KINDS OF CRABS
Crabs come in an astounding variety. Some look like terrifying monsters. Others resemble daddy long legs spiders, helmeted hockey players or wolves. The calappa species of crabs are right-handed, with the larger right claw used like scissors to cut open the shells of dextral snails. The greatest collection of specimens is at the National Museum of Nation History in Paris.
Some species have Velcro-like hairs and attach sea weed and even sea anemones to themselves as camouflage. Swimming crabs hold the fish they catch in their claw like a cigarette and then eat them head first. Arrowhead crabs a kind of spider crab, have eyes so far part they can almost see in a complete circle. Spotted crabs found in the Great Barrier Reef fiercely defend their territory during the day but at night you can pick them with your hands.
Portunid crabs (swimming crabs) are characterised by the flattening of the fifth pair of legs into broad paddles, which are used for swimming. This ability, together with their strong, sharp claws, allows many species to be fast and aggressive predators. Examples of these crabs include many well-known shoreline crabs, such as the European shore crab (Carcinus maenas), blue crab (Callinectes sapidus), and velvet crab (Necora puber). [Source: Wikipedia]
Fresh water crabs are tied to the ocean for reproduction. They return to the sea to mate and produce eggs. Young crabs go through larval stages to become crabs and then move to freshwater rivers, stream and lakes. A few species avoid returning to the sea by raising their young inside their shells while they pass through the larval stages and then releases them as small crabs. Freshwater crabs tend to near streams or marshes but can stay on land for long periods of time especially after it rains.
Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems
juvenile Galapagos crabs The infraorder Brachyura — sometimes referred to as “true crabs” — contains approximately 7,000 species in 98 families. Brachyura belongs to the group Reptantia, which consists of the walking-crawling decapods (lobsters and crabs). Brachyura is the sister clade to the infraorder Anomura, which contains the hermit crabs and relatives. Hermit crabs, horseshoe crabs, king crabs, and porcelain crabs are not considered true crabs. [Source: Chris Sergeant, Listverse, October 18, 2014; Wikipedia ]
Galapagos crabs The smallest species, the pea crab, is only a half an inch across. The large species, the Japanese spider crab, can measure up to three meters from claw to claw and have legs that are 1½ meters long and a carapace about a half meter long. Crabs have been observed at depths of 20,000 feet in the sea and 6,000 feet above sea level on land.
Spider crabs have a triangular carapace; long, slender legs; pincer-bearing length that look similar to their other legs. They often used their claws to attach pieces of sponges, seaweed, hydroid or other organic material on their shells as a form of camouflage. Some species cover their entire bodies and are quite difficult for divers and predators to find. When the move to new surroundings they will change their camouflage to match their new environment.
There are several deep sea crabs that live several thousand meters below the ocean surface. Elizabeth Kolbert wrote in the New Yorker:“Yeti crabs, first observed in 2005 on a vent system along the Pacific-Antarctic Ridge, south of Easter Island, look like hairy white lobsters. Their “hairs” are actually extensions of their shells, and along them live colonies of chemosynthetic bacteria, which the crabs scrape up and consume. Yeti crabs were found to be so evolutionarily distinctive that taxonomists had to create not just a new genus but a whole new family for them. [Source: Elizabeth Kolbert, The New Yorker, June 14, 2021]
Beach Crabs and Shore Crabs
Common ghost crabs Ghost crabs, sand crabs and other crabs that hang out at beaches tend to hang out in the hole during high ride when their holes are submerged and emerge during low tide. Their holes are often surrounded by excavated sand and little round balls that are remains of their feeding. They feed by picking up sand with their claws and sifting through it for detritus. As they do this they roll the sand into little balls. While they do this they need to keep an eye out for sea birds and other predators that feed on them.
Shore crabs in Japan live among the rocks in intertidal areas but lay their eggs in the sea. Their larvae pass through a plankton phase when they drift in the ocean currents. To get them to these currents shore crabs try to place their young as close to the currents as possible by laying their eggs as far away from shore as possible. This is during the new and full moons when the moon and earth or more or less in line and thus the gravitational pull of the moon and sun work together, producing large spring tides.The crabs lay their eggs after high tide at this time and let the receding ebb tide carries the larvae far out to sea.
