Do Crabs Have Blood?

It should come as no surprise that crabs do in fact have blood.  However a crab’s blood is much different from mammalian blood because their physiology is quite different.

All arthropods like crustaceans such as the crab and even arachnids such as the spider have a watery fluid called hemolymph which fills the inside of their shell.  Their internal organs float freely in this fluid which resembles something of half blood and half sweat.  Like sweat, it is a mixture of water and various electrolytes such as salt.  Like blood, it has oxygen carrying proteins however they are not bundled into cells such as they are in humans and the crab’s blood is also blue, not red.

These oxygen carrying proteins are known as hemocyanin and they help carry oxygen to the blood as they float freely within the hemolymph and use copper. Oxidized copper is blue, thus resulting in the blue color of the crab’s blood.  This fluid is kept circulating by the contractions of the arthropod’s legs and a long tubular heart that is open ended and also runs along the creature’s back.  It sucks fluid in at the tail end to pass it to the head end, sort of similar to the way an aquarium pump works.

If you are a crab owner of a crustacean such as land hermit crab it is very rare that you will ever see blood on your hermit crab because any time a wound is bad enough that it does not quickly clot, the crab tends to drop that wounded limb to avoid bleeding to death.  This rare occurrence of discovering blood on crabs and their physiological differences that make that blood so much different from mammals could be the reason many think to ask this question.

How Do Crabs Breathe?

Crabs are somewhat similar to fish in that they are equipped with gills for breathing.  The gills themselves are feathery type structures located at the top of the crab’s walking legs.  They are derived from part of the jointed walking leg.  In order to breathe on land, crabs must keep these gills moist so that oxygen in the air can still be absorbed.  Gills work because oxygen is a very small molecule.  During respiration these oxygen molecules first dissolve into a layer of moisture surrounding a thin membrane.  Then these molecules, (because they are so small) cross through the membrane and enter the circulatory system (blood) of the crab. The source of oxygen can then either be used as gas in the air or already dissolved into another liquid such as the sea.

It does not necessarily matter where the oxygen originally comes from as much as that the surface of the oxygen molecules cross is wet.  Crustaceans that reside in the water do not typically have trouble when it comes to keeping their gills moist.  Terrestrial crustaceans can usually keep their gills wet by using fluids from inside of the body and by having their protective chambers well sealed so that very little moisture is lost.  Crabs such as the hermit crab use their claws and antennae to keep themselves moist by placing water onto themselves.

Because these creatures need to keep their gills moist to breathe properly, it is important that a water source of some kind always be nearby.  One of the most common reasons crabs such as the hermit crab dies in captivity is because crab owners neglect to provide them with this. The difference between marine crabs and land hermit crabs is that land hermit crabs have much smaller gills but also need air to breathe.  They must be kept moist to survive but would drown if submerged completely in water.  This is why it is also important when keeping them in captivity to provide a broken up piece of sponge in their water dish so that they have something to grasp onto to keep from losing their footing and falling into the water and/or drowning.  If the crab’s gills become too dry, the crab will die.  Another interesting fact about hermit crabs is that they carry an extra supply of water around in their shells to help keep them moist.

How Do Crabs Reproduce?

Crabs reproduce by laying eggs.  When crabs reproduce they assume the doubler position.  In this position the male crab carries the female.  Mating between the two usually lasts for at least five and a half hours however the crabs maintain this position for up to three days afterward.  The female crab then stores the male’s sperm on the underside of her abdomen where it will be used for two more spawning.  These spawning occur within her three year lifespan.

The female crabs then migrate to slightly saltier waters after mating and attach between 100,000 to 2,000,000 eggs to the sperm stored under the abdomen.  The crab incubation period lasts for about two weeks until the crab larvae hatch and is then released into the ocean to fend for themselves.  The crab larvae will continue to grow for the next 40 days until they reach the adult crab stage.

