Crabs are a delicacy in some places, and have their place in folklore too. There are just under 7,000 known crab species worldwide, and while there are a few species living on land, most of these animals spend at least part of their time in the water. Some live in shallow tide pools, and others live in freshwater environments. Still others spend their lives thousands of meters beneath the surface, safe from the grasp of all but the most dedicated marine researchers. Playing an important role in the environment, these animals often seem to fade into the background the minute something more spectacular swims by; but spend a little time watching, and you’ll be rewarded with a view into the intriguing lives of some of the most interesting crustaceans on the planet.
Crabs: The Basics
Of all the many species of crabs identified, most are true crabs, with hard shells. Others are hermit crabs, which do have a protective covering but which hide inside the discarded shells of other creatures, moving to new, larger shells as they grow. There are about 500 different species of hermit crabs.
Other crabs, such as King Crabs, are not considered to be true crabs, although they are part of the crustacean family and are closely related. Porcelain crabs, found around the world in all but the Arctic and Antarctic regions, are not considered to be true crabs either; their fragile bodies do resemble those of true crabs, but these animals are more closely related to hermit crabs. Finally, Horseshoe Crabs are crabs in name only; they are not even classified as crustaceans – instead, they are arthropods.
All true crabs have hard exoskeletons and ten jointed legs in pairs which are used to perform a variety of functions. Most crabs have three pairs of walking legs that extend from the centers of their bodies, plus a pair of swimming legs, which are located behind the body, and which curve inward, forming a slight U shape. The fifth and final pair of legs are the chelipeds, which are also called pincers or grasping claws.
While land crabs enlarged modified cavities perform a lung-like function, marine crabs breathe via gills located in cavities beneath their protective carapaces. While about 850 crab species are either terrestrial, semi-terrestrial, or live in freshwater environments, the remainder of the world’s known crab species are ocean dwellers.
As with many other crustaceans, most crabs exhibit sexual dimorphism, with females having rounder abdomens and males often have at least one enlarged claw which proves useful in attracting a mate. Males often fight with one another to gain access to their female counterparts, and in areas where crab populations are heavily concentrated, crabs will fight with one another for the right to occupy a desirable hiding place.
Omnivorous, feeding on plant and animal matter alike, crabs often feed on algae, smaller creatures such as worms and other crustaceans, and detritus such as fish and marine mammals that have died. They like to occupy rocky hiding places, wrecks, and other areas where they are safe from predation.
Commonly Encountered Crabs
Colorful and fun to watch, crabs are plentiful in many places. One crab Caribbean divers will commonly encounter is the Batwing Coral Crab, a small creature with a mottled shell in shades of red, white, yellow, and orange which helps provide camouflage. Growing to just about five inches across these active crustaceans tend to be found in depths up to 15 meters, and are sometimes referred to as Queen Crabs or Red Coral Crabs.
The Common Blue Crab, which is distributed throughout the Atlantic, Florida and the Bahamas, Brazil, the Gulf of Mexico, and the Caribbean, is usually between 4 and 6 inches across and can be found in depths from two to fifteen meters. Easily identified by its bright blue legs, this crab is a close relative of the Ornate Blue Crab, which is distributed throughout the same regions, and which is distinguished by a large orange spot on the carapace.
Tiny, but very beautiful, the Blotched Swimming Crab can be found throughout the Caribbean, in Florida and the Bahamas, and in the Gulf of Mexico. Usually between two and three inches across, these little crabs can be found in shallow areas as well as in water up to about 300 feet deep. With a slightly iridescent gleam and pronounced purple stripes, they have delicate legs with bright magenta tips on their claws and walking legs.
Nimble Spray Crabs are even smaller, growing to be less than two inches across at maturity. Slender and capable of flattening themselves or tucking themselves into very tiny spaces, they have pronounced bands of red, yellow, and brown on their legs, which are quite long in comparison to their tiny brownish red bodies. Living at depths to about 23 meters, they can be found in the Caribbean, Florida, and the Bahamas.
The Red Swimming Crab, commonly found throughout the Indo-Pacific and Hawaii, grows to be about three inches across, are particularly active at night and are commonly encountered on muck dives after dark. Very colorful, with red and orange patches separated by white bands, this is an extremely fierce species which has been known to go after fingers if provoked.
In the Pacific Northwest, the Dungeness Crab, which is a large species with a carapace about 8 inches across, can be found in most places, along with several different types of hermit crabs and the Flattop Crab, a ½ inch long species that is usually only found by hunting beneath rocks.
These are only a few of the most commonly encountered and most notable crab species you might encounter while diving. Remember, even the smallest crab can inflict a mighty pinch; respect these animals and don’t forget that you are visiting them in their living space. Fun to find and photograph, many species are hard to identify without the help of a chart; studying a region’s marine life before diving can make the experience even more fun and memorable.