yeK rewsnA
_____________________________:
emaN ruoY
snaecO gniviL eht gnirevocsiD
Nature’s One-Man Band
Lab Activity: The Blue Crab, a Representative Crustacean
The primitive ancestors of Phylum Arthropoda were probably
animals whose bodies were composed of a series of identical Segments,
each bearing one pair of swimming limbs, sort of like modern day
centipedes, but with flat, flimsy paddles in place of legs. They lived
underwater, breathed with gills, and produced a hard Exoskeleton made
of the carbohydrate chitin. This simple, segmented body plan proved to
be exceptionally adaptable, for in time the different body segments
Speleonectes, a primitive
changed, each becoming specialized for a particular function. The paired
cave-dwelling crustacean.
The earliest arthropods may
swimming limbs also specialized, transforming into all sorts of gizmos and
have looked something like
gadgets: pincers, antennae, fangs, stingers, oars, propellers, sucking
this – lots of segments, all
straws, web weavers, jaws, teeth, brushes, hooks, tweezers, egg cradles,
similar to one another, and
each with its own pair of flat
forks, spoons, knives, tongs, shovels, scoops, scrapes, probes, clubs,
paddles for swimming (from
daggers, drumsticks, musical instruments, glue guns, velcro grips,
Buchsbaum et al, Animals
Without Backbones)
strainers, baskets, combs, pogo sticks, jackhammers, syringes, stilts,
stakes, braces, plows, switchblades, sex organs, and so on. Here lies the
secret to the spectacular success of arthropods: their ability to adapt their limbs and segments
for almost any function in any niche. This has enabled them to invade and establish
themselves in virtually every habitat on earth, and they now make up more than 80% of all
animal species! Most of these belong to Class Insecta, the insects. Many others belong to
Class Arachnida, the arachnids: spiders, scorpions, ticks, and mites. Most marine arthropods,
however, are members of Subphylum Crustacea.
Krill
Copepod
Amphipod
Barnacles
Sketches from Marine Life by James Sumich
Many familiar seafood items are (were) crustaceans,
including shrimp, lobster, and crabs. Others, though, are
much more important ecologically. Flea-sized Copepods,
which are probably the most abundant animals on earth,
and Krill, the swarming inch-long shrimp that are the main
diet of baleen whales, are herbivorous zooplankton, tiny
grazers that make up the vast majority of first order
consumers in pelagic food webs. Feeding on microscopic
phytoplankton, these small crustaceans form the crucial link
to larger marine animals.
Other crustaceans include
amphipods, isopods, and ostracods. And oddly enough,
barnacles are actually a sessile, filter feeding crustacean
that builds a limy shelter around its segmented body.
Three essential traits distinguish all Arthropods: (1) a Segmented Body, (2) Jointed
Appendages (limbs), and (3) a chitinous Exoskeleton. The exoskeleton not only provides
body support and protection, but also serves as the framework to which muscles attach.
Together, the muscles and skeleton operate as a complex system of pulleys and levers, giving
arthropods a capacity for coordinated movement superior to that of any other invertebrate.
Imagine yourself inside a medieval suit of armor. Now imagine your muscles detached from
your bones and reattached to components of your armor. By tugging on the armor, your
muscles could move you just as effectively as before. But to grow, an arthropod must
periodically “molt,” shedding its old exoskeleton, swelling with water, and then producing a new
one. Many arthropods can Regenerate lost limbs with each molt, as the ensuing comic
illustrates…
Discovering the Living Oceans
Complex Invertebrates
Bloom County
by Berke Breathed
In this lab you’ll dissect the blue crab, Callinectes sapidus. The genus name means
“Beautiful Swimmer” and the species name means “Tasty.” Blue crabs are indeed delicious,
supporting the largest single-species fishery in the world. Most are steamed as “hard crabs,”
but some are kept in captivity until they molt and then served as the delicacy “softshell” crab.
Be aware that if you ever order a softshell crab sandwich at a restaurant, it will come with legs
dangling out of the bun!
