Crabs: Characteristics, habitat, alimentation habits and more…

It is time to learn about crabs, which are crustacean that belong to the decapod order. These nice and even weird creatures have been used a lot for advertising, children’s stories, and animated films, and besides their delicious taste they also offer the same health benefits than other marine species.

It´s time to learn more about crabs..

Crabs Main Characteristics

It is time to go deep into the most outstanding features of these weird and funny creatures

Anatomy

Crabs belong to the arthropods´ family that is, they have an  external exoskeleton that protects them.  In many cases, this exoskeleton is mineralized by the action of calcium carbonate, which makes of it a sort of armor.

As in the case of any other arthropod, their exoskeleton changes and molts when they grow, and they occasionally take advantage of this phenomenon to reproduce.

Every time crabs molt their exoskeletons, they increase their sizes considerably, but they are also exposed to many risks , because while their bodies remain soft, they are at the mercy of their predators.crabs: anatomy

There are people who mistakenly call some crabs, «soft-shell», but this is because they are in this phase of transition.

As any other decapod, crabs have 5 pairs of legs, although the 2 frontal have evolved over time, to become a pair of pincers that they employ to capture their prey, to fight against their enemies and even to carry out courtship.

Their abdomens are usually small, and in many species are not used for locomotion, because they are folded under their bodies, being employed mostly to carry their eggs . This feature is more evident in crabs that do not have a lapel-shaped tail, such as hermit crabs.
There are more than six thousand species of  these animals and most of them inhabit in the sea, although there are also freshwater crabs, and others that live in the sand near the beaches.

These unique creatures are related to lobsters and shrimps, but unlike them, they can run and walk laterally, and also dig.

Other interesting Characteristics

In terms of behavior crabs develope different patterns, although most of them are quite active and even aggressive among themselves, when they are looking for a couple.

This species, which can be of fresh or salt water, is characterized by its special locomotion mode, since when we observe them walking, they seem to be moving sideways, even backwards for some people.

The estimated life span for crabs ranges  between three and twelve years.

Let´s watch this video to know the way they walk

Where do Crabs Inhabit?

These creatures belong  to the water, although they can also be seen loitering frequently on the ground, and in some cases, they climb palms, as the case of the coconut crab. When staying on land they usually dig deep holes to look for some water.

The precise location depends on the species, since they can be found in estuaries and coral reefs, while other crabs prefer to live in the intertidal zone, which means that they live between the two tides, both high and low.

The king crabs can survive without any problems in  high depths of the ocean, and we can’t forget about the 8 different families of crayfish

The blue crab which is one of the most wanted types of crabs, lives in estuaries and salt lagoons along the coast that goes from Nova Scotia, to the Gulf of Mexico.

In turn, the yeti crab, which gets its name from its furry legs like those of the caricature, lives in the depths of the South Pacific Ocean.

We can conclude by saying that they are always close to the water. If you want to learn about the precise habitat or location of every species of crab, you just have to read the several articles about these creatures that we have in our blog.

Crabs´Reproductive Methods

The first thing we should know is that all species of crabs have separate sexes. The mating occurs only when the female has just molted her shell, and the new one is still soft, since in normal conditions the exoskeletons of these creatures are quite inflexible.

Crabs are characterized by having a complex  courtship behavior,  and can even fight for a female

After copulation, the female can store the male’s sperm for a long time, until an opportune moment to fertilize it being able to put up to two hundred thousand (200,000) eggs. The eggs are usually sticky, and are caught on bristles on  the  of the female´s  abdominal appendages, who carries  them for several weeks until they hatch  into free-swimming larvae

Crabs usually carry out sexual reproduction, although there are some species that also carry out asexual reproduction, which is called parthenogenesis. This type of asexual reproduction is based on the development from unfertilized eggs.

The sexes are separated, but there are some parasites individuals, which causes the development of hermaphrodites specimens, that perform a crossed fertilization.

It is believed that some of these species live in male and female pairs, or are gregarious, although this does not mean that they form well-organized societies.

Crabs use different ways to attract their couples. For example in the case of species that spend more time in the water, the attraction of the female is produced by the use of pheromones, which are thrown into the water.

In the case of those who live on land, the courtship occurs through visual actions, sounds and vibrations, which entail the use of their pincers

The male is the one who seduces the female, waving his pincers, or making them sound and hit the ground to conquer her, until both of them enter into their cave, to begin the mating.

It is easy and fast to learn how they molt their shells

Crabs´Alimentation Habits

Most crabs are omnivores, although some families have more specialized diet. In the case of the species pertaining to the Portunidae family, they are known by being mainly carnivorous, preying on slow-moving invertebrates such as mollusks and crustaceans. However, this diet often varies depending on the specific habitat and the environmental conditions where they inhabit, and the corresponding availability of food and preys.

