Today we will learn about the sea crabs, which are beautiful and very interesting crustaceans. There are more than four thousand species that we can call crabs , where besides from sea crabs, we also find crayfish and mangrove crabs.
Probably some of our readers thought that they are all the same in terms of anatomy and habitat, but there is a huge variety of these crustaceans.
We invite you to read our articles crayfish and mangrove crabs to learn about other species of crabs
Table Of Content
Sea Crabs Main Characteristics
All crabs belong to the decapods´ order, which means that they have five pairs of legs inserted into their abdomens. The first pair of legs evolved into a pair of pincers or claws, which depending on the crab, can change in size and power, but all equally are used to capture, cut, and manipulate their food and even to defend themselves, and to court their couples.
The aquatic characteristic of these crabs can change a bit depending on the species. For example, the hermit crab is a sea crab, which can adapt to live on land, as long as he can live near the coast so that the female can spawn.
Another distintive characteristics related to the sea crabs, is that they are not usually good swimmers, but on the contrary, they use their legs to wander on the rocky surfaces of the underwater habitats where they live. This fact makes some of them benthic creatures. Few species have managed to develope nektonic habits spending their lives swimming between two different depths of the oceans.
Sea Crabs Common Properties
As sea crabs are arthropods, they have an exoskeleton composed mainly by chitin, which, when being mineralized with calcium carbonate becomes hard and resistant, so in most cases it is a carapace (shell) for the crab. When these crabs grow and develop, they need to molt their exoskeleton because they no longer fit in it. For this reason, sea crabs make many molts in their lives.
This process represents a danger for sea crabs, first of all, because they are vulnerable to the attack of their predators, who await the molting process in order to have easy access to them.
Another important aspect is that if the molt is not carried out properly, the crabs can get trapped in their old shell and die. Before the molting the crab ingests a large amount of water, to increase the size of its old shell, and once it feels that it has loosened, it begins to slowly withdraw its body. Once it has completely left the old shell, it is again filled with water, in order to expand the new one a little more, so that it allows it to continue growing comfortably.
This molting process can last several hours; for this reason the crabs hide to be safe from their predators, until the new shell is hard enough.
Hermit crabs employ snails´ shells or other mollusks to cover their bodies, which unlike that of other crabs is soft and does not harden.
This crustacean is characterized by having a distinctive shell in the shape of a horseshoe
Let´s watch one of these wonderful crustacean molting.
Sea Crabs´ Alimentation Habits
No matter what kind of crab it is, these animals are omnivorous by nature, it is to say, they are capable of consuming any type of organic matter. They have powerful pincers, which are sharp, to facilitate the capture and handling of food.
The bigger the crab is, the stronger and bigger its tweezers become, since as well as they acquire more size, they also acquire more strength. The claws of the sea crabs are used by them to chop and crush food, such as crustaceans, fish, small creatures, and algae.
Most crabs are opportunists, since they hunt their food, but they also take advantage of creatures that are moribund, or simply become scavengers, when they get corpses of animals that are on the shore of the beach. On some occasions, these animals can even consume waste from humans, which unconsciously throw into the sea.
In any case, there are sea crabs, which in addition to feeding themselves in the way we already mentioned, also do so by filtering organic particles from the water.
In the specific case of the yeti crab, it has a particular way of feeding very different from other crustaceans of this species; which can catch a prey of the ideal size to be able to eat it in a single bite. Remember that the teeth of most crabs are in their stomach. But the yeti crabs feed on the filamentous bacteria that inhabit their pincers, and that is what allows them to inhabit relatively calmly in the depths of the sea.
Sea Crabs Digestive System
Sea crabs have a digestive system very different from that of humans, since this is made up of a straight tube that ultimately becomes a gizzard that has the purpose of crushing and shredding food.
In addition to this, the sea crabs have two glands which segregate digestive juices, which collaborate with the metabolism of food, so that the crustaceans can absorb their nutrients. These animals have an excretory structure, which acts as a kidney, since it purifies their blood.
What the sea crabs eat in captivity?
