Cardisoma guanhumi: The Blue Land Crab

Image Meera Mahase, David Forgenie, Ronell Marshall and Dannielle Newallo

GENERAL INFORMATION 

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A Male Blue Land Crab An Adult male Cardisoma guanhumi Source: The Twitching Sailor, 2012
Kingdom: Animalia
Phylum: Arthropoda
Class: Malacostraca
Order: Decapoda
Family: Gecarcinidae
Genus: Cardisoma
Species: Cardisoma guanhumi
Common Reference Name: Blue Land Crab

Throughout the Caribbean and Latin American region where Carcinoma guanhumi is found it is referred to by many names. Some of the common Caribbean Community and Common Market (CARICOM) names and Latin American names are listed below.

CARICOM Countries:

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  • The Blue Land Crab
  • Blue Crab
  • White Crab
  • Great Land Crab

Countries in Latin America:

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  •  Giant Blue Land crab
  • Juey
  • Cangrejo Azul
  • Guaiamum

According to the International Union for the Conservation of Nature (IUCN), the Blue land Crab is a green listed species. This means that the species is not endangered and it is free to be hunted and traded in the CARICOM and Latin American region where it is most predominantly found. In addition, according to the Conservation on International Trade in Endangered Species (CITES), Cardisoma guanhumi is given no special status as it is not an endangered species. It is a species that can be hunted and traded in the region where it is predominantly found.

GETTING TO KNOW THE ANIMAL

  •  Physical Appearance and Description:
Cardisoma guanhumi grows roughly 127 mm wide and 102 mm long, weighing 400 g on average, though males are generally larger than females. As a juvenile, it is typically tan or brown in color with orange legs. As an adult, its coloring ranges from blue to violet, though some females are white or ashy gray. Its body, protected by a smooth carapace, consists of a cephalothorax and abdomen. Connected to its cephalothorax are five pairs of biramous appendages covered with tactile setae, each of which are sturdy and longer than the width of the cephalothorax. At its front are two pairs of antennae and two pincers, one larger than the other. The larger pincer in males can reach 300 mm in length. The difference in pincer size notwithstanding, C. guanhumi is bilaterally symmetric.
  •  Diet and Digestive System:

Cardisoma guanhumi is an omnivore (Hill 2001). It occupies a range of dietary niches. In the wild it generally feeds on leaves, fruits and grass. C. guanhumi also feeds on insects, terrestrial non-insect arthropods, carrions and at time will even feed on feces. To forage the animal typically does not stray too far from its burrow and uses light and sound to locate it food source. The Blue Land Carb prefers to avoid direct sunlight and prefers to stay in the shade while foraging or they wait until the night (Hill 2001). After foraging, C. guanhumi carries its food back to the burrow using its cheliped where it eats and saves whatever it did not consume for later (Hill 2001). In captivity, the Blue Land Crab would eat almost anything (Lutz and Austin, 1980), however, they are fed mostly plant foods. For instance at the Emperor Valley Zoo in Trinidad, the Blue Land Crab is fed leaves and fruits along with vegetable such as carrots. They are fed decaying plant matter at time also. Cardisoma guanhumi is a mono-gastric animal. The digestive system of C. guanhumi is one that is made up of three parts: the mouth, the stomach and the intestines.

The digestive system of blue land crab is similar to that of another common crab in found in the Caribbean region, the blue crab or Callinectes sapidus. Information on the digestive system of C. guanhumi was not available; however, the blue land crab and the blue crab are closely related and share similar characteristics such as digestive systems. The digestive system of C. sapidus consists of three parts, the mouth, the stomach and the intestines. Digestion first starts in the mouth where the claws, also known as the chelicera, bring the food towards the mouth. Inside the mouth there are three jaws, one holds the food, then moves the food to the second and third jaws where it is torn into smaller pieces. Here mechanical digestion is at work.  It is then sent down the esophagus to the stomach. The stomach of the crabs such as C Sapidus is very unique.

