Crabs: Characteristics, Behavior, Mating | Sea Life, Islands and Oceania

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CRABS

Southern Pacific porcelain crabs — Porcelain crabs resemble crabs, but are more closely related to squat lobsters and hermit crabs.

Crabs are decapod crustaceans of the infraorder Brachyura. "Decapods" — or "10 limbed" creatures —include crabs, lobsters, prawns, shrimp and crayfish. Most crabs are scavengers that feed on detritus, but some feed on plant material and others feed on small sea creatures. There are marine crabs, freshwater crabs and terrestrial crabs. They can be found in coral reefs, fast-flowing rivers and deep-sea hydrothermal vents.

Crabs are basically marine or at least water creatures They breath by passing oxygen-laden water through gill chambers within their shells. Crabs that spend a lot of time out of the water take in water when they are submerged and retain water within their gill chamber inside their carapace when they leave water. This a relatively small volume of water. Many species beat the water into a froth so it can hold more oxygen — which is drawn in from the air — which is absorbed in the gill chamber. Their exoskeletons prevent them drying out.

The basic design of crabs has not changed much in 200 million years and they have many similarities with insects. Their evolution has been characterized by the increasing of robust body, and the decreasing of the abdomen. Crabs have evolved and adapted to many environments. Some crabs have become so adept at living outside of water they don't like the water. During high tide some crabs immerse themselves in their holes and plug them and place a bubble of air underneath the plug to sustain them until tide goes out.

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 ]

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

Crab Characteristics

Blue crab dorsal view, from University of Oklahoma

Crabs generally have short, projecting tails and abdomens hidden underneath their thorax. They have an exoskeleton and compound eyes like insects, one set of claws, four pairs of legs, eyes mounted on stalks and a hard carapace enclosing gills and a soft body. Chelipeds are the appendages bearing a "chela" or pincher-like claws. Crabs scuttle along sideways on land but there movement in the water is more complex and varies according the conditions in the water. Portunids (swimming crabs) have a paddle-shaped rear legs to accommodate swimming.

A carapace is the hard upper shell of a tortoise, crustacean, or arachnid. Crab carapaces are often about twice as wide as they are long. They can be round, triangular, rectangular or oval. The abdomen is short, flat and tightly curled so it fits under the carapace. Crabs molt by squeezing their bodies through a crack across the rear of the shell.

Crabs can regenerate legs or pinchers lost while fighting or protecting themselves. They are cold blooded (ectothermic, use heat from the environment and adapt their behavior to regulate body temperature) and heterothermic (have a body temperature that fluctuates with the surrounding environment). Sexual dimorphism (differences between males and females) are sometimes present. Many crabs are bottom-dwellers. Some use beds of submerged aquatic grasses as sources of food, nursery habitat for young, and shelter during mating and molting. [Source: Samantha Bodden, Animal Diversity Web (ADW) /=]

Blue crabs have different names depending on their sex and where they are in the molting and reproduction processes: 1) Hard-shells (not molting): 2) Peelers (about to molt): 3) Busters (started to molt): 4) Soft-shells (just molted: not yet hard): 5) Jimmies (adult males): 6) Sooks (adult females): 7) She-crabs or Sallies (immature females) and 8) Sponges-Sponge crabs (females carrying eggs)

Crab Perceptions and Behavior

Blue crab ventral view, from University of Oklahoma

Crabs like to have a good place to hide whether it burrow on a beach, a crevasse in reef or rock or layer of sand or sedement on the ocean bottom. Scientists who conduct experiments with crabs often offer a place to hide as a reward. Crabs communicate with vision, touch and chemicals usually detected by smelling and sense using vision, touch and chemicals usually detected with smell. To smell, crabs that live in water have specialized organs called aesthetascs on their antennae to determine both the denseness and the direction of a scent.

Michael H. Robinson wrote in Smithsonian magazine: When I was studying the defensive behavior of crabs in Panama, I found an interesting variation in the flight responses of mangrove crabs. Those that I watched lived close to a major road. They did not flee down into their burrows in response to passenger cars but did retreat when large trucks went by. Trucks were rarer than cars, in a ratio of about 1 to 40. Interestingly, the escape response to trucks persists in the absence of real danger. No crab has ever been attacked by a truck when standing outside its burrow. It is appropriate, however, that escape responses to large moving objects should be difficult to extinguish. But, because hundreds of cars passed by throughout the day, running for shelter every time a car passed wouldn't be energy efficient. In the absence of attacks, many organisms learn to ignore stimuli that are constant or occur frequently. This is a behavioral process called habituation. It occurs widely in the animal kingdom. Scarecrows cease to scare. Plastic owls on buildings finally fail to exclude roosting starlings, and recordings of their alarm calls eventually fail to rid airports of feeding seagulls. [Source: Michael H. Robinson, Smithsonian magazine, September 1997]

Crab Reproduction

Crabs often mate belly to belly. Female crabs can only mate during the short period after she sheds her shell and is waiting for her new shell to harden. Males are alerted by biochemical signals given off by female when this occurs. The first male on the scene climbs quickly on her back and fights of rivals until mating takes place. Crabs engage in internal reproduction in which sperm from the male fertilizes the egg within the female and young hatch from eggs. They engage in year-round breeding and females release huge numbers of eggs into the sea. [Source: Samantha Bodden, Animal Diversity Web (ADW) /=]

Males often can mate more than once, during each mating season, where females often only mate once in their lifetime. For blue crabs the breeding season is all year, but spawning occurs mostly during the months of December until October. The number of offspring ranges from two million to eight million, with the average number of offspring being one. The gestation period ranges from 14 to 17 days, with independence occurring on average at two months. Both males and females reach sexual maturity at 12 to 18 months.

