14 Fascinating Facts About Crabs

Crabs possess highly advanced sensory organs. Their sense of touch and taste are facilitated by sensory bristles found on their antennae and other appendages. These bristles are connected to nerve cells beneath the epidermis. The crab’s balance organ, called the statocyst, is located at the base of the first antenna and contains sensory bristles along with tiny sand particles gathered during molting. Crabs are also able to detect sound through specialized organs located at the joints of their legs.

Crabs have compound eyes, each consisting of:

• A transparent chitinous membrane formed by cells that secrete chitin.
• A conical lens beneath the chitin membrane, surrounded by pigment cells.
• A photoreceptor layer arranged in a star-shaped pattern along the vertical axis, surrounding a rod-shaped structure secreted by network-like cells.
• The innermost part of the photoreceptor cells connects to the optic nerve.

All of these nerves together form the optic nerve, which leads to the optic ganglion along the length of the eye’s stalk.

As crabs transition from larvae to adulthood, they must molt several times. Crabs, like other crustaceans, are covered by an exoskeleton made of chitin, which becomes hardened through the absorption of calcium. Because the exoskeleton is rigid and does not grow, it must be periodically shed to accommodate the crab’s growth during its metamorphosis (from free-swimming larvae to mature adult) or when it simply outgrows its shell.

Before molting, crabs absorb calcium from their old exoskeleton and secrete enzymes to separate the old shell from the skin or epidermis. The crab’s new, softer shell forms underneath, which is thinner than the previous one. A day before molting, crabs begin absorbing seawater, causing their bodies to swell like a balloon. This helps crack open the old exoskeleton along a small line running across the body. The crab then wriggles and pushes itself out of the old shell, starting from its legs and finally separating completely. This entire molting process typically takes about 15 minutes.

In order to grow, crabs must undergo several molts. The time between molts can vary depending on the crab's stage of life. Larvae molt every 3-5 days, while adult crabs molt about once every two to four weeks. During molting, crabs may lose limbs such as claws or legs, but they can regenerate these parts in subsequent molts.

Before molting and mating, male and female crabs pair up anywhere from 2 to 10 days in advance. The male, typically larger than the female, uses his walking legs and claws to tightly hold onto the female's back. He leads her around, sometimes for up to 3 to 4 days or longer. Just before the female molts to mate, the male steps aside and waits nearby. Once the female has molted, the male immediately embraces her, pressing their bellies together and detaching her exoskeleton to mate.

The mating process can last anywhere from 5 hours to an entire day. Afterward, the male releases the female but stays nearby to guard her. Following mating, the sperm is stored in the female’s reproductive organs for an extended period, fertilizing her eggs. During the breeding season, the female may lay eggs 1 to 3 times, fertilized by sperm stored from earlier in the season. Our research showed that egg-carrying crabs kept in a cement tank (without males) from July to October laid eggs three times, and the eggs developed into larvae.

After mating, the female’s eggs continue to grow and mature. She lays her fertilized eggs while anchoring herself to the bottom with her legs. Her abdomen opens, and her pleopods are raised to assist with egg release through the oviduct, fertilized by the stored sperm.

The eggs, now contained in the female's abdomen, are encased in two membranes; the outer membrane swells when absorbing water. Between the membranes, there is a mucous substance. As the female moves her abdomen, the eggs adhere to the hair-like structures on her pleopods and stretch into “egg stalks,” preventing them from sticking together. These crabs are called egg-bearing crabs, and they continue to protect the eggs as they develop until the larvae are ready to emerge, at which point they are referred to as 'baby crabs.'

The world of crabs is incredibly diverse in both shape and size. Some species are terrifying, others are colorful, and many are quite bizarre.

Halloween Crab: This species is known for its vibrant colors. Halloween crabs are found primarily in the waters of Mexico, Central America, and possibly even Peru. They are nocturnal and typically roam forests eating leaves and seeds. Although they live mostly on land, they must return to the water to spawn.

Coconut Crab: Among the largest land-dwelling arthropods, this crab can weigh up to 4.1 kg. Coconut crabs are mainly found on islands in the South Pacific, feeding on coconuts, nuts, and occasionally other fruits. Their respiratory system is unique, with a structure that is neither gills nor lungs, but something in between. Their sensory organs resemble those of insects rather than other crabs.

Furry King Crab: This crab is entirely covered in spiny armor, which not only helps with camouflage but also serves as a defense mechanism against predators. They live at depths of up to 730 meters underwater, feeding on starfish, other crabs, and organic debris that falls from above.

