Crabs are crustaceans characterized by their hard outer shell and their distinctive claws. Crab species are commonly found in both freshwater and saltwater environments and play an important role in many ecosystems as predators and scavengers. Crabs have a long history, with fossil evidence dating back over 150 million years. They are also an important source of food for many cultures and are a popular seafood item around the world.
There are many different species of crabs found around the world, each with their own unique characteristics and adaptations. In this article, we will discuss some of the most common species of crabs.
Blue Crab (Callinectes sapidus)
Carapace width: Up to 9 inches (23 centimeters).
Habitat: Found in coastal and estuarine habitats, including brackish lagoons, along the Atlantic coast of North America, including the Gulf of Mexico, and as far south as Uruguay.
Blue Crabs are characterized by their striking blue legs and claws, contrasting with their olive to grayish-blue bodies. Their blue coloration arises from a pigment named alpha-crustacyanin which becomes deactivated in heat, turning the crab red when cooked.
Blue Crabs thrive in shallow waters during warmer months, seeking refuge in deeper sediment beds as temperatures drop for the winter. As bottom-dwellers, the crabs use submerged aquatic grasses for food, nursery grounds, and shelter during mating and molting.
Their diet is opportunistic and includes clams, mussels and other crustaceans. In turn, they fall prey to larger fish, certain fish-eating birds like great blue herons and sea turtles. They are also known for their aggressive behavior and their ability to quickly shed and regrow their limbs. Despite being a significant species both commercially and recreationally, it is considered an invasive species in the Mediterranean.
Alaska Snow Crab (Chionoecetes opilio)
Carapace width: Between 3 to 6 inches (7.5 to 16 centimeters).
Habitat: Found in the cold waters of the North Pacific and North Atlantic oceans, with a preference for sandy and muddy bottoms less than 650 feet deep.
Alaska Snow Crabs are brown on top and lighter underneath with a hard rounded shell. They have four pairs of walking legs, one pair of claws and greenish-blue eyes. Interestingly, this crab is named for the appearance of its delicate meat which becomes white and snow-like when cooked.
Like other crabs, they play an important part in the ecosystem. They are opportunistic feeders, consuming a wide variety of prey, both live and dead. They have a variety of predators including seals, sea otters, octopi, other crabs and fish.
Alaska Snow crabs are commercially harvested and are popular in many seafood dishes. Although the population is not overfished, it is still in the process of rebuilding. They are typically caught using traps or pots, which are set on the ocean floor and baited with fish or other food.
Fiddler Crab (Uca spp.)
Carapace width: About 1 to 2 inches (2.5 to 5 centimeters).
Habitat: Inhabit tropical and sub-tropical intertidal zones worldwide, specifically muddy or sandy shores, mangrove forests, estuaries, and salt marshes.
Fiddler Crabs are very common and there are over a hundred different species. They are a small, burrowing crab species known for their oversized claw, exclusive to males. Males use their large claws to intimidate other males. They are highly territorial and aggressively fight each other to protect their burrows. These burrows provide refuge from predators, access to water during low tides, and can be used for breeding.
Males also wave around their oversized claw to attract females and sometimes defend multiple burrows to attract more females. Females make decisions regarding mate choice based on the size of a male’s oversized claw as well as burrow location.
Their burrowing activity creates a network of tunnels that aerate the marsh grasses and seagrass meadows, providing essential oxygen to these ecosystems. Together with their fecal matter, burrowing activity helps organic matter to decompose and release nutrients, enhancing the overall productivity of the wetlands.
Japanese Spider Crab (Macrocheira kaempferi)
Carapace width: Up to 15 inches (38 centimeters).
Habitat: Native to the Pacific waters surrounding Japan, inhabiting deep-sea environments on rocky and sandy bottoms at depths between 160 and 2,000 feet.
The Japanese Spider Crab is the largest crustacean in the world, known for its unique body structure and long legs that resemble those of a spider. Its pear-shaped body is orange and cream with a hard shell. It has ten legs, eight for walking and two “chelipeds” which are used for capturing prey.
These crab species are primarily scavengers that play a crucial role in maintaining the ocean floor’s cleanliness by consuming dead animals and decaying matter. They are also opportunistic feeders and also consume plant material like algae.
The Japanese Spider Crab faces several threats to its population, including overfishing due to its culinary appeal, habitat destruction from activities like trawling, ocean acidification which can impact their exoskeletons and development, and rising sea temperatures caused by climate change that can negatively affect reproduction and growth.
Hermit Crab (Paguroidea spp.)
Carapace width: Up to 6 inches (15 centimeters).
Habitat: Inhabit various marine environments, including oceans, seas, and tropical and subtropical regions worldwide.