Blue crabs belong to the Portunid family, the strongest, quickest, most agile swimmers of
all arthropods. They spend most of their time, however, skulking on the soft seafloor of Atlantic
estuaries from Nova Scotia to Argentina. They are a tough, opportunistic, omnivorous species,
able to tolerate salinities from ocean water to nearly fresh, and adapted to eat everything from
live prey to rotting carrion to vegetable matter.
PROCEDURE
1. Get a crab and dissecting equipment. As you gather your utensils, ponder the fact that
arthropods have invented all those same tools on their own!
2. Locate these external appendages:
(a) Two Compound Eyes (pry them forward and notice that they sit atop stalks)
(b) Two long Antennae (just inside the eyes)
(c) Two shorter Antennules (tucked away between the eyes, near the center of the head;
use a probe to extend them)
(d) Two Chelae or Chelipeds (the claws)
(e) Six Walking Legs
(f) Two Swimming Legs (or “back fins”)
Each of these appendages is a specialized tool, a gadget reshaped by evolution from an old
swimming paddle (except for the eye, which has a different embryonic origin) to serve a new
role in the crab’s niche. A basic principle of evolutionary biology is that “Form Follows
Function.” Form refers to the shape, structure, and physical design of an organism’s body
or body parts. Function refers to the role and reason for that form. To say that “Form
Follows Function” means that a body part’s function determines its shape and structure.
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Complex Invertebrates
For example, your skull is thick, hard, and bony (= form) in order to protect the brain (=
function). Function comes first, and form follows from it!
In the table below, sketch each limb, study and analyze its shape and its position on the
animal, and describe it as a case of “Form Follows Function.” You are strongly encouraged
to pluck off these limbs, especially the smaller ones, and study their sophisticated form with
a magnifying glass or binocular scope!
Sketches
“Form Follows Function”
Positioned atop a turret (with multiple lenses) for
360º vision.
Compound Eye on Stalk
Long and thin to flexibly reach out and feel/taste.
Antenna
Jointed and feathery to hoist up into the water for
smell/taste or to detect vibrations in the water,
much like a satellite dish.
Antennule
Jointed, sharp, and robust for a powerful pinch, and
toothed for a firm grip.
Chela
Jointed and stilt-like for walking in soft sediment.
Also laterally streamlined for sideways walking and
swimming.
(The hairs are sensory; they can taste w/ their feet)
Walking Leg
Flat, flexible, and robust for sculling and paddling.
Swimming Legs (“back fins”)
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Discovering the Living Oceans
Complex Invertebrates
The Far Side by Gary Larson
3. Compound Eyes are unique to Phylum Arthropoda.
Remove one and observe it beneath a binocular
scope, focusing clearly on the eye’s textured surface.
Notice the hundreds of dimples. Each of these is a
separate Lens.
Now slice the eye in half. Notice that the eye is
covered by a layer of exoskeleton, just like the rest of
the body. When the crab molts, the outer eye must
be shed, too! Peel away a piece of this outer layer
and scrape away the black matter clinging to its inner
surface. Place the exoskeleton in the well of a
depression slide, add a drop of water and a cover
slip, and study it beneath a compound microscope.
What geometric shape are the lenses?
Hexagonal
We vertebrates have a “camera eye” with a single, flexible lens. The “compound eye” of
arthropods, by contrast, has multiple lenses, each locked in place with a fixed focal length.
What advantages might this type of eye give over the camera eye? What disadvantages?
Advantages: panoramic vision; sensitive to slightest movements
Disadvantages: unable to adjust depth/distance of focus
4. A nifty trick to impress your prom date at the seafood
restaurant: Pluck a Chela (or pincer …and yes, it’s
spelled “pincer” not “pincher”) off the crab, and then use
scissors to cut out a section as diagrammed to the right.
Remove the piece of exoskeleton to expose the
underlying muscle. Resist the temptation to eat the
Cut and Remove
meat, as it’s either raw or marinated in embalming fluid.
