Sea Animal.
Have you ever walked along the beach and stumbled upon what looked like an orange spider? If so, you’ve probably seen a sea crab!
Sea crabs are a type of crustacean, meaning they have no backbone and are covered with a hard shell. Sea crabs live in the seas and oceans around the world, but can occasionally walk onto the shore.
e like creepy-crawly spiders, sea crabs actually have more legs than spiders do. These crustaceans have five pairs of legs – the back four are used for walking, and the front is where their claws are located.
also known as pincers, are used as weapons. They can fight off predators, or catch their prey when it’s time to eat. So, if you ever see a sea crab scuttling along the beach, you might want to stay away so you don’t get pinched!
id you know that there are thousands of species of sea crabs? Now that you know what a sea crab is, let’s take a look at some of the different types!
bs are generally covered with a thick exoskeleton, composed primarily of highly mineralized chitin, and armed with a single pair of chelae (claws).
rab are found in all of the world’s oceans, while many crabs live in fresh water and on land, particularly in tropical regions. Crab vary in size from the pea crab, a few millimeters wide, to the Japanese spider crab, with a leg span of up to 4 metres (13 ft).
Biology Of Crab
Evolution
The earliest unambiguous crab fossils date from the Jurassic, although Carboniferous Imocaris, known only from its carapace, may be a primitive crab.
concurrent radiation of bony fish, crabs’ main predators.
Sexual dimorphism
Crabs often show marked sexual dimorphism. Males often have larger claws, a tendency which is particularly pronounced in the fiddler crabs of the genus Uca (Ocypodidae). In fiddler crabs, males have one claw which is greatly enlarged and which is used for communication, particularly for attracting a mate.
Another conspicuous difference is the form of the pleon (abdomen); in most male crabs, this is narrow and triangular in form, while females have a broader, rounded abdomen. This is because female crabs brood fertilised eggs on their pleopods.
Reproduction and lifecycle
Crabs attract a mate through chemical (pheromones), visual, acoustic, or vibratory means. Pheromones are used by most fully aquatic crabs, while terrestrial and semiterrestrial crabs often use visual signals, such as fiddler crab males waving their large claws to attract females.
of brachyuran crabs have internal fertilisation and mate belly-to-belly. For many aquatic species, mating takes place just after the female has moulted and is still soft. Females can store the sperm for a long time before using it to fertilise their eggs.
When fertilisation has taken place, the eggs are released onto the female’s abdomen, below the tail flap, secured with a sticky material. In this location, they are protected during embryonic development. Females carrying eggs are called “berried” since the eggs resemble round berries.
When development is complete, the female releases the newly hatched larvae into the water, where they are part of the plankton. The release is often timed with the tides. The free-swimming tiny zoea larvae can float and take advantage of water currents.
They have a spine, which probably reduces the rate of predation by larger animals. The zoea of most species must find food, but some crabs provide enough yolk in the eggs that the larval stages can continue to live off the yolk.
Behaviour
Crabs typically walk sideways (a behaviour which gives us the word crabwise), because of the articulation of the legs which makes a sidelong gait more efficient. However, some crabs walk forwards or backwards, including raninids, Libinia emarginata and Mictyris platycheles.
Some crabs, notably the Portunidae and Matutidae, are also capable of swimming, the Portunidae especially so as their last pair of walking legs is flattened into swimming paddles.:96
Crabs are mostly active animals with complex behaviour patterns. They can communicate by drumming or waving their pincers. Crabs tend to be aggressive towards one another, and males often fight to gain access to females.
On rocky seashores, where nearly all caves and crevices are occupied, crabs may also fight over hiding holes. Fiddler crabs (genus Uca) dig burrows in sand or mud, which they use for resting, hiding, and mating, and to defend against intruders.28–29, 99
Crabs are omnivores, feeding primarily on algae, and taking any other food, including molluscs, worms, other crustaceans, fungi, bacteria and detritus, depending on their availability and the crab species. For many crabs, a mixed diet of plant and animal matter results in the fastest growth and greatest fitness.
However, some species are more specialised in their diets. Some eat plankton, some eat primarily shellfish like clams, and some even catch fish.:85