Brown Box Crab: A type of king crab, this species has a peculiar breathing system. When it moves its front claws through its shell, a V-shaped groove between the claws and adjacent legs allows water to flow into the lungs, which are located beneath a layer of sediment.

Soft Coral Crab: These bizarre creatures excel at camouflage, blending seamlessly into soft coral reefs where they feed on plankton.

Zebra Crab: Found mainly in the Indian and Pacific Oceans, including Australia, these crabs have an unusual appearance, with dark and white stripes resembling a zebra pattern.

Orangutan Crab: A strange little creature related to both spiders and crabs, this crab has a thick orange coat of fur and long legs that resemble the arms of an orangutan.

Porcelain Crab: Not technically a true crab, the porcelain crab is an evolved type of shrimp with a crab-like appearance. It is fragile and easily loses legs when escaping predators, but they grow back over time. This species feeds on mucus produced by sea algae, using it to clean itself while the algae protects it from threats.

Crabs mainly consume plankton. Most crab farms today focus on feeding them fresh animal-based foods. The primary foods for crabs include crushed fish, crabs, mud crabs, fish heads, clams, snails, shrimp, and other crustaceans.

Plant-based foods include vegetables, aquatic plants, yams, cassava, rice bran, and other by-products from farming.

• Live food: This consists of animals that are whole and either alive or recently deceased, as long as the meat is still fresh. Avoid using spoiled or salted meat that can't be thoroughly rinsed. The following animal products are commonly used for feeding crabs:
• Fresh fish: Typically, various marine species such as broken fish, anchovies, and small fish.
• Mollusks: Such as clams and snails.
• Crustaceans: Primarily cheaper shrimp and crabs.
• Other animals: This often includes inexpensive animals not used for human consumption or leftovers from food processing plants like fish, shrimp, and squid.

Hong Kong Hairy Crab: Also known as Shanghai Hairy Crab, this species is harvested around September to October each year. Compared to other crabs, the Hairy Crab is much smaller, weighing around 150–200g, and covered in soft, fuzzy hair. Despite its small size, it is considered one of the finest crab varieties, beloved by connoisseurs. Its firm, smooth, and naturally sweet meat is complemented by rich roe that isn't too greasy, earning it the title of the 'king' of crabs.

Australian Snow Crab: Also called the Australian Crystal Crab, this species is found in the remote waters of Western Australia. Its white, large, and translucent shell gleams like crystal on the ocean floor. Due to the deep dives required to harvest it, this crab is quite expensive. It is prized for its unique, sweet, and firm meat, unmatched by most other crabs.

Tasmanian Crab: This crab hails from the seas surrounding Tasmania in southern Australia. Though it resembles the crabs of Vietnam, its size is vastly larger, with mature crabs weighing up to 10kg. With its unique flavor, delicious roe, and naturally sweet, tender meat, the Tasmanian Crab is highly sought after by seafood enthusiasts.

Alaskan Crab: Found in the deep waters off North America at depths of 200–400m, this large crab has a rounded shell resembling a giant beetle or turtle. Its head is covered with long, downward-facing spines. Known for its rich, fatty flavor, tasty meat, and firm roe, the Alaskan King Crab is a prized delicacy despite its high price.

Ruby Alaskan Crab: A relative of the King Crab, Ruby Alaskan Crab is found in the cold waters of Alaska. While similar in appearance to the King Crab, Ruby crabs have shorter legs. Their white, sweet meat is firm and crispy, making them a highly desired delicacy.

Emperor Crab: Found in some remote islands and coastal regions of Central Vietnam, the Emperor Crab is a large crab with a unique shape, characterized by a long body and a reddish-pink shell like boiled shrimp. Its claws and legs are short, while its broad, U-shaped shell is adorned with numerous antennae. A mature crab can weigh up to 1kg. Its superior taste, sweetness, crisp texture, and richness make it a sought-after delicacy for seafood lovers.

Devil Crab: The Devil Crab is a poisonous species found in the waters from Da Nang to Vung Tau, typically inhabiting shallow reefs and tidal zones. This small crab, roughly the size of a palm, is distinguished by its lumpy, flat shell and vibrant colors, setting it apart from other crabs. Its flesh and shell contain dangerous neurotoxins such as Tetrodotoxin and Saxitoxin, which can be lethal to humans, even in minute amounts. The toxins are mainly found in the meat, eggs, and the legs and claws. These toxins are extremely potent, similar to the venom found in pufferfish, directly affecting the nervous system and inhibiting respiration. Currently, there is no known antidote for Saxitoxin. Without prompt medical intervention, poisoning can be fatal, so it is crucial to avoid this deadly crab.

