Crab
Brachyura
More than 6,700 species of crabs have been identified. Some crabs live exclusively in the ocean, while others live along the shoreline, and some crabs live in freshwater instead of the saltwater environment of the ocean. Still, others live full time on land, but always near water of some type.
Crabs are very important to the environment because they help to keep things clean. They are also an important source of food for a wide range of other creatures, including humans.
• Crabs help coral reefs to survive by cleaning away debris that might otherwise kill the reefs.
• Crabs have been around since Jurassic times, more than 200 million years ago.
• Most crabs walk and swim sideways.
• Male crabs of some species fight with one another over mates and hiding places.
• Crabs have 10 legs, but the first two are claws and not used for walking.
Crab Scientific Name
Since there are so many different kinds of crabs, they have thousands of common names, such as king crab, horseshoe crab, blue crab, snow crab, coconut crab, and more. Yet, they all belong to the scientific order Decapoda, which comes from the Greek words “deka” meaning ten, and “pous” (poda), meaning feet.
Most crabs belong to the Brachyura family. This term is based on the characteristic of crabs to have a short, hidden tail. The word Brachyura comes from the ancient Greek terms for short, “brachys”, and tail, “oura.” However, not all crabs belong to this family, and some of the better-known species, such as the king crab, are in the Lithodidae family. This name comes from the Greek word “lithodes,” which means stone-like, because they have very hard, stone-like shells.
Each kind of crab has a unique appearance that separates it from all other crabs, though some look enough alike that only an expert can tell them apart. In general, a crab has a rounded or oval-shaped body that is sometimes smooth and sometimes covered with protrusions of varying lengths that offer the crab some protection from predators.
Crabs have ten legs, five along each side of the body. The pair of legs in the front has evolved to become pincers that the crab can use for defense or to feed itself. In some crabs, the pincers are of roughly equal size, but in other species, such as the fiddler crab, one pincer is much larger than the other.
Crabs come in a wide range of sizes. The smallest known crab is the pea crab, Pinnotheres pisum, which measures as little as 0.27 inches (0.68 cm) across. That’s about half the size of an aspirin tablet.
The largest crab is the Japanese spider crab, which can grow as much as 13 feet (4 m) wide when its legs are spread – about the length of a Volkswagen. The heaviest crab ever found was a king crab, which weighed in at an amazing 28 pounds, about the same weight as a corgi or a miniature poodle.
The average crab falls in between these two extremes and is only about 15.74 inches (40 cm) in diameter, or about one-tenth the length of a Volkswagen.
A crab’s body is covered with a hard shell called an exoskeleton. This protects the crab during most of its life, but because the exoskeleton can’t grow with the crab it must be shed, typically once per year, to allow the crab to grow. This is a very vulnerable time for crabs and they usually try to hide at this time.
The exoskeleton on a crab is the part that determines what color it is. Crabs come in many different colors, depending on the species and where they live. Many are shades of red or blue, but crabs are also brown, white, yellow, tan, or a combination of colors. A crab’s color can help to protect it by offering it some camouflage.
However, sometimes the color is very distinctive, such as with the bright red Christmas crabs from Christmas Island. In these cases, the color helps crabs find one another or warns other creatures to stay away.
Crabs can also have smooth shells or they can be covered with spiny bumps that deter predators or help them hide in coral reefs or rocky niches.
Crab Behavior
Different kinds of crabs have different lifestyles. Some crabs live alone, only meeting with other crabs when it’s time to mate. Other types of crabs live in large groups called “casts” all of the time. These groups can have hundreds or even thousands of crabs in them. Living in a group makes it easier for a crab to find a mate, and it also makes it harder for any one crab to be selected as prey by a predator, so it helps to keep them safe.
Crabs tend to be shy and will usually run from danger. Despite the fact that crabs have pincers that they can use to hurt a predator, the injury is usually not serious and most crabs would rather run than fight. Some crabs, though, such as the coconut crab, have big, strong pincers that are strong enough to break a person’s finger. These animals live on land and can be aggressive. They will even attack small animals such as dogs and cats if they encounter them.
One type of crab, the horseshoe crab, is actually not a crab at all. In fact, it’s not even a crustacean. This is an ancient species that has been mostly unchanged since the time of the dinosaurs. People continue to call them crabs because they live in salt and brackish water and act a lot like crabs, but they aren’t. Surprisingly, their closest living relatives are not other crabs, but spiders.
Crabs typically live around water, especially saltwater or brackish water. They are found in every ocean on earth. Some live in the water all of the time, while others live at the edge of the water, in and among the rocks or the sand along the shores. Some types of crabs live only in freshwater and would die if they were put into the ocean.
Other types of crabs live entirely on land, though most of these live at least some part of their lives in the water. Often, they seek the water to breed, and the babies are born there and live in the water until they are developed enough to come out onto the land. Sometimes land crabs migrate in huge groups to the ocean when it’s time to breed, as with the red Christmas crabs that seem to take over everything where they live until breeding season is over.
