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Blue Crab
Background:
Crabs belong to a group of animals called Crustacea, which also includes lobsters and shrimp. They are
decapods, meaning they have ten legs. There are more than 50 families of crabs and thousands of species
worldwide, including blue crabs, shore crabs, hermit crabs, and spider crabs, to name a few. They have
excellent eyesight and certain species can detect movement from as far as 20-30 yards away. Crabs can hear and
make different kinds of sounds which are used in courtship or to intimidate a competitor.
Habitat:
Only a small number of species live in freshwater. Marine species live in all oceans, and may live anywhere
from the shoreline to the very deep parts of the sea. Some crabs can tolerate wide ranges of temperature and
salinity, but others need more specific parameters.
Molting:
When walking along the shore, you have probably seen crab shells washed up on the beach. A crab's hard
exoskeleton doesn't actually grow, but a soft shell grows inside it, and when the crab gets too big, the
exoskeleton must be shed. This process is called molting or ecdysis (ek`-da-sis). In preparation for growing the
new shell, the crab absorbs the calcium from the old shell into its blood, so that it can be used for the new shell.
The old, hard shell cracks, and the crab, now soft and wrinkled, frees itself. At this point, the crab absorbs large
amounts of water and expands to its new size. While the new shell is hardening, the crab is vulnerable to
predators, so it tries to stay hidden, and sometimes will not eat for several days. Molting frequency decreases
with a crab's age, and some crabs may molt up to twenty times in a lifetime. The exoskeleton is made of chitin.
Reproduction:
Mating usually takes place between a newly-molted, soft-shelled mature female and a larger male with a hard
shell. The male carries the female around before she molts, and may continue to do so after mating. This
protective behavior not only helps the female, but guards the male's genetic investment, and decreases chances
of his being displaced by another male. Females mate once and spawn twice, using the sperm from the single
mating for both spawns. Males deposit sperm in the seminal receptacle of the female. Eggs are fertilized as
they move through the oviduct to the exterior. When the eggs are released (spawn), they attach to the setae of
the pleopods where they develop. When the eggs hatch, usually in the warmer months, free-swimming larvae
spend several months as plankton before they go through a number of molting stages, eventually settling down
on the sea bed.
Feeding:
Crabs are not fussy eaters and will dine on worms, mollusks, other small crabs, algae, decaying fish, or anything
else they can catch. The crab has a hearty appetite, and some adults may eat forty half-inch clams per day, and
may even eat crabs their own size. It finds food by using smell detectors on its antennae, and other detectors on
its legs that tell the crab when it makes contact with a food source.
Crab Anatomy Glossary
EXTERNAL FEATURES
Antenna (pl. antennae)
The long segmented appendages located behind the eyestalks. These allow the crab to interact
with its environment by touch and chemoreception.
Antennule (pl. antennules)
Shorter segmented appendages located between and below the eyestalks, sensory organs; these
also use chemoreception to "smell" and "taste".
Appendages/ Pereon
Ten legs (five pairs) including a claw-bearing pair with spines used for feeding and defense,
followed by three pairs of sharply pointed walking legs, and a pair modified as flat swimming
paddles at the rear, swimming legs.
Apron
Abdomen of a crab, which is folded under the body; male's is shaped like the Washington
Monument or an inverted Y. An immature female's is triangular (pyramid shaped) and mature
female's is semicircular, like the dome of the capitol building.
Carapace
The shell covering the body that provides rigidity and protective covering. It is made of chitin
and hardened by calcium carbonate and is the part of exoskeleton (hard outer covering) that
covers the head and thorax (center) of the crab.
Cheliped (see appendages)
The first pair of legs, carries the large claw which is used for defense and obtaining food. Male's
claws are blue tipped with red; female's are red.
Eyes
Visual organs mounted on the ends of eyestalks. The eyestalks contain cells that release
hormones that inhibit molting.
Lateral spines
Paired points on the widest outside edges of the carapace.
Mouth
Opening to the digestive system, located between the antennae. The mouth contains jaws that
hold and push food into the esophagus.
Swimmerets (pleopods)
Paired abdominal appendages under the apron of the female crab on which the eggs are carried
until they hatch.
Walking legs (see appendages)
Used for movement; crabs are capable of walking forward or diagonally, but usually they walk
sideways.
INTERNAL FEATURES
Gills
Place of respiration and filtration, consisting of many plume-like filaments arranged around a
central axis. There are eight gills on each side of a blue crab's body.
Heart
The pump of the circulatory system. It is broad in size and located in the lower center part of the
body.
Hepatopancreas (midgut gland)
Extremely large organ with several functions, including the secretion of digestive enzymes and
absorption and storage of digested food. It fills most of the area around the stomach, depending
on its contents of food reserves and water.
Intestine
Portion of the digestive system through which digested food passes.
Stomach
The organ of the digestive system that breaks down swallowed particles of food. It is lined with
small hard plates and projections which aid digestion.
Testes
Part of the male reproductive system, located on top of the hepatopancreas on either side of the
stomach.
Cartilage - Encases muscles that aid in movement of the legs. The muscles are the edible portion of the crab.
Male Crab
Female Crab
Blue Crab Dissection
Objectives: Students will be able to:
 Identify the crab’s major external structures
 Identify the crab’s major internal organs
 Describe how crabs respire, feed, reproduce
Procedure: Identify the following:
External Anatomy:
 Abdomen- male or female?
 Carapace
o Lateral spines
 Rostrum
 Eyestalks
 Antennae
 Antennule
 Pereon: Cheliped
o Pereopods
o Swimming leg
 Maxillipeds (3 pairs)
 Maxilla (2 pairs)
 Mandible
 Mouth
 Inhalant aperature
 Sternite
 Gonopod (1st pleopod) and 2nd Pleopods (in males)
 Telson
 Anus
Internal Anatomy:
 Epidermis
 Hemocoel
 Stomach
 Gills
 Gill cleaner
 Heart
 Stomach
 Hepatopancreas
 Intestine
Blue Crab Dissection Questions- answer on a separate sheet of paper
1. What is the meaning of “arthropod?” What are 4 distinguishing characteristics of this phylum?
2. Explain how the body structure of crabs different than other arthropods.
3. Explain the process of molting and why is it important for crabs.
4. Explain the process of reproduction for crabs.
5. Explain the function of Antennae and Antennule.
6. Explain the difference in the shape of the abdomen for a male and female crab.
7. Describe the number, shape, and function of cheliped, pereopod, and pleopod.
8. The exoskeleton of crabs is made up of which two materials? What other organisms had these materials
and where in the body was it found?
9. Explain how crabs respire.
10. What is “autotomize?” How is it important adaptation of crabs?
Blue Crab Dissection Questions- answer on a separate sheet of paper
1. What is the meaning of “arthropod?” What are 4 distinguishing characteristics of this phylum?
2. Explain how the body structure of crabs different than other arthropods.
3. Explain the process of molting and why is it important for crabs.
4. Explain the process of reproduction for crabs.
5. Explain the function of Antennae and Antennule.
6. Explain the difference in the shape of the abdomen for a male and female crab.
7. Describe the number, shape, and function of cheliped, pereopod, and pleopod.
8. The exoskeleton of crabs is made up of which two materials? What other organisms had these materials
and where in the body was it found?
9. Explain how crabs respire.
10. What is “autotomize?” How is it important adaptation of crabs?