Many people have the bad habit of removing crabs from their natural habitat, to exhibit them in their fish tanks and aquariums, where they are given any food, without taking into account that they have a digestive system very different from that of humans. This is one of the main causes of death among these crustaceans, since they will eat any food you offer to them.
But there are foods that do not provide the nutrients they need to grow and develop, and the worst is that are harmful for their health.
It is best to leave these animals at their natural habitat, where they are able to get their food, and discard what they cannot eat. Also, when you are visiting the beach, you should avoid leaving garbage on the sand, which can be harmful to sea crabs.
Sea Crabs´ Reproductive Method
Most crabs are oviparous, and all them have to fertilize their eggs to procreate new creatures. Sea crabs attract their partners by releasing pheromones that attract those of their own species, to let them know that they are ready to mate. Depending on the species, this mating period varies a lot, since it can be carried out during spring; others especies begin in the summer and some others three days after the new moon has started, or when the tide is at its peak.
It is very difficult to describe each of the reproductive processes; however, what is invariable is that it takes place just after the female sheds her exoskeleton. This is because at this time is when the new shell is softer, and it is much easier for both. It is the male who has to court the female, and she decides with which of them she stays.
The females in turn store the sperm, during a time that varies according to the species, since in some species it can take two months, while others can carry them up to a year. When the ovules have fertilized, they go down the abdomen of the female, and they are located under the tail, where they are fixed with a sticky material.
Then they will stay in this place, until the female moves to the sea to deposit them there.
But not in all cases happens in the same way, since in the case of the horseshoe crab, for example, the ovules emerge through short ducts, and then flow into the genital operculum. The mating of this species takes place on the shore of the beach, where the male arrives first, and unlike other species, copulation is not performed belly to belly, but since the male is small; it mounts on top of the female, fixing on her shell.
Meanwhile, the female is responsible for opening a hole in the sand, where she will put around two thousand eggs,. Once the process is over, they cover the eggs and they go away without returning. Instead of spawning their eggs in the sea like most crabs, this species does so in the sand of the beach, about twenty centimeters deep, and with a diameter of thirty centimeters wide.
Instead of looking for a specific depth to spawn, the horseshoe crabs require sediment and porous soil, with good drainage, to contain and cover the eggs, in order to guarantee the life of the offspring.
Sea Crabs Life Cycle
The crabs´ incubation period can also vary from species to species, but in general, these larvae go through several stages before they reach adulthood. The first of them is the Zoe, where the compound eyes, the telson, the abdomen, and the periopodos, develope.
After having carried out a series of molts, they go to the stage called megalopa larva, where they have a larger and thicker exoskeleton than before. In this stage the structure of the crab changes, since the rostral and posterior spines disappear, and their shell round off. Periopods, and pleopods, as well as cephalic appendages are similar at this stage, than those of juvenile crabs.
During this phase the crabs begin to swim, and they have maxillipeds, which are designed to capture their food. They also migrate to the coast during the megalopa larva stage, and after passing the next moult they are called immature crabs. After successive shells molts, the development of the adult state of the crab begins.
Sea Crabs Common Habitats
These beautiful animals are distributed throughout the world, there is not a sea on the planet, that does not have at least one species of crabs that live in it. If they don´t live indefinitely, at least they do it temporarily, because of their migrations. But in all the seas of the world, there are crabs.
These marine animals can adapt to any type of habitat as long as they can find all their food requirements, If the conditions of an environment guarantee their existence, the crabs will stay for a long time.
Sea crabs can live even five kilometers from the seashore, as long as they can return to it, and there are foods available for them in this new environment, such as small worms, algae, crustaceans, and also rocks where they can hide to be safe from predators.
There are also species that can live both at the bottom of the ocean, and in bottoms full of mud near the coastal areas, at a depth of no more than fifty meters.
Sea Crabs Distribution
Just as these fantastic animals are present all over the world, many of them are usually habitual neighbors of the coasts of the Mediterranean sea and the Atlantic ocean. However, and even if they are scattered, the crabs are in all seas, especially in the tropics, where the temperature of the water appeals to them because they are more favorable to their needs.
