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Marine Biology--Mr. Nelson
PHYLUM ARTHROPODA, CLASS CRUSTACEA--CRAB DISSECTION
Introduction:
Of all the phyla in the animal kingdom the Arthropoda must be considered one of the
most successful from the standpoint of diversity of species and number of individuals.
Members of the Phylum Arthropoda account for over three-fourths (about 1,000,000+) of all
animal species described. The arthropods are the only group of invertebrates to have invaded
the terrestrial and aerial environments on a highly successful scale.
One of the most distinctive characteristics of the arthropods is a jointed external
skeleton composed of a complex polysaccharide called chitin. This exoskeleton is not only
useful as a defensive structure but also serves as a site of attachment for muscles. However,
the exoskeleton does have some drawbacks. Movement of the body and appendages is limited
by thin, flexible joints between segments or sections of the exoskeleton.
To allow for growth, arthropods must shed their exoskeletons in a process known as
molting. During the molting period, the animal rids itself of its exoskeleton and proceeds to
form a new, larger exoskeleton. Molting is a vulnerable time for arthropods. The old
exoskeleton is partially digested from the inside by specialized cells in the underlying tissue.
The chitin is partially absorbed into the bloodstream and redeposited as a new exoskeleton
begins its formation within the old one. The new exoskeleton is not fully developed at this
time. This soft, pliable exoskeleton allows the animal to extricate even its finest appendages as
it casts off the old exoskeleton. However, in this state, the newly molted animal has neither a
solid surface for muscle attachment nor a suitable defense against predation. The length of
time necessary for the new exoskeleton to completely harden depends on the species but
generally takes from a few hours to a few days.
On the basis of the structure of arthropods' mouth parts, antennae, and other
appendages, some taxonomists separate the living members of this phylum into two subphyla,
the Chelicerata and the Mandibulata. The following is a synopsis of the most common living
arthropods.
Phylum Arthropoda
Subphylum Chelicerata
Class Arachnida- scorpions, spiders, ticks, mites
Subphylum Mandibulata
Class Crustacea- shrimp, lobster, crab, barnacle
Class Insecta- ants, butterflies, grasshoppers, bees
Class Chilopoda- centipedes
Class Diplopoda- millipedes
The subphylum Mandibulata is by far the most successful of all the arthropods, for this
group includes the classes Insecta, Crustacea, Chilopoda (centipedes), and Diplopoda
(millipedes). Usually, members of these classes possess segmented antennae and several pairs
of feeding appendages near the mouth. The centipedes and millipedes are of minor
significance on land, and none are marine. As numerous and abundant as insects are on land,
very few of them have adapted to a completely marine environment. In contrast, the
crustaceans are abundant in both marine and freshwater habitats. In fact, very few species of
crustaceans live on land. The most common one in North America is the pill bug, Oniscus.
Like the bodies of other arthropods, the bodies of crustaceans are segmented, with each
body segment commonly bearing a pair of appendages. All crustaceans are equipped with
two pairs of antennae, and most sport a broad shield like carapace covering the head and
thorax. Many of the smaller forms of planktonic crustaceans, including copepods,
euphausiids, and cladocerans (fig. 10.3 A, B, C) have thin exoskeletons with hairy or feathery
appendages for feeding and for flotation. The larger and heavier crabs (fig. 10.3D), shrimps,
and lobsters find suitable niches on the seafloor. Generally, barnacles (fig. 10.3E) also make
their home on the seafloor, attaching to rocks and other solid objects. A few are planktonic,
attaching to floating objects, and some are commensal, hitchhiking on large animals, such as
whales. The Class Crustacea is composed of about 26,000 species. Obvious and well-known
crustaceans include shrimps, crabs, and lobsters. Other smaller, less obvious forms are a major
component of the zooplankton.
Procedure:
1. Earlier, during the 3rd quarter, we saw organisms that looked like A, B, and C, from the
diagram above. Are these organisms zooplankton or phytoplankton? Explain.
A. External Anatomy of a Crustacean
The Atlantic blue crab, Callinectes , will be used for dissection and study of crustacean
anatomy. This decapod crustacean exhibits many features common to all other crustaceans.
Examine the external features of preserved crabs using figure 10.4 as a guide.
2. The crabs and lobsters have 10 walking legs, hence the order name Decapoda (deca, ten;
podus, foot). Some species with reduced or vestigial legs may appear to have fewer. All the
legs are basically alike, but each is modified for a specific function, and although it may no
longer be used for walking, it is still considered to be a leg. Carefully remove the five legs, at
their base, from one side of the body and examine them.