During this time of growing, before they reach adulthood the crab larvae go through several molting stages.  First they morph into what is known as the megalapore stage.  This is the second larval stage and is characterized by a larger and thicker exoskeleton than the first larval stage.  At this point the megalopae often resemble a cross between a crab and a lobster.  The megalopae then can be found migrating to shore where they will then continue the next molting stage, after which they are classified as immature crabs or “first crabs”.  This occurs about two months after the first larvae are hatched.  These “first crabs” or immature crabs usually molt at least 18 times before reaching adulthood.

Crabs such as the hermit crab have been known to live for 30+ years.  In captivity, they can even live long lives if cared for properly.  Some crab owners have reported crabs living to be the age of 16 or so.  Mating between hermit crabs occurs when a spermatophore is transferred by a male to the female as both crabs partially emerge from their shells.  Depending on the size of the female, she can lay thousands of eggs and then deposit them with the aid of the gill grooming appendages on her pleopods along the left side of her abdomen.  She then carries the eggs as they mature for about a month during which time the eggs will change from brick red to a dark gray color, as the embryos deplete their yolk supply.

The female hatches her eggs in the ocean by passing clusters of eggs from pleopods using her gill grooming appendages to her maxillipeds and forming clusters that are then passed to the tip of the claws and flung out to sea.  The crab eggs immediately burst open upon contact with salt water and the new larval hatchlings known as the zoea float amongst the plankton.  Each zoea will pass through 4 to 6 different stages that usually last anywhere from 40-60 days and follow the same metamorphosis as other crabs.  Something to keep in mind if you are a crab owner is that although hermit crabs do mate in the wild if conditions are right, they will not breed in captivity.  They only mate within their natural environments.

In very rare occurrences some crab enthusiasts that live in places like Southern Florida and have kept their pet hermit crabs in outdoor cages that are very large have been lucky enough to have several pregnant crabs from time to time.  Unfortunately the zoeas did not survive.

Are Crabs Arachnids?

Crabs are not arachnids; they are crustaceans. Arachnids are animals such as spiders.  Arachnids, insects and crustaceans all belong to the family group arthropod.  Arthropods are animals with exoskeletons, segmented bodies and jointed legs.  They are the largest group of animals on Earth. Most crustaceans such as the crab are aquatic animals living in either marine or fresh water environments.  While a few crustaceans such as terrestrial crabs and/or hermit crabs have adapted to life on land.  Other crustaceans you may be familiar with include lobsters and barnacles.

Crustaceans can be recognized by their hard, external shell which protects their body. They are equipped with a head and an abdomen.  Their heads include antennae which are a part of their sensory system.  Their abdomen includes the heart, digestive system and reproductive system as well as appendages such as legs for crawling, swimming and climbing.  Crabs love to climb.  Many crustaceans have separate sexes and also reproduce sexually while a few others are hermaphrodites.  Others have been known to change their sex during the course of their life.

Crustaceans such as the crab often have claws which help it crawl, eat, and defend itself against predators.  Crabs usually pack quite a pinch.  They burrow and can also run or walk sideways.  These creatures can often live in extreme areas due to their ability to adapt to the changing environment.  (An example of this would be the hermit crab which requires a temperature of 70+ degrees Fahrenheit and lots of moisture in order to live).

Do Crabs Feel Pain?

Virtually all animals, even crabs are able to feel pain.  In fact, Not only do crabs feel pain, they are also able to retain memory of it.  Both pain and stress are also key survival mechanisms for the crab.  There have been experiments where hermit crabs have been electrically shocked and then provided with vacant shells afterwards.  In which case they were all found to abandon their old shells and enter new ones, exhibiting stress related behaviors such as grooming of the abdomen or rapping of the abdomen against the empty shell.

Grooming for a crab can also be used as a protective motor reaction viewed as a sign of pain.  Crabs are also equipped with a central nervous system and receptors that help them learn to avoid negative stimulus after experiencing such.  Like humans if they are hurt they will limp or rub the appendages that are in pain.  Studies have also shown physiological changes that occur within the crustacean including the release of adrenal like hormones that occur when pain or stress is suspected in the creature much similar to that of a human.  Unfortunately for crustaceans such as the crab, they often become food for humans and sometimes are even boiled alive.  In which case, they would feel pain!  Perhaps this is something most of society should begin to consider.