The omnivore species usually feed on algae and other organic matters that they found in the surroundings.

Despite of their strong behaviors, these creatures usually share their food with others crabs, especially with their pairs.

Most Common Species of Crabs

Now it is time to learn, about the characteristics of differents species of crabs

Freshwater Crabs (Crayfish)

These crustaceans belong to 8 different  super families, and they are freshwater crabs, characterized by having gills  quite similar to feathers, which they employ to breathe

These crabs are quite abundant in streams and rivers, especially in places where they can hide from their predators. Thus, they can be found in all the freshwater of the world, although the icy temperatures of some season of the year represent a big risk for this species.

Their bodies are elongated and a little different than those of others crabs, and they have a cephalothorax and an abdomen. On average, the river crabs grow to about 17.5 cm in length.

In the same way than any arthropod, they molt their exoskeletons.

In terms of diet they are omnivorous; animals, plants, organic matter and carrion, are the main elements they feed on.

The males and females of this species usually carry out a brief fight when trying to reproduce. This weird ritual usually causes several wounds to each one of them, and then if they remain alive, they perform the copulation.

Unfortunately this species is considered in risk of extinction, due to a disease called afonomicosis, which is caused by a fungus named  Aphanomiyces astaci. Moreover the construction of reservoirs, the illegal fishing and the pollution of their environment, are also harmful aspects for the freshwater crabs.

This species is very popular in the gastronomy worlwide, and every region has its particular preferences, although the presence of tomato and spicy is a constant in every dish.

To know if a crab is edible or not, it recommended to observe the meat of the tail: if it is soft it is usually an indication that this specimen is not suitable for consumption.

Only a small part of the body of the crayfish is used. In the case of dishes, such as soups, bisques (creamy soup of French origin) and étouffées (specialty of the Creole cuisine of Louisiana) only the cola is employed. In other recipes they are served whole, but only some parts are eaten.

bon appétit

Anomurus or hermit crabs

These creatures belong to the Paguroidea family, and their names are related to the unusual shape of their asymmetric bodies.

The individuals of this species lack of abdominal exoskeleton, that is to say, decalcified and therefore soft.  This fact makes them vulnerable to predators, which forces them to seek refuge and in the empty shells of mollusks

The hermit crabs have five pairs of legs with a pair of pincers each one. The first pair of legs is employed to eat and defend themselves, the second and third ones perform displacing functions, and the fourth and fifth pairs remain inside the shell.

Some of the hermits also live inside the land, leaving from there only to mate. They also have gills located in a gill chamber, which perform the function of lungs. The most primitive specimens are lobster-shaped, while the porcelain crabs resemble the true crabs.

In terms of behavior, they are nocturnal animals that develop symbiotic relationships with other species such as the anemones. These creatures locate on the shells of the hermits crabs, in a very strong relationship that is maintained even when the crustacean molts its shell. This is an advantage for both of them, since the anemone helps defend the crab from its predators, and in turn it feeds on the remains of food that the crab consumes.

The hermit crab has an omnivorous diet which includes small crustaceans, dead animals, worms, and the remains of some plants.

This type of crab is oviparous, and it only reproduces twice a year; releasing the eggs into the sea after having remained under the abdomen of the female. This species can live between thirty and seventy years.

The main predators of the hermit crabs are the people that feed on them, as well as seagulls, yellowfin tuna, and some other amphibian, although they are not considered as an endangered species.

Brachyuros

This group is comprised by most of the marine crabs, as well as other species.  They have five pairs of leg, and as in the case of most crab´s species, the frontal ones have the shape of pincers, and they are employed for several purposes such as defense and to capture food. The rear legs are mainly employed for displacing, and even for holding their eggs after mating.

These creatures are also characterized by being skilled swimmers, and most of them are amphibious. They employ their flat rear legs to control their movements when they are swimming. In terms of size we can say that it is quite variable, since there are specimens of only one centimeter in length, and some others much bigger.

The brachyurans are generally benthic creatures, although in few occasions some individuals can develop nektonic habits.

One of the most distinctive individuals of this species; is the arrow crab or Stenorhychus seticornis in scientific terms. It has quite long and slim legs very similar to those of a spider, being the main reason of its name.

This crab has a long, spiny face and a nasal appendage that it uses as a harpoon to catch its potential prey. Both the hearts and the legs are reddish brown in color, while its pincers have violet-gray tonalities. It also has live red joints with fine lines sometimes brown, and in other black.