On the Crab Dissection Lab Drawings had copy sheet, draw each leg in Table 1,
and from their shape/structure attempt to deduce their function. Remember, in
Biology, structure is related to function!!!
3. Are any appendages missing from your crab? Crustaceans have the ability
to regenerate missing parts with subsequent molts. Do any appendages show
signs of regeneration?
4. At the joint, remove the chela from the rest of the leg. Have Mr. Nelson show you the
example he has and with a pair of scissors, start at the severed joint and cut along the dorsal
and ventral surface of the chela to about 10 mm behind the hinge of the pincer. Next make a
cut across the inside of the chela (about 10 mm behind the hinge of the pincer) connecting the
dorsal and ventral incisions, and remove the loosened piece of exoskeleton. Carefully, tease
away the underlying muscles and expose the two central ligament-like chitinous plates (whiteish in appearance). The dorsal plate is very narrow and relatively fragile, whereas the ventral
plate is large and broad. Invite Mr. Nelson to your lab bench to talk about how muscles are
engineered.
5. Based on your discussion with Mr. Nelson and your observations in #4,
what is the function of the dorsal plate?
6. Based on your observations in #4, what is the function of the ventral plate?
7. Explain why the ventral plate larger than the dorsal plate?
8. Now examine the head region. The most anterior appendages are two pairs of antennae
that are sensitive to underwater sounds and chemicals. Behind the antennae is a pair of
stalked compound eyes. Remove one eye stalk and examine it under a dissecting microscope.
You should be able to see the many lenses that make up the eye. With a scalpel, slice off the
end cap of the lenses from the eye, and clean out some of the inside material with the tip of one
of the forceps. Place the lenses on a microscope slide and observe them, using compound
microscope. The surface of the eye will be covered with numerous clear corneal lenses. These
lenses have a definite shape, interlocking with adjoining lenses.
On the Crab Dissection Lab Drawings hard copy sheet, sketch the lenses, using medium
power, in the space below (pay attention to the geometry of each lense).
9. Each lens, its crystalline cone, and the associated retina cells are called an ommatidium.
The compound eyes of both insects and crustaceans are made of numerous ommatidia and
give these animals exceptional ability to detect movement. This feature, together with the
curvature of the eye's surface, enables these animals to readily detect movement in a very wide
field of vision. The construction of the lenses is such that adjustment of focal length is
impossible. Therefore, the animals do not have the ability to focus on objects at varying
distances. Color vision is known to exist in a wide variety of insects and crustaceans, and the
eyes of crustaceans living in the very dim light found deep within the ocean are able to
concentrate what light does reach their habitat to be able to see.
Obviously, the human eye is not constructed so that we can see objects in all directions
at once, but our eye does have advantages over the compound eye found in crustaceans and
insects.
10. What are those advantages our eyes has over the compound eye?
(It might help to look at the scissor pivot screw at the center in fig. l0.5A with a dissecting
microscope. Mr. Nelson has placed a figure 10.5A on each lab bench)
DID YOU KNOW? The corneal lenses have an exoskeleton origin and as a consequence are
replaced when the animal molts its exoskeleton.
11. Posterior and ventral to the eyes are five pairs of small appendages that function to collect
and break up food and pass it to the mouth. Examine these appendages. The first three pairs
are maxillipeds. They have a multitude of functions, some external, some internal. They are
hinged in the middle so that when the animal performs a particular function externally, such
as manipulating food toward the mouth, it can simultaneously perform another task
internally. The next two pairs are the maxillae. Like the maxillipeds, the maxillae have a
variety of functions. They may taste food as well as manipulate it toward the mouth. The pair
of mouth parts immediately surrounding the mouth are the mandibles. These function as
teeth and also have an internal role to play in aiding the movement of the stomach to help
grind up the food once it is swallowed.
Touch them with a probe, they should be hard. Why?
12. Mandibles are “crushing jaws”! Politely ask Mr. Nelson to come
over to your group and enthusiastically point out the mandibles to
him. He will make a special symbol below.
14. The dorsal surface of the crab's cephalothorax is covered by a single, broad exoskeleton
structure called the carapace. The border is fringed by spines and serrations. The posterior
end of the ventral side of the crab is covered by the reduced abdominal segments. These
segments form the genital plate, which covers and protects the genital openings. The genital
plate of the female is wider and U-shaped compared to the narrow and V-shaped genital plate
of the male. Mr. Nelson has made sure that there a both sexes in the class. Use your text book
and the other crabs in the class;
Is your crab a male or female?