Masked Crabs (Scientific name Corystes Cassivelaunus) are a small species of burrowing crab that grows to a maximum size of five centimeters (2 inches) and is found throughout the North Atlantic, North Sea, and some parts of the Mediterranean. It gets its name from the patterns on its carapace (shell) which are said to resemble a human face. Among the most interesting features of this crab are its two elongated antennae at the head end of its carapace that serve as breathing tubes, drawing oxygenated water down into sediment. The antennae allow the crab to breath while buried under the sand and while hunting within the seabed for polychaete worms and bivalve mollusks. [Source: Chris Sergeant, Listverse, October 18, 2014]
Ghost crabs live in holes and feed on organic matter found in the mud. They keep the lookout for predators with eyes stalks covered by multiple eyes that allow the crabs to see 360̊. They sometimes climb trees to get at insects. Ghost crabs are generally small, with carapace length that reaches around five centimeters (2 inches) at maturity. They are usually straw-colored or grayish-white and have a quadrate carapace, large club-shaped eyestalks, unequal chelipeds (claws) and long walking legs. Males are generally larger than females.
Atlantic ghost crabs (Scientific name: Ocypode quadrata) are found in the area above the spring high tide line of tropical and subtropical sandy oceanic and estuarine beaches from the water line up to the dunes. They inhabit the eastern coasts of North and South America. And range from Block Island, Rhode Island to Santa Catarina, Brazil.[Source: Lisa Izzo and Nikhita Kothari, Animal Diversity Web (ADW) /=]
After hatching from an egg, Atlantic ghost crabs go through five zoea stages and one megalopa stage to develop into an adults. The larvae develop in saline water. Metamorphosis into the first crab stage takes place at the surf-beach interface. The megalopa stage, which requires at least 35, is one of the largest of the true crabs. Unlike other crab species, ghost crabs can mate even when the female’s shell is hard, which means that they can mate anytime of the year after they reach sexual maturity. This is an adaptation to terrestrial life. Typically mating occurs in or near the burrow of the male. The male often produces a copulatory plug. The seminal fluid that contains the sperm becomes hard, preventing the sperm of rivals from reaching the female’s ova. /=\
Ghost crabs are primarily nocturnal. The Atlantic ghost crab constructs new burrows or repairs older ones during the morning. In the early afternoon it plugs the burrows and stay in them until after sunset. Burrows range from 60 centimeters to 1.2 meters in length and their width is equal to or slightly larger than the carapace size of the burrower. Younger, smaller crabs tend to burrow closer to the water. While foraging at night, a crab can travel up to 300 meters and often it returns to a different burrow. Ghost crabs in the U.S. hibernate in their burrows from October to April. Ghost crabs communicate using a variety of sounds, including striking the ground with their claws, stridulation (rubbing together) their legs and making a “bubbling sound”. Males compete in a ritualized matter that avoids the need for physical contact. /=\
Ghost crabs are both predators and scavengers, and they feed at night. Their prey is often influenced by the type of beach they live on. Crabs on oceanfront beaches tend to feed on bean clams and mole crabs, while crabs on more protected beaches may feed on the eggs and hatchlings of loggerhead turtles. Ghost crabs have few terrestrial predators. They are largely nocturnal, which reduces the risk of being eaten by shorebirds and gulls. When they leave their burrows during the day, they are able to slightly change their color to match the surrounding sand. Raccoons and burrowing owls are known to sometimes prey on ghost crabs. /=\
Pom-pom crabs (Scientific name: Lybia Tessellata) are also known as boxer crabs. They are a small species from the Xanthidae family found across shallow reef environments in the tropical Indo-Pacific. They are noted for their symbiotic relationship with the Triactis producta sea anemone. When threatened, the crabs grasp anemones between their modified front claw and shake them around like pom poms. The anemones also sting and immobilize prey with their venom, enabling the crabs to help themselves to easy meals. In return the crabs transport the anemones around, giving them access to a larger array of food sources. Neither species depends on the other, If no anemones are present, the crab moves around small sponges or pieces of coral instead. Pom pom crabs are also popular aquarium species.