The Hermit Crab has a soft, asymmetrical abdomen that is protected by a salvaged empty seashell. They are commonly found in shallow ocean waters as well as freshwater habitats, including rivers and lakes. Hermit Crabs are unique in that they do not grow their own shells but rather rely on the shells of other mollusks, such as snails, for protection. As they grow, they must continually search for larger shells to inhabit.
These crab species are omnivores, feeding on plankton, small fish, and other invertebrates. They are also known to scavenge on carrion and decaying organic matter. This makes them important as nutrient cyclers in the environments to which they belong, adding to local ecosystem productivity.
Hermit crab populations are threatened by habitat loss and harvest for the pet trade. Plastic pollution in the ocean also poses a dual threat to Hermit Crabs, making it challenging to find suitable shells and leading to accidental entrapment.
Red King Crab (Paralithodes camtschaticus)
Carapace width: About 7 to 11 inches (18 to 28 centimeters).
Habitat: Found in the cold, deep waters of the northern Pacific Ocean and the Bering Sea.
Red King Crabs are dark red or burgundy in color, covered in sharp spines, and have three pairs of walking legs. Their most notable feature is their pair of claws, one large and heavy-duty for crushing prey, the other smaller and more delicate for handling food. Adding to their unique look, their abdomens are fan-shaped and tucked beneath the rear of their shell.
While adult Red King Crabs are only susceptible to predators immediately after molting, younger crabs seek refuge in shallow-water habitats like shell hash and algae to avoid becoming prey. As they mature, these crabs congregate in massive pods, sometimes numbering in the tens of thousands, as a means to deter predators.
They are the largest commercially harvested crab species and their meat is considered a delicacy in many parts of the world. To mitigate bycatch and habitat impacts, regulations mandate the use of escape mechanisms on pots and fishing in soft sediment areas. Though some bycatch of other species occurs, the overall ecological impact of the fishery is considered minor.
Ghost Crab (Ocypode spp.)
Carapace width: About 1 to 4 inches (2.5 to 10 centimeters).
Habitat: Found in sandy beaches and coastal areas around the world.
The Ghost Crab, also known as Sand Crab, is named for its ability to blend into the sandy beaches and dunes it calls home. It is a relatively small crab species with a pale, almost translucent body with flecks of pink and yellow. This camouflage, combined with running speeds of up to 10 mph, allows it to disappear into its surroundings in a flash.
Ghost Crabs are nocturnal, spending their days burrowed deep in the sand and emerging at night to hunt. As opportunistic feeders, their diet includes both terrestrial and marine prey. Their eyes are perched atop long stalks, granting it a 360-degree view of its environment. They also have the ability to extract moisture from damp sand using fine hairs on their legs, eliminating the need for frequent returns to the water.
Despite being relatively common, these crab species are sensitive to ecological disturbances and are considered an indicator species of coastal ecosystem health. Their presence can be negatively impacted by human activities such as vehicle traffic, beach nourishment, and coastal modifications.
Ocean Insights: Hear From Our Experts
By Dr. Rabiah Ryklief
Principal Scientist and Lecturer of Marine Science, with a PhD in Zoology
Carcinization is the convergent evolution of a crab-like body plan in crustaceans. This has occurred independently at least five times, in both true crabs (Brachyura) and false crabs (Anomura). The crab-like form is characterized by a flattened carapace, a reduced abdomen tucked beneath the thorax, and four pairs of walking legs. This body plan is thought to be advantageous for maneuverability, defense, and exploitation of various ecological niches. While all Brachyura exhibit this form, Anomura display greater diversity, with some lineages evolving towards a crab-like form while others retain ancestral traits. The repeated evolution of this body plan suggests strong selective pressure favoring it in certain environments. However, the exact factors driving carcinization remain an area of ongoing research
FAQs
How do crabs reproduce?
Crab reproduction begins with a “belly-to-belly” mating embrace where the male transfers sperm to the female. The female then carries the fertilized eggs, attached to her tail, and broods them for about two weeks. During this brooding period, the egg mass changes color from yellow to a chocolate brown, signaling the eggs are ready to hatch. Once hatched, the larvae are released into the water to begin their life cycle.
What is the rarest species of crab?
The rarest crab species is the Louisea yabassi, a small, brown and rusty colored crab native to Ebo Forest, Southwest Cameroon. This freshwater crab was recently rediscovered after being presumed extinct for over a century.
What is the European Green Crab invasion?
The European Green Crab is an invasive species because it is a highly adaptable predator with no natural predators in its invaded regions. It disrupts ecosystems by damaging eelgrass habitats, outcompeting native species, and preying on shellfish, impacting both biodiversity and local fisheries.
