of the Month
Common Name(s): Emerald crab,
green clinging crab, jade crab,
mithrax crab, hardback crab,
Range: Tropical western Atlantic
Ocean: Florida Keys and the
Natural Environment: This large-bodied crab, with sizeable forearms and
claws, is usually found living on algae-covered rocky coastlines, backwater areas
with algae-covered rock surfaces and
rubble, and in seagrass beds.
Captive Care: These heavily armored crabs have a shiny-green carapace and claws, with hairy, dark-green walking legs. Their claws
have blunt tips used for feeding on different forms of algae, including bubble algae Valonia. Though this crab is often sold as an herbivore, it
is actually omnivorous, and while thought to be reef safe, larger specimens—i.e., about 3 to 4 inches in diameter (from leg tip to opposite leg
tip)—may turn into predatory creatures and eat small fish and possibly other small animals in the aquarium. In fact, anything with a claw
that does not have its fill of its normal foods found in the wild will turn toward other sources of food, tasty or not! Therefore, this is not a
totally reef-safe animal, and even if kept in a fish-only aquarium, thought should be given to what size and type of fishes are maintained in
that system. As for feeding, they are not fussy eaters, as small members will scavenge various forms of algae, but more mature ones may
turn to other foods if they do not have an adequate supply of algae. Therefore, in systems somewhat devoid of algae, plant rocks—e.g.,
those covered with different forms of algae (micro or macro)—may be a way of satisfying their nutritional needs, thereby preventing them
from turning to other forms of food for nourishment.
Water Quality Requirements: Calcium 380 to 430 ppm, alkalinity 2. 5 meq/l, pH 8. 1 to 8. 2, specific gravity 1.024 to 1.026, and
a temperature range of 75° to 82°F.
Note: How to catch unwanted crabs is a question I’ve fielded numerous times. Besides the baited trap that drops a door sealing
off an escape route, the least expensive way I’ve found to rid some unwanted specimens comes with the use of a large, tall drinking
glass. Simply place a tasty morsel at the bottom of the glass and place it at the aquarium bottom against some rock in the late
evening. The crab will sense the food at some time in the evening or early morning hours, and it will use the rock surface to gain
entrance to the lip of the glass.
Once it falls to the bottom of the glass, it won’t be able to climb out. This is not always 100-percent effective, as some crabs are fairly
good swimmers, but this method has resolved some terrible crab problems in some aquariums.