Gaudy clown crabs (Scientific name: Platypodiella Spectabilis) are small but colorful crab. With a carapace that only reaches a maximum of around 2.5 centimeters (1 in), it lives in or near the intertidal zone and coral reefs in the Gulf of Mexico, the Caribbean, and the eastern Atlantic as far south as southern Brazil. According to Listverse: The patterning on their carapaces vary, but individual clown crabs tend to be consistent in color, with a yellow carapace covered in orange or red markings and black outlines, interspersed with small black and yellow spots. Despite their eye-catching coloration, they are extremely elusive, preferring to venture out onto the reef flats under the cover of darkness to feed. Colony-forming Zoanthid corals are their favorite meal. The crabs store the toxins produced by these corals and use them as a weapon against potential predators. [Source: Chris Sergeant, Listverse, October 18, 2014]
Arrowhead Crabs (Scientific name: Stenorhynchus Seticornis) are slender and have “triangular, almost guitar-shaped body, tapering out into an elongated rostrum with serrated edges”. They have long, spider-like legs, which can reach up to 10 centimeters (4 inches) in length and three times the length of their bodies. They can be aggressive scavengers, with feather duster worms and bristle worms being their favored prey. They sift through detritus although too and have been observed eating corals and attacking slow-moving or sleeping fish. Arrowhead crabs are known for being intensly territorial, especially with members of their own species. They are mostly nocturnal and spend the day hiding in crevasses of reefs or rock walls. They are found in the western and eastern Atlantic Ocean and Caribbean Sea to depths of about 10 meters (33 feet). Their strange appearance has made them a favorite among aquarium enthusiasts
Orangutan crabs (Scientific name: Achaeus Japonicus) are small species of spider crab or decorator crab native to tropical Indo-Pacific waters. Named after the thick, red-and-brown orangutan-like hair that covers its legs and carapace, they are often found among grape or bubble coral on protected reef flats. The hair traps various materials which help camouflage the crab from predators. Like other spider crabs, they have long arms in relationship to their bodies. Their pincers are often at work pulling organic stuff collected in their hair.
Flower moon crabs (Scientific name: Matuta Planipes) are also known two-spined burrowing sand crab or reticulated surf crab. Native to the Indo-Pacific and Australia, they reside in sandy sea-grass beds from the low tide mark down to around 15 meters (50 fee). They are most active at night, spending the day buried under sediments and sand. According to Listverse they are distinguished by their rounded beige to yellow carapace with a pair of elongated spines on either side. These crabs hold their pincers close to their body, in a similar manner to box crabs, and also resemble swimming crabs with their flattened carapaces. However, their limbs are flattened as well, with hind leg paddles that swim, dig, and burrow backward into the sand. Their hind limbs also lack the fringing hairs present in other crab species, which aids their digging abilities further. Unlike many other crab species, flower moon crabs take up oxygen-rich water through openings near their eye sockets, drawing it through hair-lined channels to their chelipeds. There, unwanted particles are filtered out using a dense mat of hair. [Source: Chris Sergeant, Listverse, October 18, 2014]
Raspberry crabs (Scientific name: Nucia Speciosa) are also known as the red leucosiidae crab. They gets their name from their color. Found between the high and low tidal zones in shallow waters over a wide area, including the Red Sea and waters off Mozambique, Mauritius, India, Japan and Hawaii, these small crabs are opportunistic carnivorous feeders, consuming small invertebrates and other organic materials. Their carapace is uneven, with 12 symmetrically arranged protruding nodules around the edges. They have strong, short, legs and chelipeds, which maneuvers food into their mouth. This crab is similar to but should not be confused with the strawberry crab (Pelia mutica) from the Gulf of Mexico and the Northern Atlantic.
Shame-faced crabs (Scientific name: Calappa Granulata) are also called flame, smooth or red-spotted box crabs. Relatively small and found across the Mediterranean Sea and the eastern Atlantic Ocean from Mauritania to Portugal, they get their name, according to Listverse, “from their characteristic stance, holding their chelipeds close to their carapace and covering their face, as if in shame”. These crabs spend the day buried in the sand with just their eyes and upper carapace and claws visible and hunt at night. Their enlarged front claws are very powerful and are used to crack apart the shells of their mollusk prey on and to dig up from the sand. When the crabs are buried in the sand, the claws also provide an air pocket within which to breathe.
Hairy crabs (Scientific name: Pilumnus Vespertilio) are also called teddy bear crabs. They a carpet of setae (long, silky hairs) covering their carapace and legs. These hairs protect them against predators, camouflaging them in their rocky reef environment and breaking up the outline of their body. The setae also trap dirt, mud particles, and other materials which provides them with additional cover. Hairy crabs be found across Indo-Pacific region, in the Red Sea and off Australia, and Japan mostly in low intertidal reefs, rocky and pebble shorelines, and seaweed -covered areas. They feed mostly on larger pieces of algae during the low tide. Females carry 300–800 eggs at a time and breed continuously between March and December.