Sea crabs are able to adapt to any habitat, although they tend to stay in the places where they feel best, where they can get enough food, where they can swim with complete freedom, and where they do not feel the human beings´ threat, which are their fearest predator.
Most Common Types of Sea Crabs
Let´s learn about different types of sea crabs
Moruno or Velludo
(Eng) Warty crab (Fr) Crabe verruqueux
It inhabits between algae and shallow rocks, up to 15 m, feeding on mollusks and worms. It can reach 7 cm in length and 9 cm wide. Its color is reddish or greenish brown with yellowish spots.
Coastal or Sand Crab (Liocarcinus vernalis)
(Eng) Smooth swimcrab (Fr) Etrille lisse
It is Brown, with its legs and part of its body covered with hairs, and has the tip of the last two legs in the form of an oar. It usually lives half-buried.
King Crab (Chaceon affinis)
(Eng) Deep-sea red crab (Fr) Crabe rouge profond
It lives in the area of the Canary Islands, in deep waters from 500 m. It is reddish brown in color. The king crab lacks of hairs on the body and legs while their limbs are long and thin.
Horseshoe Crabs
Horseshoe crabs are also known by other common names, such as casserole crabs, as they look like an inverted pot, Moluccas casserole, in reference to the Indonesian archipelago; crab from Japan, small sea casserole, bayonet crab, marine cockroach, given its similarity to this creeping animal, and small sea tank, as they look like an old war tank.
This crustacean is characterized by having a distinctive shell in the shape of a horseshoe, which originates its name. In addition, such carapace is characterized by being smooth and convex.
Japanese Spider Crab.
This crab is a crustacean decapod, which belongs to the group of spider crabs, so it is characterized by having a triangular shell and long legs.
The legs of the Japanese spider crab can reach more than two meters in length, the longest being the front ones. Therefore, the legs in conjunction with their bodies can reach a total lapse (from chelated to chelated) of 3.8 meters in diameter and approximately twenty kilograms in weight.(we invite you to read our article Japaneese Spider Crabs to learn more about these crustaceans)
Let´s watch them
Sea Crabs´ Predators
It is very difficult to determine the life expectancy of an animal that has about four thousand species. However, by general measure, sea crabs can live for between three to fifteen years, but there are also several species that can exceed thirty years of life
However, these animals are usually hunted and devoured by marine animals such as otters, octopi, sea turtles, dogfish, sharks, and even other species of larger crabs. Other times, the same crabs of their species commit cannibalism when they lack of food.
When the crabs come to the seashore, there are also many animals that take advantage of them, not only of their eggs or larvae, but also of themselves. Even larvae can be consumed, by the plankton and other forms of marine life; while on earth are the cats and dogs, and occasionally the birds, who feed on them.
At the moment when the crabs are molting their exoskeleton, they are easy prey for sharks, eels, striped bass, and drum fish, which are their main predators in the sea.
It is for this reason that the crabs remain hidden as long as possible when they shed their shell to avoid being eaten by predators, there is even a species that eats its exoskeleton to feed itself, while the days pass so that its new shell hardens.
These poor animals have many other predators such as the octopus, the sea seal, the squid; and in addition to having marine and terrestrial predators, they also have flying predators, such as the bald eagle, hawks, gulls, herons, who not only take advantage of the floating larvae, but also of the crabs that are on the shore of the beach.
When the hermit crabs first go out to the shore in search of their first terrestrial shell, they become a feast for the birds that wait for them anxiously.
As you can see, there are a lot of predators of these creatures, but the most bitter of them shamefully is man. In the case of marine predators, it is normal, since crabs are part of the food chain of the ecosystems where they live; and just as they are predators of some species, they are also the food of others.
But this species is also used as food, because its meat provides vitamins, minerals and a delicious flavor.(We invite you to read our article crabs to learn more about the properties that these crustacean offer for humans health)
Crabs even represent a delicacy for many people, for which they pay a large amount of money in different parts of the world.
For this reason it is a quite profitable business fishing these crustaceans, where you do not invest money or time to raise and feed them, many times without even letting them finish developing.
It is true that human beings can enjoy all the benefits of nature, even in the nutritional aspect, but we must remember that all living species are essential for our planet.