Its two sensorial antennas as well as its calcified exoskeleton, are two other distinctive features of the arrow crab. Its nervous system is compact and centralized, especially when compared with most arthropods. The reproductive organs of this crab, are located in the frontal portion of its shell.

In terms of size this is very small creature, whose shell measures around one centimeter, and its total length is 6 cm when most.

In the same way than all Brachyurous, the availability of sun light and the temperature of the environment, determine the reproductive activity of the arrow crab.  Then, they are born in the form of planktonic larvae that experiences several morphologic changes, to become an adult arrow crab.

You can distinguish the sex of this species by carefully observing their bellies; since those of the females are folded forward and in a triangular shape, while the ones of  males are little wider, and rounded shaped.

In addition to their terrifying legs which are quite similar to those of a spider, these crabs also have a head shaped like an arrow, which they use as a harpoon even against small fish.  This fact makes of this species a little dangerous for human beings, since this appendix is quite sharp, and can easily be stuck into the person’s skin, causing a painful wound, susceptible to infection.

This species of crab can be mixed without any problems with other species inside an aquarium, but small fish often become in their preys.

Properties of Crabs for Health

There are many health properties provided by the consumption of most crabs. Let´s learn more about them

Promotes Bone Health

The crabs´ meat is quite high in phosphorus content, which is quite useful for the growth of body cells and tissues, especially those of teeth and bones. In conjunction with calcium, phosphorus provides strength for different tissues..

 

Improves Mental Activity

The meat of crabs contains a wide range of nutrients, such as cooper, vitamin B2 and omega-3 acids, which are  also quite important for the proper function of our brain.

 Omega-3 Acids.

The omega-3 fatty acids have very good properties to balance the cholesterol levels in our body, to control the blood pressure and to prevent diseases such as atherosclerosis.

Immune System

Crabs also contain high amounts of selenium, which is a stimulator of the immune system and an excellent antioxidant. .

Body Detoxification

The phosphorus also helps to improve the functioning of the kidneys by speeding the release of toxins, which results in in better metabolic functioning.

Iron

Finally but not less important, we must point out the benefits of the iron contained in this species, which has excellent effects in the production of red blood cells, improving also the circulation of blood in our body.

More interesting facts about Crabs

  1. There are more than six thousand species of crabs, which are quite similar to each other.
  2. In spite of being mainly aquatic creatures, crabs are not usually good swimmers, but they are very skilled to walk or move on their legs at the bottom of the sea, and also on the ground, as well as being able to climb palm trees.
  3. Crabmeat is an excellent source of several minerals and vitamins. In this sense we can mention niacin, zinc, iron, as well as large amounts of the antioxidant mineral selenium.  But its biggest claim to nutrient fame is its vitamin B12 content, and we can´t forget about it also provides omega-3 fatty acids
  4. The shell of the crabs is mineralized by calcium carbonate and it is quite hard and resistant.
  5. The life expectancy of crabs ranges between 3 and 12 years, depending on the species.
  6. In certain parts of the world, crabs cross big distances to reach the sea, in order to carry out their reproduction. We can mention the phenomenon known as the red tide, which occurs at the Christmas Island. (Indian Ocean). Millions of  red crabs walk along the streets in their way to the sea.
  7. As a result several regulations have been implemented, as is the case of one which obligates the cars to stop, in order to wait for the red tide to pass,
  8. For this reason, when this phenomenon is about to occur, people and authorities take their forecasts, in order to not cause any damage to the red crabs.
  9. The Birgus latro or coconut crab,  which is very common in some islands of the Indian Ocean, is one of the largest in the world, reaching a meter in length, and weighing between four and ten kilograms.
  10.  The largest specimens of all are the Japanese Spider crabs, which can reach even 4 meters in leg span , and twenty kilograms in weight.(See the Article Japanese Spider Crabs for more information)
  11. On the other hand, the smallest crab in the world is known as the violinist crab, which only gets to measure between two and a half, and three centimeters.
  12. Crabs have been used in countless film and television commercials, in addition to being the protagonist of many children’s films.
  13. Crabs represent a twenty percent of  all the marine crustaceans  consumed in the world
  14. Only in Spain, one and a half annual tons of crabs are consumed.
  15. There is a practice which consists in turning and removing the claws of the crabs, returning them to the sea later, since these extremities will regenerate. As its meat is so appetizing, this has become a very sustainable industry.
  16. A group of crabs is also called a cast.
  17. The king Crabs  and the Hermit crabs  are often called false crabs..
  18. The crabs are known for walking sideways. The main reason of this fact lies in their legs, which attached to the side of their bodies. Moreover their articulations  bend outwards enabling the crab to displace sideways only. There are a few  species of crabs that can walk forward.

Believe it or not

 

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