With a probe, tease away the muscle to uncover the
thin, chitinous plate within. Now, wiggle the claw’s “thumb” a few times to loosen the joint.
Finally, grasp the internal plate with forceps and push/pull it left and right.
Explain the specific function of this plate (Hint: it’s a “tendon”). Why is it so wide?
Muscle attachment. It’s wide for maximum musculature, hence strength.
5. Study the five pairs of mouthparts. Move them around with
If available, feed a live blue
probe or forceps to get a sense of how they work together
crab and watch those crazy
to collect, manipulate, dismantle, feel, and taste their food.
mouthparts go, go, go!
Blue crabs are omnivorous, opportunistic feeders. They
prefer live prey – especially small clams and snails – yet often scavenge for dead, decaying
animals, and will even eat plant matter. They are also quite cannibalistic, with smaller crabs
making up as much as 20% of an adult’s diet!
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Discovering the Living Oceans
Complex Invertebrates
The two outermost pairs of mouthparts are the 1st and 2nd Maxillipeds. The next two pairs
(more delicate) are Maxillae, and the innermost pair (hard) are the Mandibles. Remember,
all these used to be flat, flimsy little swimming paddles, but have since been fashioned
through natural selection into utensils adapted for feeding! Based on their form, speculate
on the current function of each:
Maxillipeds:
Handling food, manipulating it, shoveling, spooning, shredding…
____________________________________________________
Maxillae:
Sorting food (edible from inedible), tasting, slicing...
____________________________________________________
Mandibles:
Crushing, grinding, chewing…
____________________________________________________
6. To the left and right of the mouth are openings into the Gill Chambers. See ‘em? _______.
7. The entire dorsal surface of the crab is covered by a single broad piece of exoskeleton, the
Carapace. Projecting sharp spines and jagged edges, it makes the blue crab nearly
impossible to swallow. On the underside, find the Genital Plate, or “apron.” Males (often
nicknamed “Jimmies”) have a slender genital plate resembling the Washington Monument.
Immature females (“Sally crabs”) have a triangular apron, like an Egyptian pyramid.
Sexually mature females (“Sooks”) bear a broader, dome-shaped apron, like the U.S.
Capitol Building, fit for carrying eggs. Is your specimen a Jimmy, Sally, or Sook?
___________________.
Lift the genital plate to expose the Genital Openings in the abdominal groove beneath.
Behind the apron of Jimmies are two brush-like appendages called Pleopods (also known
as gonopods, or “sperm legs” …yet another pair of modified swimming limbs!). Sooks and
Sallies have pleopods, too, but theirs are feathery. Crabs mate by facing each other, belly
to belly, aprons open. The Jimmy ejects sperm from the tiny genital openings under his
apron. He then inserts his two pleopods into the she-crab’s genital openings, using them to
transfer sperm into her body cavity, where a mass of eggs waits to be fertilized. Crabs
therefore reproduce through internal fertilization.
A she-crab will mate only once in her life, and only after her final molt. When crabs molt,
they are soft and vulnerable. Thus when a Jimmy meets a soon-to-molt, ready-to-mate
she-crab, he will “cradle carry” her with his legs, swimming for days, even weeks, to find a
sheltered area such as an eelgrass meadow where she can shed her exoskeleton in safety.
When at last his “wife” (as crabbers call her) molts, the Jimmy fertilizes her eggs and then
cradle-carries her for a couple more days while her new exoskeleton hardens. The
following winter she buries herself in mud. In spring, she emerges and expels her eggs
through her genital openings, and they cling to the feathery pleopods beneath her apron. At
this stage crabbers call her a “sponge crab” because of the spongy orange egg mass
bulging from her apron.
Because blue crabs are not fully adapted to estuarine life and must begin life in saltwater,
the sponge-bearing sook swims all the way to the ocean to release her eggs. The young
crabs hatch out as Zoea larvae (say “zo-EE-uh”) that drift among the plankton. Several
weeks later they metamorphose into the Megalops larval stage, before finally adopting their
familiar benthic body form and lifestyle. When megalops swarm Atlantic beaches each
summer, they nip wading tourists with their tiny claws, prompting complaints about “water
fleas”!