Majoid Crabs — Decorator Crabs
Majoid crabs — known as decorator crabs — are well-known among marine scientists for adorning their surface with items secured from their surroundings. About 75 percent of majoid crab species decorate themselves with sponges, algae and other marine debris. Scientists are uncertain what physical and environmental factors drive this decorating behavior, though it appears to be used as a means to hide from, or deter, predators. University of Delaware marine scientist Danielle Dixson studied the majoid species Camposcia retusa to identify the factors that determine patterns of, and investment in, decorating. "The decorator crab is a perfect study example because the IndoPacific species has velcro-like substances on its shell and hooks on its appendages that enable it to secure items on its exterior," Dixson said.[Source: University of Delaware, December 8, 2017]
The researchers ran a series of experiments with decorator crabs that were placed in individual containers and provided with craft pom-poms that had been soaked in water so they would sink to the bottom. Half of the crabs were given a shelter for habitat to see whether having somewhere to hide affected how much or how fast the crab decorated. Over a 24-hour period, the team photographed the crabs every hour for the first 12 hours, and at hour 24, and analyzed the images to determine where the crabs decorated, whether they rearranged things and what parts they decorated first.
In the study, all of the crabs were fully decorated within 24 hours. Most of the crabs were decorated within six hours of having access to the pom-poms. According to Dixson, this shows that decorating is an important predator adaptation because the crabs do it very quickly. While other species of decorator crabs adorn their body first, the UD research team's study showed that Camposcia retusa decorated their appendages (arms/legs) first when a habitat was present.
This was different than other crabs that typically protect their vital organs first, but according to Dixson, still made sense because when they hide, a little bit of Camposcia retusa's arms remained outside their enclosure."This tells us they decorate the parts that stick out," Dixson says. But more decoration is not necessarily better. For the decorator crab, more decoration means the animal requires more energy to move around, and the slower they will be to escape predators.
Dixson and her students are also investigating whether the crabs can actually see and choose items based on color — meaning they are visually hiding themselves — or whether their decorating habits are motivated by smell, known as chemical camouflage. Sea sponges, for example, emit a scent that the crab may be using to chemically mask or camouflage itself from predators like eels, which have terrible eyesight but are known to hunt through smell. They also plan to explore what could make the crabs decorate faster, such as if it could see an eel in the next tank or if the predator smell suddenly was introduced into their environment and the stakes were higher.
Invasive, Cannibalistic Green Shore Crabs Can Eat Through Their Gills
Green shore crabs are a species native to the North Sea that are the cannibalistic and were introduced to Canada's west coast around the 1980s and quickly became a dominant species there. In 2017, scientists at the University of Alberta announced that they had discovered that these invasive, cannibalistic 'supervillain' crab crabs can 'eat' by absorbing nutrients across its gills — the first demonstration of this ability in crustaceans. [Source: Jennifer Pascoe, University of Alberta, December 6. 2017]
"People just assumed that because of their hard body, that crabs would not be able to access nutrients present in the water via the gills, and therefore they could only ingest nutrients through their digestive system," said Tamzin Blewett, a postdoctoral fellow at the University of Alberta A and lead author of the study. "We found that their gills, these specialised and delicate tissues that are designed for transporting things in and out of the body, are way more important than we originally thought," said Blewett. "While we knew that the crab gill takes up oxygen and deals with ions and toxicants in the environment, they are also being used for nutrition. They're super-critical organs."
Blewett and her colleague, Greg Goss, professor in the Department of Biological Sciences, observed the absorption of the amino acid leucine through the green shore crabs' nine sets of specialized gills, tucked away under its hard exoskeleton at the Bamfield Marine Sciences Centre on Vancouver Island. She said she suspects the findings are applicable to other crabs as well as to other amino acids, possibly pointing to the crabs' ability to survive and thrive as a species.