The stomach grinds the food using hard plates and bumps, as well as digestive juices to breakdown food so it can be absorbed into the hepatopancreas. Here both mechanical and chemical digestion is at work. The hepatopancreas also referred to as the digestive gland or the midgut gland is an organ located in the digestive track of arthropods and molluscs and crustacean. The crustacean hepatopancreas has different functions including absorption, storage of nutrients and vitellogenesis during growth, and ovarian development (Wang 2014).  It also provides the same function that the liver and the pancreas provide to mammals. The partially digested food is then passed to the intestines.  The intestines simply complete the digestion of the food with the aid of enzymes and other digestive fluids. The nutrients are then absorbed by the animal and the undigested food is then sent to the anus for excretion.

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A side view of the internal anatomy of a typical crab, drawn as if the animal were cut open just to the side of the midline. The location of the claws and walking legs are given. The heart and major blood vessels are shown in blue, the gut is in green, and the nervous system is in red. Specific structures are labeled. Source: (Shimek 2008)
  •  Reproduction and the Reproductive System:

The male and female Blue land Crab takes approximately 4 years to reaches sexual maturity. Cardisoma guanhumi reproduce base on seasonal breeding. Breeding takes place in the based on seasonal weather conditions as well as lunar phases which varies by latitude. At the turn of the rainy season in the Caribbean which is marked at June to December the female searches for a male. This is done by the migration of the female crab from its natural habitat (burrow) to the seashore. The full moon and new moon lunar phase which is accompanied by heavy rainfall causes a mass migration of the species. This migration can be seen towards the Eastern Coast of Trinidad in places such as Manzanilla.  At this time the weight of the female crab greatly increases as forage intensity is increased for the first few weeks of the migratory period. In addition, it releases pheromones and tactile and auditory behaviours to attract males. During this stage of the reproduction cycle the female is larger than the male in size. This is seen as the start of the reproduction system of the animal. Males actively court ripe females during this period. After this, breeding takes place. Mating by sexual reproduction is done through a system known as polygynandrous. This is where a female pairs with several males, each of which also pairs with several different females.

Fertilization is done internally after which the female carries the eggs on their backs. The crab produces approximately 20,000- 1, 200,000 eggs per year. The average gestation period of Cardisoma guanhumi is 2 weeks after which the eggs begin to hatch into larvae so they are released into the ocean by the female. The Blue land Crab is oviparous meaning, the development of offspring occurs outside the mother’s body. The reproduction system is connected to the life cycle of the animal and will be discussed later in the paper. The reproduction system of Cardisoma guanhumi is both terrestrial and marine.  

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A. Ventral view of the male blue crab Callinectes sapidus (The reproductive anatomy of the male Blue Land Crab was not found. B. The Reproductive System of the male crab Oziotelphusa senex. C. Ventral view of the female crab Callinectes sapidus (The reproductive anatomy of the female Blue Land Crab was not found) D. The Reproductive System of the female Crab Callinectes sapidus showing the ovary.
  • Animal Health and Disease:

 There are no known genetic diseases that affect this species. However, lethargic crab disease affects the mangrove crab Ucides cordatus of Northeastern Brazil (Memórias do Instituto Oswaldo Cruz 2005) but this disease does not seem to have any effect on the C. guanhumi. With respects to where the crab lives, there are many beetles and insects together with worms that reside with the animal. However, their existence is one that is symbiotic with that of the blue land crab (Hill 2001). Cardisoma guanhumi is also not prone to any viruses/bacteria or parasites. However, there is a mutualistic and communistic relationship between flies, mosquitos, and small insects. Since there is no record of any diseases affecting the animal then there are not precautionary measures that need to be taken with respects to the animal health. However, with respects to human health, the animal should be cleaned properly before utilized by humans to prevent them from becoming ill. This is because if the animal resides in dirty water then it can get E. coli and other waterborne pathogens that can cause health issues to humans if they consume it (Blackman 2015).