Sketches by Consuelo Hanks, swiped from William Warner’s Pulitzer
Prize winning book, Beautiful Swimmers
Zoeae
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Discovering the Living Oceans
Complex Invertebrates
8. If available, place a Zoea larva in the well of a depression slide and study it with a
compound microscope on LOW power. Take your time, slowly dialing the fine adjustment
knob up and down to alter your depth of focus. You should clearly see that the larva’s body
is segmented, with jointed limbs. Next, examine a Megalops under a binocular scope.
Again, take your time, alter the depth of focus, and move the specimen around to inspect
the larva’s entire body.
Notice that the Zoea is a furry, feathery fellow, with many tiny projections all over.
Megalops, too, is quite hairy and spiny. The megalops is also rather flat and leaf-like. What
might these structural adaptations accomplish for these drifting larvae?
Help it stay adrift in the water column, catching currents like a kite while avoiding sinking.
9. Observe the contrast in coloration between the crab’s underside and its back. The ventral
surface is light, while the dorsal surface (the carapace) is dark. Why? (Hint: these crabs
are portunids, often swimming up in the water column.)
The dark dorsal surface blends in with the seafloor, and while swimming,
the light underside blends in with sunlight above (countershading)
10. Lay your crab belly-down in the dissecting tray.
With scissors (blunt-tip down), cut around the rim
of the carapace (the broad “shell” on top) just
inside the spines. Gently lift off the severed piece
while peeling away the underlying tissue.
Snip and
Remove
11. Use the diagram on the next page to identify internal structures. First examine the Gills
(“dead man’s fingers”). How are they built for maximum gas exchange?
Folded and feathery for amplified surface area.
Find the Gill Bailer, the brush-like tool along the gills. Maneuver it to learn its range of
movement. Strip away the gills on one side to find a similar tool underneath. These
devices are yet another set of ancient swimming limbs, now modified into a new gadget
…for what obvious function?
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Discovering the Living Oceans
Complex Invertebrates
Cleaning the gills, and also circulating water across them
(these are actually internal extensions of the maxillipeds)
Notice that beneath the gills there is another wall of exoskeleton, which separates the gills
from the internal organs like the stomach and heart. Why are the rest of the organs housed
in a chamber separate from the gills?
Gills are “external” organs that must be in contact with seawater, which is not good for the other
organs, which are truly internal.
Diagram from Invertebrate
Zoology by Ruppert & Barnes
12. Around the base of the gills are the Gonads, which produce the gametes to be released
through the genital openings under the apron. In males the gonads – or Testes – are a pair
of coiled, zigzagging ribbons; in females, a mass of eggs, the Ovary. Extensions of the
Jimmy’s testes may also be seen as whitish or pinkish structures between the heart and
stomach. In this same area, a Sook has Sperm Storage Sacs, where her mate’s sperm
may be stockpiled for years and repeatedly used to create new “sponges” of fertilized eggs
…as many as 6 or 8 broods during her life, all from her one and only “roll in the eelgrass”!
13. Carefully strip away the thin membrane (mesentery) covering the internal body cavity. This
is the Coelom (say “sea-loam”), a sealed off chamber where internal organs are free to
move and operate. In the central dorsal area of the coelom you can locate the Heart
(roughly between the points where the tips of all the gills converge). Notice that quite unlike
our own, the heart resides in the lower back (holding a live softshell crab - one that’s just
molted - you can feel the heart beating beneath its new, paper-thin carapace). The heart
pumps oxygenated blood into a handful of arteries, which empty into open spaces within the
body (“sinuses”). After delivering oxygen to the tissues, blood passes through channels in
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Discovering the Living Oceans
Complex Invertebrates
the gills to pick up fresh oxygen, and finally returns via veins to the open space around the
heart. Blood reenters the heart through small holes called Ostia, which you should be able
to see.
