Crabs are amazing animals that can be found in a vast array of sizes and forms. These crustaceans inhabit freshwater environments, oceans, and even land. They are easily recognized by their hard shells and characteristic claws, but there is still a great deal to learn about them.
You may be surprised to learn that, depending on the species, crabs can walk in various directions. Although most crabs crawl sideways, some are able to move backwards and forwards. In addition, crabs are renowned for their amazing capacity for regeneration—they can regrow lost limbs like legs or claws!
Crabs are vital to their ecosystems because they eat dead animals and decomposing vegetation, which keeps their surroundings clean. There is always something new to discover about these incredible animals, as there are over 6,800 species. Let’s investigate some of the most surprising and fascinating facts about crabs!
In the animal kingdom, crabs are unusual and fascinating animals with distinctive qualities. These crustaceans have adapted to survive in a variety of environments, from the deep ocean to sandy beaches, thanks to features like their sideways walk, hard exoskeleton, and capacity to regenerate lost limbs. Crabs come in a wide variety of sizes, shapes, and colors, with over 6,800 species. Each species has unique behaviors and survival techniques. Examining these strange but true facts provides an insight into the fascinating world of crabs and their significance to marine ecosystems.
- Crabs have a hard and durable exoskeleton
- Crabs have a broad, flattened body and a short, symmetrical abdomen
- Some species of crabs are active predators
- When moving on land or the seabed, crabs usually move sideways
- Crabs exhibit some sexual dimorphism
- Mating is usually preceded by a short courtship period
- Crabs are among the most common marine invertebrates
- Crabs can reach enormous sizes
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- Interesting facts about crab
Crabs have a hard and durable exoskeleton
The Brachyura order comprises all members with this type of shell. A broad, flattened cephalothorax is formed by the union of the head and thorax. Crabs have multiple sections to their shell, each with a few distinct grooves dividing it. For feeding purposes, one to three anterior thoracic appendages develop into maxillofacial appendages.
Large species can have highly developed, widely venous circulatory systems, but they are still regarded as open. The nervous system has a large central component. A paired ventral nerve cord that spans the whole body connects the brain to multiple ganglia.
Crabs have a broad, flattened body and a short, symmetrical abdomen
Due to these two adaptations, they can fit into cracks and beneath rocks in order to find food and shelter. The abdomen takes up the majority of the crab’s body. The abdomen has a small head with short eye stalks that are positioned in unique nests on the shell. Additionally, there are multiple pairs of antennae with different maxilliped lengths. Compared to the rest of the body, the first pair of walking legs is larger and ends in pinching claws. We call these chelipeds in common parlance. The tips of the remaining four pairs of legs in the majority of species terminate in points.
Some species of crabs are active predators
Crabs are omnivores and scavengers, just like many other crustaceans, but many of them are also carnivores and some are vegetarians. Small fish, mollusks, worms, and other crustaceans are the food source for carnivorous species. When they eat, they break up food with their chelipeds, pass it to their jaws in small portions, and then push the food down into their pharynx.
David Clode took this picture.
Others use their chelipeds like a shovel to gather large amounts of mud as they feed on debris. The food particles are then removed by filtering, and the inedible materials are disposed of. Certain crab species that burrow into the soft seabed do so to create a current of water that filters out food particles. Additionally, the tiny bristly structures that fringe their chelipids aid in removing larger materials from the water flow while leaving behind small food particles.
While not all crabs are parasitic, some certainly are. A prime example is the small soft-bodied pea crab (Pinnotheridae), which coexists with a range of other mollusks in their shells and feeds on them. The growing algae, zoophytes, and sponges that many spider crabs (Majidae) cover their shells with provide them with excellent camouflage.
When moving on land or the seabed, crabs usually move sideways
Crabs typically move by walking or crawling, and the majority of this group share the well-known lopsided gait of the common shore crab. Crabs have four directions of locomotion. Walking involves pushing off with the legs on the other side pulling the body forward. When moving quickly, certain crab species may only use two or three pairs of legs, occasionally turning to rearrange their leg arrangement.
