Atlantic Spider Crab (Libinia emarginata)
Very common in many areas, the spider crab reveals itself to be a fascinating animal to those who take the time to know it. These interesting crustaceans are members of a family containing over 600 species, the largest of which, the Japanese spider crab, reaches a leg span of 8 feet.
The Atlantic spider crab is a scavenger and uses its tiny, pointed claws to probe into nooks and crannies below seashells and aquarium decorations, performing therein a valuable service. Younger animals have the endearing habit of jamming algae, vegetation, and whatever else they can find into the crevices of their shells. Animals so decorated appear to be walking plants and, once they settle down, the camouflage effect is readily apparent. They will also “harvest” from this portable garden, nibbling at it when they are hungry. For some reason, spider crabs seem to forgo this habit when they reach a carapace size of about 3 inches.
In general, crabs that possess flattened paddles on the rear legs, an adaptation for swimming, are extremely aggressive and are difficult to keep with any other animals. “Walking” crabs, on the other hand, such as spider crabs, are generally much less aggressive and more easily assimilated into a community tank.
Spider crabs make inoffensive aquarium inhabitants, and consume all sorts of foods, including flakes, pellets, black worms, and frozen food. They also enjoy algae and all sorts of green leafy vegetables. Be aware that terrestrial plants have a more rigid structure than do aquatic plants. To increase the digestibility of foods such as kale and spinach, soak the leaves in very hot water for a few minutes before feeding them to your pets.
Spider crabs are extremely slow moving, and may be easily collected in tidal pools and eelgrass beds. Although they get along fairly well with each other, care must be taken that they have a place to hide when the shell is molted, because at that time they are soft and vulnerable to attack by other animals. Spider crabs seem to possess a suicidal impulse to leave the aquarium at night, so be sure that your tank is well covered.