Which of the two basic types of circulatory system do arthropods possess?
Open
____________________.
14. The stomach is the bag-like structure in the head region, just behind the mouth. Open it.
Can you identify any contents?
Inside is the Gastric Mill, hard toothy structures for grinding food (bet you wish you had
teeth in your stomach, eh?). Behind the stomach and beneath the gonads are the brownish
or yellowish Digestive Glands (or “hepatopancreas” …this is the soupy “mustard” inside a
steamed crab). These secrete enzymes for the chemical breakdown of food. Feces
(undigested food) follows a narrow intestine from the stomach to the anus, which you can
find at the very tip of the genital plate (the apron). Did you? __________.
15. Carefully remove the stomach and look for the Brain, a small white mass in the head with
white nerve cords exiting in various directions (it’s hard to find). A pair of major Nerve
Cords run down the animal’s ventral surface.
16. On either side of the brain is a pair of Green Glands (they aren’t green). These help to
regulate the concentration of certain salts in the blood, and they also filter out some nitrogen
wastes (ammonia-like compounds), which they expel through openings at the base of the
antennae (yes, crabs urinate from between their eyes). It is the gills, however, that excrete
the majority of cellular wastes and that primarily control osmotic balance. In fact, unlike
most crustaceans, blue crabs are good osmoregulators. Given that the blue crab is an
estuarine animal, living where the salinity is ever shifting, and given that the soft crustacean
body is encased in an inflexible exoskeleton, why is it crucial that the blue crab have an
ability to osmoregulate? (Hint: think about the effect that changing salinity has on
osmoconformers.)
If the blue crab were an osmoconformer, the ever-changing salinity in an estuary would cause its
soft body to shrink/swell against/away from the rigid exoskeleton. Shrinking and swelling is
especially traumatic if your exoskeleton can’t shrink or swell with you.
17. Here’s another trick to impress your dinner date: In the area beneath the stomach (now
removed), find a pair of tendons that connect the mandibles (“jaws”) to the mandibular
muscles. These ligaments move on a pivot. Wiggle them to make the mandibles move.
Notice that unlike our own, a crab’s jaws chomp side-to-side.
18. Gut your specimen and cut into the chambers in the ventral region of the body. Leg
muscles are rooted in each chamber. Notice that the heftiest (and most delicious!) muscle
is that of the “back fin” or Swimming Leg. Account for its large size:
The muscle is large for powerful, sustained swimming, able to hoist the entire animal off the
seafloor against the pull of gravity.
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Discovering the Living Oceans
Complex Invertebrates
In terms of the evolutionary history of crabs, why are the limbs rooted in separate
chambers? (Hint: think about the ancestral arthropod, about which you can re-read in the
lab’s opening paragraph).
This is a trace of their segmented past.
Metamerism is the condition of having segments, or the evolutionary process of developing
those segments in the first place. It was a trick by which evolution lengthened or expanded
the bodies of some primitive animals, achieving size and complexity simply by “repeating”
the body parts that were already there. After metamerism had occurred, different segments
could evolve into specialized body parts, as you have clearly seen with the blue crab. Look
at your crab’s underside and you can easily spot the vestiges of metamerism. Internally,
you see it in the repetition of the gills.
Tagmosis is sort of the opposite of metamerism. It is the process by which an animal’s
body segments may later fuse together or vanish altogether – a reduction in segmentation.
This simplifies the animal, trimming the many repeating segments into a handful of major
body regions. You can see this, too, in the crab’s single body cavity and collective gill
chambers.
But what about us Vertebrates? We seem to be quite different animals from the segmented
Arthropods and Annelids, as we lack any obvious external segmentation. Think below the
skin, though. Do we bear any internal remnants of metamerism and tagmosis? Describe
fully.
Vertebrae, ribs, phalanges, and the bones of the arms and legs reveal our segmented heritage.
Less obviously, so does our musculature. The single thorax shows later tagmosis.
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