Environmental factors (natural or man-made) that can affect species health:

  1. The destruction of forests and coast areas – When human being clear down forests and coastal areas they obstruct the migration path of the blue land crab from borrows to the coast. This makes them prone to being killed by vehicles and it can reduce the amount of crabs reaching the sea to release eggs.
  2. Alterations to estuaries – Humans can alter estuaries and in the process damage them in efforts to prevent flooding. As such since estuaries are natural habitats for the Blue land crab this can have a major effect on their health and survival.
  3. Damage to river banks – When people go to rivers they may damage river banks. This is another habitat where the crab has it borrow.  Also, in the event of flooding flood waters may destroy the crabs borrow as it erodes river banks. This can affect the animal’s health as its home may be damaged.
  4. Hurricanes and Tropical Storms – Hurricanes can damage coastal areas as storm surges hit the coast. This damages the habitat of the animal and can in turn affect their health.
  • Housing and Environmental Requirement:

As adults, Blue land crabs are terrestrial (land-dwelling) animals and are found as far as 5 miles from the shoreline, returning to the sea only to drink or breed (Hill 2001). They live in burrows several feet deep or at least to a level that will allow water to seep in for moisture. Their burrow is made from mud, straws and grass material. They usually dig their burrow in shaded areas. For the first two periods of the life cycle, the blue land crabs resides in the water. Then in the juvenile stage and the adult stage they reside in their terrestrial burrows.

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A juvenile C. guanhumi

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An adult C. guanhumi in a burrow             A blue land crab in the water environment

In the captivity, the habitat of the animal is usually a mound that is built using mud, twigs and straws. For instance at the Emperor Valley Zoo in Trinidad the exhibit that hold the blue land crab is built out of mud and twigs in the form of a mount that is islanded by water. In captivity water is essential for the survival of the blue land crab. This is because the crab requires water to facilitate reproduction and in also in the early stages of its life cycle. The crab also needs mud and twigs in order for it to build its burrow. However, in captivity the crab does not usually reside in a burrow this is more of a natural environment characteristic.

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An adult blue land crab at the Emperor Valley Zoo, Trinidad, in an exhibit Source: David Forgenie 2015
  •  Animal Behaviour and Psychology:

Cardisoma guanhumi communicates in various ways, but mainly with visual, auditory, and chemical signals. To attract mates, females release pheromones. To orient itself, depends on polarized light or the light from the horizon at dawn or dusk. It also uses the setae on its appendages for tactile purposes. It is heavily sensitive to vibrations. The animal is also territorial, they will attack other intruder crabs and will defend themselves using their large cheliped against predators or persons who try to handle them.

Communication Channels: visual; tactile; acoustic; chemical

Other Communication Modes: pheromones

Perception Channels: visual ; polarized light ; tactile ; acoustic ; vibrations ; chemical

The blue land crab is usually a passive animal but can become aggressive if disturbed. This is to defend themselves against danger. However, with respects to territorial aggression, they are aggressive to defend their territory. The main weapon of the blue land crab is it giant cheliped, to avoid being caught with them the animal can be handled safely by placing the thumb and index finger over the back just behind the cheliped. This would prevent the animal from getting hold of you since the cheliped is not very flexible.

  •   Habitat and Housing:

Cardisoma guanhumi lives within several hundred meters of the shore, particularly along estuaries and river banks. It burrows in dense shrubbery, mud, or coastal sand above the tide line. Burrows can extend up to six feet underground, and contain small pools of water at the bottom. Cardisoma guanhumi shares its burrow with insects and other small arthropods. The atmospheres within such burrows typically have very high carbon dioxide concentrations because they are often sealed with mud.

