Download Blue Crab Dissection 2016

Blue Crab Dissection
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Examine the crab’s external features. How is it different from the typical “head, thorax,
abdomen” body shape? How is it different from the crayfish?
Crossing the midline of the carapace, just posterior to the middle, is a short, shallow, transverse
groove. This is the cervical groove and it is the cross-bar of an H-shaped set of grooves. It marks
the approximate division between head and thorax. What is the cephalothorax covered by?
Why do you think it has “teeth” on it?
The antennae are next to the eyes and are very hard to see. Describe the eyes of the crab. How
are they attached to the body?
One pair of maxillipeds acts as a door over the mouth. They are the appendages of the third
thoracomere and together they resemble a pair of doors protecting the mouth field and hiding the
other mouthparts. The mandibles are the ones that are very calcified and are used to chew. The
two holes on either side of the mouth are where water exits the gill chamber. What appendages
are on the cephalothorax?
On the ventral surface locate the abdomen and its appendages are pleopods. How are the
pleopods different from the crayfish’s?
The small, triangular, terminal portion of the abdomen is the telson, which is not a true segment
or appendage. The transparent intestine runs along the ventral midline of the abdomen, under the
thin layer of exoskeleton, and ends at the anus on the telson. It may be filled with dark feces in
which case it is easier to see. Press its posterior end with a probe to get feces from the anus,
thereby confirming its position.
Begin with the pleopods, or abdominal appendages, and work your way forward through the
pereopods, maxillipeds, and mouthparts, to end with the antennae. What appendages are
missing in these crabs?
Which of the pereopods are the chelipeds?
Note the slight asymmetry of the two chelipeds. The left, or cutter cheliped, is smaller and its
teeth are a little smaller and sharper. The right, or crusher cheliped, is a bit larger and has larger
and slightly more rounded teeth. This may be reversed in some individuals and it is more
pronounced in many other crab species. What would be the advantage of these two different
types of claws?
Pereopods have a special fracture plane at their base so that the appendage can be voluntarily
autotomized or removed. Why would the crab want to do this?
How/when does it get its appendage back?
What is the sex of your specimen? Determine it with the shape of the abdomen.
Males have only two pairs of pleopods and they are located anteriorly on the abdomen. Both
function in the transfer of sperm to the female during copulation. The long, curved, tubular first
pleopod is the organ used to deliver sperm in a package called a spermatophore. The second
pleopod is much shorter and functions as a piston to push spermatophore through the hollow core
of the first pleopod.
Females have paired pleopods on their abdominal segments but they are hidden under the flexed
abdomen which must be extended to reveal them. After release from the body, the eggs attach to
the long setae of the pleopods where they are ventilated by movements of the abdomen and the
pleopods. Why would the eggs need to be ventilated?
Turn the crab so its dorsal side is up. Insert the tip of the scissors beneath the lateral, posterior
edge of the carapace and make a cut around the periphery of the carapace on its dorsal surface.
Be careful that you cut only the heavy calcified exoskeleton and not the organs beneath it. Keep
your scissors about 5 mm from the edge of the carapace and cut completely around it. Use a
knife to separate it (by scraping, not cutting) from the underlying tissues. Carefully remove the
carapace, in pieces if necessary, with minimal disturbance to the underlying tissues. The
epidermis attached to the exoskeleton has chromatophores. What are these for?
Notice two small projections on the inner surface of the carapace almost exactly in its center.
What do you think is their function in the exoskeleton?
If your specimen is a mature female, the orange ovaries may cover and obscure other structures.
The smaller, white testes of the mature male do not obscure other structures. It may be necessary
to remove the ovary (but nothing else) from one side in order to see the stomach beneath. The
stomach is a large, bulging, transparent, thin-walled sac lying dorsally on the midline in the
thorax region. The digestive ceca are large, soft, yellow or greenish organs on either side that
connect to the pyloric stomach. They may be completely obscured by the ovary in mature
females. What do you think the ceca do?
The stomach is the largest and most conspicuous part of the gut. It is an exceptionally complex
structure whose walls bear some 40 calcareous ossicles and 80 muscles. It is divided into a
large, dorsal cardiac stomach (or anterior chamber) and a smaller, ventral pyloric stomach (or
posterior chamber). The cardiac stomach is the large balloon-like structure in the anterior
thorax. It lies dorsal to the mouth to which it is connected by the short esophagus. Its walls are
made of exoskeleton and are used for:
What happens to the stomach during ecdysis?
The pyloric stomach is the much smaller ventral region of the stomach. It lies posterior and
ventral to the cardiac stomach and is hidden by it.
The large, triangular, firm, beige or greyish mass of gills are on the outside of the crab. Move
the mouthparts and observe how the crab would chew. Watch the upper branches of the
appendages. What are they doing? What would they be doing in the living crab?
Separate the gills and examine them. Where are the gills attached?
Move the legs to see if the gills move. The gills are covered by a very thin, transparent
membrane of exoskeleton. What happens to this membrane during ecdysis?
Water normally flows into the crab from the posterior and ventral side, up through the gills and
then out dorsally and anteriorly. The water flow is generated by rhythmic undulations of the gill
bailer. Reversing the beat of the bailer reverses the direction of flow over the gills. This
backflushes the gills to clean them or to respire at the surface of poorly oxygenated water. Crabs
attempting to respire out of water or in very shallow water may blow bubbles out beside their
mouth. Where are the gill bailers attached?
Move the gill bailers and gill scrapers. What are the mouthparts doing?
A tiny parasitic barnacle sometimes lives attached to the gills of several species of crabs. You
may see some of these on your crab’s gills. Why would they want to live there?
Posterior to the gills is a heavy endoskeletal plate that covers the powerful swimming muscles of
the legs. These muscles are called "backfin" crabmeat in the seafood industry.
The soft, white or gray heart lies on the midline posterior to the stomach. There are arteries
attached to the heart, but by the time the blood gets to the tissues it is not in vessels any more.
What kind of circulatory system is this?
Crab blood contains the pigment hemocyanin, which is colorless when deoxygenated. What
color is it when it is oxygenated?
Now look at the internal reproductive structures of both a male and female crab.
If you have a male crab, the two long, paired, white or grayish, testes lie dorsally in the anterior
body where they may be difficult to distinguish from the digestive ceca beneath them. The white
color is due to white spermatophores which are formed here and look like tiny white eggs. What
are spermatophores?
If you have a mature female, the orange ovaries may be small or so large they obscure the other
organs. In immature females, the ovary is beige or white and much less conspicuous. The right
and left ovaries are connected across the midline of the crab to form an "H". The ovary is
normally white but will turn orange as eggs are formed. At the end of the ovary is the seminal
receptacle. What is its function?
The excretory system consists of two soft, grayish or pale greenish-white glands located behind
the second antenna. They may be difficult to find, and the bladder is so tiny it cannot be seen.
Urine is released behind the antennae, but nitrogen is removed from it in the gills. The glands
also are osmoregulatory organs and blue crabs are tolerant of a wide range of salinities. Why
would this be important?
The nervous system is hard to see unless ethanol is applied, it will become opaque and white.
We will sacrifice one crab for this purpose. There is a brain and a large ganglion. The brain is
located dorsally in the head immediately posterior to the rostrum, between the two eyestalks, and
on the midline. It lies under a layer of muscle and connective tissue that must be removed before
it can be seen. Nerves branch out from the brain for the sensory organs and from the ganglia for
the limbs. One nerve connects the two eyestalks. Why would this be important?
After you have looked at everything else, remove the stomach and look inside at the “teeth” or
ossicles. How does the crab eat and how do these help?