It’s a common misconception that all crabs can swim. The last pair of walking legs of the swimming crab family, Portunidae, are flattened and function as paddles, giving the group an oval body shape. The European green crab (Carcinides maenas), the calico crab (Ovalipes ocellatus), and the blue crab (Callinectes sapidus) are some examples.
Crabs exhibit some sexual dimorphism
In all species, males have a narrow abdomen, with the first pair of anterior abdominal appendages modified into reproductive structures designed to aid in sperm delivery. The abdomen of adult females is broadly rounded, with its width almost equal to the width of the ventral surface of the body. Pleopods (abdominal appendages) are used to hold fertilized eggs during incubation. Immature females have a wider abdomen than males, but narrower than mature females. In many species, male claws are much larger than females. Males use their enlarged claws for display, defense, or combat, and for handling food. Often, one claw is larger than the other; this can happen in both sexes. Body size, shape, patterning, and color can be important in identifying the species.
Mating is usually preceded by a short courtship period
Female crabs lay 1,000–2,000 eggs at a time shortly after mating. Until the larvae emerge, these stay on the female’s body.
Bob Montanaro is pictured.
Similar to other crustaceans, nearly all crabs have young that are significantly different from their parents as soon as they hatch from their eggs. The larval stage, sometimes referred to as the "zoea," is a tiny, transparent creature that swims and consumes plankton. It has a rounded body and no legs. The growing crab goes through multiple phases of moulting before entering a stage called megalopia, also called postlarva, where its body and limbs start to resemble actual crabs. An immature adult form and an adult form that is more developed are the outcomes.
Some crabs, particularly those found in freshwater environments, do not develop into free-living larvae but rather emerge from their egg shells as tiny adults.
Crabs are among the most common marine invertebrates
One of the most varied groups of crustaceans is the crab family, which includes 98 families and roughly 7,000 species that are known to exist in freshwater, marine, and terrestrial environments. The majority of species favor marine environments. Even land crabs, which are common in tropical regions, do, however, occasionally enter the sea to spend their formative years there.
Normally, crabs breathe through gills, which are found in two cavities beneath the shell walls; however, in land crabs, the cavities have undergone modifications and enlargements, functioning as lungs. Removal of nitrogenous waste most likely also happens through the body wall or gills.
Furthermore regarded as the most prevalent introduced and invasive species are crabs. Numerous introduced species can already be found in one or more of the northern European countries, and it’s possible that additional introduced species are making their way from nearby nations.
Crabs can reach enormous sizes
The two biggest crabs in the world are the giant Tasmanian crab (Pseudocarcinus gigas) and the Japanese deep-sea spider crab (Macrocheira kaempferi). When the former is fully extended, its size can reach up to 4 meters. The Tasmanian crab has a weight limit of more than 9 kg. Its body, or carapace, can reach 46 cm in diameter in a very large specimen, and its legs are much shorter and more robust, with a main claw that can reach 43 cm in length. In contrast, the pea crab, which is the tiniest crab in the world, has a diameter that ranges from 1.02 to 1.52 cm.
| Crabs have been around for more than 200 million years. | There are over 6,800 species of crabs. |
| Crabs can walk sideways, but they can also walk forward and backward. | Some crabs, like the coconut crab, can climb trees. |
| Crabs have a hard exoskeleton that protects their body. | They molt, or shed their exoskeleton, to grow. |
| Crabs use their pincers for feeding, fighting, and mating. | Some crabs are scavengers, feeding on dead plants and animals. |
| Fiddler crabs have one claw much larger than the other. | Crabs can regenerate lost limbs over time. |
Crabs are unusual among animals because of their special adaptations, which make them fascinating creatures. For those who are willing to look a little closer, they have plenty to surprise you with, from their sideways walk to their amazing ability to regenerate lost limbs.
Whether it’s clearing out trash in the ocean or giving other animals something to eat, these crustaceans are vital to their ecosystems. With over 6,800 species dispersed across a broad range of environments, from sandy shores to deep oceans, their diversity is astounding.
You’ll know there’s much more to crabs than meets the eye the next time you encounter one, whether it’s at the beach or in a documentary. Not only are they fascinating to observe, but they are also vital to the wellbeing of our marine ecosystems.