  • Life Cycle and Development:
Cardisoma guanhumi has a complex life cycle that begins during the rainy season (which varies by latitude) when females spawn their eggs into the ocean, strictly coinciding with the lunar cycle. Once hatched C. guanhumi molts through five larval periods before it undergoes metamorphosis into a megalops, the stage when the appendages first appear. The nest metamorphosis produces the juvenile crab, which closely resembles the adult form. Each metamorphosis is accompanied by a molting of the cuticle. The following molt, named the puberty molt, precedes full sexual maturation. Molting regulates the life cycle of C. guanhumi. Generally, C.guanhumi molts 60 times during its lifespan, compared to 20 times, which is typical of other crab species. After achieving sexual maturation, C. guanhumi becomes primarily terrestrial, returning to the ocean only to spawn and begin the life cycle once again.
  • Lifespan:
Many giant land crabs do not survive the larval stage. Those who reach adulthood achieve sexual maturity in approximately four years. Unfortunately, there is not much further data available regarding the lifespan of C. guanhumi. However, biologists hypothesize that the lifespan of a species of land crab is inversely proportional to its growth rate. In other words, the faster they grow, the shorter they live, and vice versa. From this, it is probable that C. guanhumi has a relatively longer lifespan than other land crabs, as it grows more slowly and molts three times as much (averaging 60 molts per lifetime as opposed to 20). In fact, the largest female C. guanhumi kept in captivity lived 13 years. Albeit biologists admonish against extrapolating laboratory findings to a species’ actual environment, I have given some rough estimates of lifespan below based on given data regarding the lifespans of C. guanhumi and other land crabs.
  •  Behaviour:
Cardisoma guanhumi is not a social species, spending most of its time in its burrow when not foraging or migrating to mate and spawn. Burrows range from one to eighteen centimeters in width, extending up to six feet into the ground. At the bottom of each burrow are one to two liters of water. In particularly abundant habitats, there can be up to 7,500 burrows per acre. Cardisoma guanhumi fiercely defends its burrow, and competition for the best burrow location often results in migration away from the shore for the losing competitor. Cardisoma guanhumi typically forages at dawn and dusk, its peak activity time. Cardisoma guanhumi stays within several square meters of its burrow, except when females migrate to the ocean to release their offspring. Midday temperatures keep C. guanhumi burrowed throughout the day.
  • Predation:
Due to their size, C. guanhumi is not heavily preyed on. However, it sometimes falls prey to large birds, mammals, and other C. guanhumi. Humans are the largest threat with respect to predation, harvesting giant land crabs in massive quantities for food. Fortunately, it is fairly safe in its burrow from predation. Hence, burrowing not only provides C. guanhumi with shelter, but also protects it from predation. In fact, the absence of a burrow, in addition to physical vulnerability and other factors, explains the high mortality rates of C. guanhumi during the larval stage.

 References

Blue Land Crabs Draft Rule Review. Tallahassee, FL: Division of Marine Fisheries. 2002. Accessed 03/19/03 at http://www.floridaconservation.org/commission/2002/nov/LANDCRABDRAFTRULEREVIEW.pdf.

Burggren, W., B. McMahon. 1988. Biology of the Land Crabs. New York: Cambridge University Press.

Burggren, W., G. Moreira, M. Santos. 1993. Specific dynamic action and the metabolism of the brachyuran land crabs Ocypode quadrata (Fabricius, 1787), Goniopsis cruentata (Latreille, 1803) and Cardisoma guanhumi (Latreille, 1825). Journal of Experimental Marine Biology and Ecology, 169: 117-130.

Hill, K. 2001. “Cardisoma guanhumi” (On-line). Accessed December 04, 2004 at http://www.sms.si.edu/IRLSpec/Cardis_guanhu.htm.

Lloyd, R. 2001. “The Illusive Great Land Crab” (On-line). Accessed December 04, 2004 at http://www.mhhe.com/biosci/pae/marinebiology/casestudies/case_01.mhtml.

Pinder, A., A. Smits. 1993. The Burrow Microhabitat of the Land Crab Cardisoma guanhumi: Respiratory/Ionic Conditions and Physiological Responses of Crabs to Hypercapnia. Physiological Zoology, 66/2: 216-236.

anonymous, 2003. “Great Land Crab” (On-line). eNature.com. Accessed December 04, 2004 at http://www.enature.com/fieldguide/showSpeciesSH.asp?curGroupID=8&shapeID=1063&curPageNum=6&recnum=SC0010.

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