Crabs are often polygynous (males having more than one female as a mate at one time). When mating, a male blue crab cradles a female crab in a pose known as a “doubler” for a few days before the female’s terminal molt. They stay with her after mating until her shell hardens and to ensure another male doesn’t mate with her. In the Chesapeake Bay, blue crabs mate and spawn from spring to fall. Females migrate to the mouth of the Bay to spawn and can produce between 750,000 and 3,200,000 eggs per brood.

Spawning peaks among blue crabs are closely associated with the region they inhabit. Unlike males, female blue crabs mate only once in their lifetime — after their final molt. When approaching this stage of their lives, females release a pheromone in their urine that attract males. Male crabs compete for females. Mating occurs at this time and may last as long as 5-12 hours. Blue crab males tend to stay and protect the female until she has grown her hard shell after molting, but males have no interaction with the young. Females protects their young while they hatch, but do not have a significant role after that. This is al least in part because there are so many eggs.

Males transfer their sperm to the female for storage when the crabs mate while the female is in her soft-shell stage immediately after molting, The male then protects the female until her new shell hardens. Females spawn two to nine months. The spawning season is from December to October, with a peak both in spring and summer. When females are ready to spawn, they fertilize the eggs with the stored sperm and place them on the tiny hairs of the appendages on their abdomen. The female is called a "sponge" or "berry" crab while she carries eggs like this.

Crab Development

Crab larvae (called “zoea” ) hatch from eggs. At first they are a form of plankton. They are very small and swim freely and look like nothing like crabs and go through several larval stages, each with a bizarre-looking creature. After several moltings they begin to look like a crab. Most crab larvae are consumed by predators. Very few make it to adulthood. It takes about ten moltings for larvae to become a full fledged crab.

From Chesapeake Quarterly, Maryland Sea Grant chesapeakequarterly.net

Females releases newly hatched larvae into the water when egg development is complete within their bodies. The release is eggs is often timed with the tidal and light-dark daily cycle. The blue crab's incubation time is 14-17 days, which is when the eggs are brooded. During this time females migrate to the mouths of estuaries so that larvae may be released into high salinity waters. Blue crab larvae released in low salinity water have a low survival rate. Larval release often occurs at the peak of high tide. That way the tide can carry the larvae out to see when it recedes. Circumstances are different with other crab species. [Source: Samantha Bodden, Animal Diversity Web (ADW) /=]

Crabs are independent from the time they hatch. Freshly-hatched 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. Each species has a particular number of zoeal stages, separated by moults, before they change into a megalopa stage, which resembles an adult crab, except for having the abdomen (tail) sticking out behind. After one more moult, the crab is a juvenile, living on the bottom rather than floating in the water. This last moult, from megalopa to juvenile, is critical, and it must take place in a habitat that is suitable for the juvenile to survive. [Source: Wikipedia]

Blue crab eggs hatch into larvae that go through a series of molts in high-salinity coastal water before migrating back into estuaries. Blue crabs usually goes through seven zoeal stages that take place over 30 to 50 days and one postlarval, or megalopal stage that last for between 6 and 56 days. At this stage they begin to resemble adult crabs and returns to estuaries where they will make their home and live out their lives as adults. /=\

Crabs molt — shed their hard shell — as they grow. Because they lose hard parts during the molting process, it can be difficult to determine the age of a crab. Males molt multiple times during their lives. Females molt once, just before they are ready to mate. Blue crabs reach maturity in 12 to 18 months. Growth rates are affected by water temperature — they grow more quickly in warmer water. In the Gulf of Mexico, crabs may reach maturity within a year. But in the Chesapeake Bay, it may take 18 months. Growth is rapid during the first summer, with crabs growing from 70-100 millimeters carapace length. By the second year, maturity is reached at carapace lengths of 120-170 millimeters. Blue crabs grows to adult size after 18 to 20 molts. /=\

Crabs That People Eat

Many crabs for eaten by humans who like their taste. Crabs can be prepared in a number of ways. Hairy crabs are a Chinese favorite. Many types of crab don’t have so much meat and extracting it can be a joyous or exasperating experience depending on one’s temperament. Some people catch the crabs they eat by themselves, providing an inexpensive meal. Other people at the other extreme pay big money to have specific sought-after crabs prepared certain ways at expensive restaurants.

20110307-NOAA crab dungness crab _100.jpg
dungess crab
Commercially, crabs are often captured in trap. Those used to catch blue crabs are rectangular, 60 centimeters (two feet) wide, and are made of wire. The crabs are lured in by being baited with freshly dead fish. Crabs are also be caught in trawls and by trotlines and raised in aquaculture farms. For people who catch crabs themselves, often there aren't any harvest limits, but crabbers are encouraged to throw back females with eggs.

Most commonly human-eaten crabs
Common name(s) — Scientific name — Wild or Farmed — Harvest in tonnes (1000 kilograms)
1) Chinese mitten crab — Eriocheir sinensis — Farmed — 714,392 tonnes
2) Gazami crab — Portunus trituberculatus — Wild — 429,959 tonnes
3) Blue swimming crab — Portunus pelagicus — Wild — 180,119 tonnes
[Source: Food and Agriculture Organization of the United Nations (FAO), 2012; Wikipedia]

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