Stone Crab: The Stone Crab is a large land crab with a dark purple shell, long legs, and relatively short claws. It is nocturnal, hiding in burrows during the day and feeding on plant material. When cooked, the meat turns a reddish-orange color. Like the Devil Crab, the Stone Crab also contains harmful toxins. Its shell is distinct with a half-circle shape, measuring approximately 30mm in length and 40mm in width, covered in nodular bumps.

Stone Crabs are rare and difficult to catch, and their sweet meat has made them a sought-after delicacy. However, improper cleaning during preparation can lead to toxic contamination, resulting in severe health issues and potentially tragic outcomes.

Florida Crab: This species has a horizontally elliptical shell with a slightly raised, smooth back. Its body is marked with light blue and greenish hues, interspersed with maroon streaks, and dark claws. Found from Da Nang to Khanh Hoa, the Florida Crab was involved in a distressing incident in Da Nang when a person developed unusual symptoms, including persistent vomiting, after eating one. Immediate medical attention was required.

The Arrow Crab is an extraordinary species known for its incredibly long legs, reaching up to 20cm, five times the length of its 4cm body. Recently, the Blue Reef Ocean Park in Newquay, UK, welcomed a new and unusual resident: an Arrow Crab. This crab has a sharply pointed head and bulging eyes, resembling the tip of an arrow, hence its name.

Arrow Crabs are found in coral reefs about 10 meters below the surface in the western Atlantic, stretching from the United States down to Brazil and across the Caribbean. Their diet mainly consists of decaying marine life, algae, and plankton.

Their body colors vary widely, from yellow or cream with brown, black, or multicolored spots to blend into their environment. Their legs are typically red or yellow, while their claws are blue or purple.

Female Arrow Crabs carry eggs under their abdomen until they hatch. The larvae swim in the plankton on the water's surface before returning to the coral reefs, where they undergo several molts before maturing into adult crabs.

The mysteries of the deep ocean remain captivating, even as technology advances and submarines dive deeper. Despite this progress, much of the ocean's depths are still unexplored. Among the many species of crabs inhabiting the ocean floor, the Pacific Spider Crab stands out as the largest known species. Native to the seabeds off Japan, this massive crab is sometimes referred to as the Japanese Spider Crab.

While much remains unknown about this gigantic creature, its meat is known to be rich and nutritious, highly prized by gourmets. Scientists have determined that the Pacific Spider Crab has the longest legs of any crustacean. While common crabs might have a body span of about 10cm from claw to claw, an adult Pacific Spider Crab's leg span can reach an astounding 4-5 meters, equivalent to the length of a four-seat car.

The crab's long, spindly legs support a body that is about half a meter wide and weighs up to 20kg. Interestingly, in the animal world, crabs with long legs are often male, while the females tend to have shorter legs. Their claws are also noticeably shorter than their legs.

Despite its imposing appearance, resembling a giant, aggressive spider out of a horror movie, the Pacific Spider Crab is actually quite docile. It relies on sea sponges and other marine organisms for camouflage and hides from predators rather than engaging in fights. When threatened, it retreats into its tough, turtle-like shell for protection.

According to the AP, the newly discovered crab species was identified by Professor Ho Ping-ho from the National Taiwan Ocean University. He stated that these crabs resemble species found in the waters of Hawaii (USA), the Polynesian islands, and the island nation of Mauritius (southwest Indian Ocean). However, the newly discovered crabs have a shell resembling a clam shell, about 2.5 cm long, setting them apart from other red-colored crabs.

Wang Chia-hsiang, a leading expert on crabs in Taiwan, confirmed Professor Ho's discovery.

Professor Ho and his team found two female specimens of this new crab species in the Kenting National Park, a region rich in marine life. Unfortunately, both crabs were dead when discovered. Ho suspects they may have died due to a cargo ship running aground near the park, causing water pollution.

Crabs are omnivores, with algae being their primary food source. According to the Daily Mail, scientists have identified over 5,000 species of crabs worldwide.