Crab Diet
What crabs eat varies greatly by species, but most crabs are omnivores, meaning that they eat both plants and animals. The tiny pea crab spends its life as a parasite inside of oysters, mussels, sand dollars, sea cucumbers, and other creatures where it consumes plankton that the host brings in to feed itself. Larger crabs live on their own and often hide in burrows where they dart out to grab shrimp or fish that get too close. Crabs also eat algae, mussels, barnacles, clams, seahorses, and even smaller crabs.
Although crabs have a hard outer shell that protects them, they are still a favorite food for many animals. Newborn crabs lack a shell and usually live like free-floating plankton where they are a target for all kinds of predators, including tiny fish, corals, anemones, sea worms, and the young of most kinds of animals. As the crabs begin to develop a shell they become better-protected, but they are still vulnerable to predatory fish, otters, larger crabs, octopuses, and humans. Some crab predators use very unique tactics, such as the pistol shrimp which “shoots” high-powered bubbles that knock crabs unconscious!
Data is lacking for the International Union for the Conservation of Nature (IUCN) to classify the conservation status of all crabs, but some species are listed as near-threatened, meaning that their numbers may decline in the future. Some crabs, such as king crabs, seem to be changing their behavior in response to warming water temperatures, and this could result in problems for their survival, causing them to become threatened in the future.
Plenty of crabs still live in the ocean, and humans take advantage of this abundance by catching and consuming them in large numbers. Humans consume about 1.5 million tons of crab annually, with the Japanese blue crab topping the list for the most consumed.
If crab fishing is not controlled, some species could end up extinct. Regulating the amount of crab caught each season helps to control their numbers and make sure that crab continue to be around in the future.
Male crabs will often use their pincers to attract a mate. This is especially common in species that have one very large claw, or pincer, such as the fiddler crab. Males of some species will also fight one another over a female, with the winner getting to mate and the loser going off and looking for another female.
Crabs typically mate when they molt, because there’s no hard shell to get in the way. This is usually when both the water temperature and the outside air is warm. Many aquatic crabs mate belly to belly and the eggs are fertilized internally. The female can store the sperm until she needs it, then use it to fertilize her eggs. The fertilized eggs are placed on her underside, near her tail, and carried there until they hatch. The larvae are free-swimming and join the plankton in the water. Even crabs that live on the land must migrate to the water where their babies are born. The babies must live in the water for a time and then migrate back onto the land when they become juveniles.
Larval crabs molt several times before they start to look like their parents. As juveniles, they will begin to act like their parents as well and will either join a cast of crabs or find themselves a suitable place to live. Most species of crabs live from three to four years, during which time they must evade predators, find food, molt, and reproduce.
Crab Population
With over 6,700 species of crab worldwide, their overall numbers are considered large but unknown for most species. Many crabs are listed by the ICUN as DD, which means data deficient, because there’s not enough information about them to tell whether or not they exist in large numbers. Some types of crabs are threatened because they have limited areas in which to live, and when humans encroach upon their territory the crab numbers decrease.
Humans do monitor some species closely, as people have an interest in how well the species used for food are doing. King crabs, Opilio crabs, Japanese blue crabs, and other species that are routinely caught for humans to eat are regulated by fisheries in many countries, with strict limits as to how many can be caught as well as the size and sex of those kept. Timing of the fishing is also controlled. This helps to keep the populations healthy so that there will continue to be plenty of crabs.
Are crabs carnivores, herbivores, or omnivores?
Almost all crabs are omnivores. They eat algae and other plant matter as well as hunting animal prey.
Is crab good for you?
Crab is good for you because it contains a lot of protein and is low in fat. It also has nutrients such as vitamin B-12 and selenium. Some of the most popular crab varieties for humans to eat are blue crab, softshell crab, snow crab, Dungeness crab, stone crab, and king crab.
Do crabs feel pain?
Some scientists decided in 2005 that crabs are unable to feel pain. Newer studies done with hermit crabs, however, show that these crabs do respond to electric shocks, indicating that they can feel pain. At this point, it is uncertain if all crabs can feel pain or just some, and, if they do, how much pain they might experience.
Why do crabs walk sideways?
Although crabs can slowly shuffle forward, they walk sideways because their legs bend that way and it allows them to move much faster.
What Kingdom do Crabs belong to?
Crabs belong to the Kingdom Animalia.
What phylum do Crabs belong to?
Crabs belong to the phylum Arthropoda.
What class do Crabs belong to?
Crabs belong to the class Malacostraca.
What family do Crabs belong to?
Crabs belong to the family Brachyura.
What order do Crabs belong to?
Crabs belong to the order Decapoda.
What type of covering do Crabs have?
Crabs are covered in Shells.
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