The ocean invasion of green shore crab has been dramatic, earning the species a spot on Canada’s Department of Fisheries and Oceans' watch list. "This type of crab is so readily adaptable to extremely harsh environments, and that's why they're everywhere," explained Blewett. "They're super-tolerant to low oxygen levels and changes in salinity, and now we know they also have this ability to consume nutrients through their gills. This ability may come in handy between meals. This ability kind of makes them the superhero of the marine world-or supervillain, depending on your perspective."
Blewett said the discovery only drives further questions. "Is this ability to absorb nutrients through the gills just for food, or is it a survival mechanism for when the crabs are situated in estuaries that are constantly changing in salinity? Salinity can alter body water content, but amino acids can be used to offset these changes caused by salinity, so taking up amino acids from the water could be an adaptation to these environments" she said. The research was published in "A novel pathway of nutrient absorption in crustaceans: branchial amino acid uptake in the green shore crab (Carcinus maenas)" appears in the December 2017 issue of Proceedings of the Royal Society B.
Blue crabs (Scientific name: Callinectes sapidus) are highly sought-after shellfish for food and the most valuable fishery in the Chesapeake Bay. The state crustacean of Maryland, they are major predators of benthic communities and are prey for many other fish species. They are caught by both commercial and recreational fishermen. Their scientific name means “beautiful savory swimmer” in Latin. [Source: NOAA]
Blue crabs have grayish blue and green shell. Their legs and claws are bright blue. Females have red tips on their claws. Their average life span of one to two years but can live up to four years. Many are harvested by humans before they have a chance to die naturally. They weigh approximately 150 grams (⅓ pound) at maturity and measure up 23 centimeters (nine inches) across the carapace (shell) Their lifespan is 3 to 4 years.[Source: Samantha Bodden, Animal Diversity Web (ADW) /=]
Blue crabs are found and up and down the Atlantic Coast and in the Gulf of Mexico off New England and Mid-Atlantic and the Southeast U.S. Although their natural range extends from Nova Scotia to Argentina in the western Atlantic Ocean, they have been introduced, accidentally or deliberately, into Japan, other places in Asia, Europe and Hawaii.
Blue crabs are bottom-dwellers found in a variety of habitats ranging from very salty water in the Gulf of Mexico to almost fresh water near river mouths in bays. They are especially abundant in estuaries. Their place in the water column extends from the low tide line to waters 36 meters (120 feet) deep. Females tend to occupy the higher salinity portions of the estuary system, especially when laying eggs. During the winter they migrate to deeper water.
Blue Crab Characteristics, Feeding and Behavior
Blue crabs have three pairs of walking legs and rear swimming legs that look like paddles. They have an “apron” that covers their abdomen. Males’ aprons are thin; females’ are wider. The edible portion of the crab is 15 percent of the crab’s total weight. The crabs range in length from 120 to 170 millimeters (4.72 to 6.69 inches). The largest blue crab caught in the Chesapeake Bay weighed half a kilograms (1.1 pounds) and was 27.3 centimeters (10.72 inches) tip to tip across the carapace. Males are larger than females. [Source: NOAA].
Blue crabs eat almost anything, including clams, oysters, mussels, smaller crustaceans, freshly dead fish, plant and animal detritus — and smaller and soft-shelled blue crabs. They often skip over long-dead animals though. Crabs are eaten by large fish such as red drum and Atlantic croaker, fish-eating birds like great blue herons and sea turtles.
According to Animal Diversity Web: Blue crabs are very aggressive when threatened, except when it has recently molted and still has soft shells leaving it vulnerable. The crab will also burrow into the sand to hide. Blue crabs are an active swimmer and has its last pair of walking legs adapted to be shaped like a paddle to accommodate swimming. It also has three pairs of walking legs, and a powerful set of chelae. Since Blue crabs are highly mobile, the total distance traveled per day is about 215 meters. This species tends to be more active during the day than in the evening. The blue crab moves from zero to 140 meters per hour, with an average of 15.5 meters per hour. [Source: Samantha Bodden, Animal Diversity Web (ADW) /=]
Blue crabs are both colorful and highly visually responsive, yet almost all studies of their courtship have focused on chemical cues. In the underwater environment of Blue crabs, visual cues may function more rapidly and over a longer distance than chemical cues. Given that Blue crabs are aggressive and cannibalistic, visual cues may allow them to quickly evaluate potential mates from safer distances. The crabs will use color vision and color in mate choice with males having a preference for females with red claw dactyls. /=\
Image Sources: Wikimedia Commons, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated April 2023