Scientists from Kerala University in India have identified a strange new species of crab that lives entirely on trees in the Western Ghats forests of southern India. This discovery was reported in *Crustacean Biology* on April 3. The local Kani tribe, already familiar with the existence of these unique crabs, assisted the researchers in finding them. In recognition of their help, the crab was named *Kani maranjandu*.

This is the first time in scientific history that a crab species living exclusively on trees has been discovered. Unlike typical crabs that live on the ground, these crabs reside in water-filled hollows within trees.

The *Kani maranjandu* is not well-known due to its shy nature. The Kani tribe taught the scientists how to locate them by searching for wood chips and air bubbles escaping from the tree hollows. These crabs have dark purple shells and long, slender legs, perfectly suited for life in the trees.

Researchers emphasize that this is a significant ecological discovery. The *Kani maranjandu* serves as an indicator species in forest ecology. Areas with high numbers of these crabs signal healthy forest ecosystems. Their unique habitat, the water-filled tree hollows, is crucial to their survival.

The suborder *Brachyura*, also known as true crabs, comprises crustaceans with broad, short bodies, soft exoskeletons, ten jointed legs, and pincers that evolved from the first pair of legs. The abdomen is short, hidden beneath the thorax, and the body is encased in a hard shell. Crabs are found across all marine environments, with some species inhabiting rivers, streams, and rice paddies, such as those in the *Potamidae* and *Parathelphusidae* families, as well as a few terrestrial species like those in the *Gecarcinidae* family. The head and body of crabs are fused into a single unit, protected by their outer shell.

In Vietnamese, crabs are called by various names such as *cua*, *cày*, *còng*, *đam*, *rạm*, *dã tràng*, *ghẹ*, and *cà ra*. However, these names do not represent modern taxonomic groups, but are based on their morphology or habitat. There are approximately 6,800 known species of crabs.

The name *Brachyura* is derived from Greek, meaning "short tail" (*brachys* = short, *oura* = tail), reflecting the fact that these crabs have a very short abdomen, often concealed beneath their thorax.

The cephalothorax: The head and thorax in crabs are fused together, with no clear boundary between the segments. By examining the gill appendages, the number of segments can be determined. The head consists of five segments bearing eyes, antennae, and mouthparts. The thorax has eight segments, with legs for feeding and walking. The dorsal side of the cephalothorax is encased in the carapace (crab shell). The front of the carapace has two eye sockets, each supporting an eye on a stalk. Between these sockets, on the front edge of the carapace, there are nine spines arranged in a row. The top surface of the carapace is divided into distinct regions separated by grooves and ridges. The frontal region is called the forehead, followed by the gastric region separated by two ridges, the cardiac region, and the intestinal region. The liver region is located on either side of the gastric region, and the gill region is located on the outer edges. The ventral side of the cephalothorax has abdominal plates. These plates form a concave area that houses the folded abdomen. In females, there are two genital openings located on the third abdominal plate, covered by the folded abdomen.

The abdomen: The abdomen consists of seven segments, with reduced appendages that are folded under the cephalothorax (commonly called the crab apron). This structure allows the crab to shorten its body and become more compact, aiding movement. There are differences between the abdomen of male and female crabs. In immature females (prior to pre-molt), the abdomen is somewhat square (often called the square-shaped apron in females). After pre-molt, the apron becomes rounded (called the ballooned apron), and its size increases to cover most of the abdominal area under the cephalothorax. Male crabs have a narrower abdomen that tapers into a triangular shape and fits neatly within the concave area of the cephalothorax. In females, segments I, II, and VII of the abdomen are movable, while other segments are fixed. The abdominal legs are bifurcated and become covered with bristle-like structures to carry and protect the eggs. The anus is located at the posterior end. In males, segments I, II, V, and VI of the abdomen are movable, and the abdominal legs degenerate into copulatory pincers. The anus is at the rear end.

Appendages:

• Mandible I: Consists of a two-part base. The upper surface of the leaves is covered with numerous hairs. The inner branch of the distal part is a thin, flat blade with two segments, bearing hairs on the inner edge.
• Mandible II: Consists of a two-part base: the inner blade is shaped like a knife with many hairs on its tip; the outer blade is bifurcated at the tip, and both branches are covered with hairs.
• Maxilla I: Composed of a two-part base: the inner leaf is small and covered with stiff hairs at the tip, while the outer leaf is wide and has short hairs along the outer edge. The distal part consists of two branches: the inner branch is leaf-shaped with long hairs along the inner edge, and the outer branch is composed of three segments. The base also has a thin chitin plate shaped like a rice leaf, covered with long fibers that run outward and curve backward, called water filtering gills.
• Maxilla II: Composed of several small parts at the base, with a two-branched distal part. The inner branch has five segments with hairs along the inner edge, and the outer branch has three segments. The outer part of the base has comb-like structures resembling feathers, and the outermost part has water filtering gills.
• Maxilla III: Highly calcified, with a two-segment base. The distal part consists of two branches.
• Thoracic legs: There are five pairs. The first pair is highly developed and features pincers (crab claws). In males, the claws differ in size, with the right claw typically larger than the left. The claws serve both to capture prey and as defensive and offensive weapons. The second, third, and fourth pairs of legs are walking legs, with five segments, the last of which ends in a sharp claw. The fifth pair has broad segments, which serve as swimming paddles, and are the primary legs used when the crab swims.
• Abdominal legs: In females, four pairs of abdominal legs are used to carry eggs, and they are structurally similar, consisting of a base segment and a distal part that branches into two leaf-like segments covered with long hairs. The inner branch is segmented, while the outer branch is not. In males, only the first and second pairs of abdominal legs remain, and they are modified into copulatory organs. The base consists of three segments, and the distal part has only the inner branch, which is not segmented.

Upon removing the upper section of the crab's carapace (the hard shell on its thorax), the internal organs become visible within the cephalothorax. These organs are positioned according to the areas on the carapace. The stomach lies in the front-center of the body, followed by the heart, and then the intestinal cecum. The liver and pancreas are located beside the stomach, with the reproductive organs visible above. Beneath the liver and pancreas, within the gill chamber, the gills are attached at one end to the base of the appendages and are free at the other end.

The digestive system starts with the mouth on the ventral side, connected to a short esophagus running along the animal’s dorsal-ventral axis and leading into the stomach. The stomach consists of two chambers: the cardiac stomach equipped with a chitin-grinding mechanism, and the pyloric stomach, separated by a valve. From either side of the pyloric stomach, there are two intestinal cecums positioned above the liver and pancreas. The pyloric stomach connects to the intestines, which run posteriorly into the abdomen and terminate at the anus located at the end of the abdomen. The intestines also have long cecums. The liver and pancreas are highly developed, occupying much of the cephalothoracic cavity and consisting of numerous fibrous lobes.

The circulatory system includes the liver and pancreas, heart, gastric grinding apparatus, vas deferens, testes, intestinal cecum, gills, intestines, respiratory organs, and anus. Crab blood is colorless and transparent. The heart is five-sided and located behind the stomach, enclosed in a thin and transparent pericardium, with three pairs of heart openings: two pairs dorsally and one pair ventrally. From the heart, two large arteries extend forward and backward through the body to the organs, where blood moves through the sinuses and into the gills. In the gills, blood exchanges CO2 for oxygen and then returns to the heart through the vascular system.

The respiratory system consists of eight pairs of long gills, attached to the bases of the appendages within the gill chamber located on either side of the cephalothorax. There are also water-scraping gills. The gills have a central axis with two rows of lamellae arranged along it. Water enters through an opening beneath the claws, and the water-scraping gills circulate it in the gill chamber, creating a continuous flow. The water passes through the gills, where the blood absorbs oxygen and expels CO2, before exiting through an opening beneath the second antennae. When on land, crabs can retain some water in their gill chambers, enabling them to continue breathing for an extended period.

The reproductive system: In males, the testes are light-colored and situated on the liver and pancreas. The vas deferens coils and drains into the male genital pore on the ventral side. In females, the ovaries change color depending on the developmental stage of the eggs. Mature eggs are orange-red, and the oviduct is short and wide, draining directly into the female genital pore located on the ventral side near the base of the third pair of legs.

The nervous system: Crabs have a highly concentrated network of thoracic and abdominal ganglia. The brain consists of cerebral ganglia and a ventral nerve cord. Each branch of the nerve cord has small ganglia, which function as sympathetic ganglia. The ganglion ring connects with the thoraco-abdominal ganglia, where many nerves branch off to the appendages and internal organs. A small hole is located at the center of the ganglion mass. Behind this, two long abdominal nerve cords extend into the abdomen. In the anterior brain, around the optic ganglia and eyestalks, there are endocrine glands that secrete hormones regulating processes like molting, gametogenesis, sexual differentiation, and pigmentation: including molting hormones, growth regulators, and inhibitors of growth and molting.