known as spider crabs because of their extremely long legs, which can be three times the length of their body. Mostly legs, these crabs reach 6 and occasionally up to 10 inches in diameter and are generally easy to care for. Arrows can become aggressive toward slow-moving fish. Provide sufficient food to prevent the crabs from picking at coral in the tank. They will eat most meaty and flake foods. Arrows live for three to five years in captivity and prefer a large marine reef tank. Do not place more than one in a single tank, as they do not get along well.
Hermit crabs come in marine or terrestrial varieties and make interesting pets. One of the most readily available water-dwelling species is the dwarf zebra, which reaches only about an inch in size, and is peaceful and easy to care for. Their fondness for dining on red slime algae helps to keep the aquarium clean. Without enough algae present, they may need some extra seaweed in their diets.
Marine hermit crabs require strict control of the saltwater environment. They prefer a temperature between 72 and 78 degrees Fahrenheit and a pH between 8.1 and 8.4.
Land hermit crabs are social animals and prefer to live with others. Provide one gallon of space for every two crabs. They prefer gravel, sand or coconut bark bedding and require a non-metallic water dish. Docile creatures, they will not pinch unless provoked.
It's important to provide empty shells of different sizes so the crab can change homes as it grows. Hermits grow slowly and will only accept a new shell that it can fit inside without too much exposure to the outside. Choose shells that are similar in size and only slightly bigger than the crab's current home, and allow the crab to test each of them. Children especially find it interesting to watch the crab's process of selecting a new shell. It may often switch between several before finally settling on a new home.
Fiddlers prefer a saltwater environment with some land space above the water line. They like to dig into the tank's substrate to create hiding places. Each fiddler requires about a square foot of tank space and lives for two to three years in captivity if well-cared for. They grow to about an inch and a half in size and do well on a diet of hermit crab food served in a shallow dish. Unlike other crab species, fiddlers pick up each pellet of food individually and place it in their mouth. Several may live together in a tank, as they require so little space. They are generally non-aggressive toward other tank animals, so long as they are not attacked.
If your tank is not deep enough to provide up to two feet of digging space beneath the sand or gravel substrate, consider adding an artificial cave or rock for the crab to burrow under so it feels safe.