Using another group’s crab of the opposite sex, draw the genital plates of both
a male and female crab on your Crab Dissection Lab Drawings hard copy.
Crab Reproduction:
The female extends her hinged abdomen, exposing two genital pores known as
gonopores. The female gonopores are large triangular openings in the sterna of the sixth
thoracic segment, in line with the third pair of legs (see female anatomy for more information).
The male inserts his gonopods (see next paragraph) into the genital pores and transfers
seminal fluid which contains microscopic packets of sperm, called spermatophores, to the
female. Each spermatophore packet contains several thousand sperm cells. Copulation will last
from 5 to 12 hours.
Males have two pairs of pleopods, located towards the front of the abdomen. Both
function in the transfer of spermatophores to the female during copulation. The long, curved,
tubular first pleopod is the gonopod. The gonopod, not the penis, is the intromittent organ
used to deliver spermatophores to the female gonopore (the penis fits into a groove on the
gonopod to which it delivers sperm). The second pleopod is much shorter and functions as a
piston to push spermatophores through the hollow core of the gonopod (see male anatomy for
more information).
The sperm packets are stored inside the female in special receptacles, or sacs, known as
spermathecae, which lie just inside the gonopore. These sperm are believed to be viable for as
long as the female is alive. Although a female will mate only once, she may produce many
fertilized egg masses during her lifetime from this single mating. Fertilization occurs each time
a new egg mass is produced by the ovaries until the sperm reserves are depleted. Studies in
Florida found that some female crabs produce as many as seven broods (sponges) in one year
from a single mating, and up to 18 broods over 2–2½ years. Chesapeake Bay female crabs are
capable of producing multiple egg masses over several years, though most will not produce
more than one or two masses due to their short average life span, typically 1–2 years.
The amount of sperm that a male crab transfers to a female during mating depends on both the
size of the male crab and its mating history. Large males can product larger amounts of sperm
than their smaller counterparts. Regardless of their size, males that mate frequently will
transfer less sperm to each individual female than males that mate less often. A male can fully
recharge his sperm stores in about 10 - 20 days. For the females, larger size at maturity can
result in larger egg masses that yield more larvae. Conserving healthy numbers of females and
large males in the Chesapeake Bay is important to protecting the overall reproductive
potential of the entire blue crab population. http://www.bluecrab.info/mating.html
As the eggs from the female are extruded through her oviduct openings, they become
fertilized and then are attached to the feather like structures called pleopods located on the
underside of her abdomen. The eggs normally stay attached to these pleopods until the early
larval stages are completed. Eventually the eggs hatch, and the free- swimming larval forms,
called zoea, are released as plankton. These zoea go through several molts as plankton until
they are ready to assume a benthic role. The final larval stage is that of the megalops. The
megalops form looks much like the adult except that its abdomen protrudes and it has
swimming appendages located on the underside of the abdomen. These are lost as soon as the
next molt occurs and the crab now assumes a benthic life-style.
Have Mr. Nelson come
over to your group and enthusiastically point out the gonopods, if
you have a male, or the gonopores, if you have a female and he will
make a special symbol below.
16. Gently lift up the genital plate (abdomen) of your crab.
B. Internal Anatomy of a Crustacean
1. Using strong scissors, cut along the top edge of the carapace and remove it. Be careful to
tease away the underlying tissue from the exoskeleton. This soft tissue, which becomes
strengthened by newly deposited chitin after molting, will give you some idea of how soft a
newly molted crustacean is.
2. Find and cut open the stomach and examine the gastric mill (hard teeth like structures)
located inside the stomach. The gastric mill and the lining of the esophagus, as well as the
lining of the final portion of the digestive tract, are all connected to the chitinous exoskeleton
and are discarded during molting.
Suggest
a function for the gastric mill (remember--structure relates to
function).
3. Move the mandibles with your forceps, and look for movement inside the body cavity. To
what structure do the mandibles attach internally? What is the benefit of this
arrangement to the mechanical digestion of the crabs food?
4. Find the gills. In many planktonic species of crustaceans, the gills are exposed to the
outside and are attached to the basal portion of their swimming legs. In benthic forms, they
are usually enclosed within the protection of the body. You may have to remove a little more
of the exoskeleton. Are they still attached to the basal portion of their legs even
though they are enclosed within the exoskeleton?
4a. When the legs are moving the crab, what is the benefit of this arrangement
to the respiration process?
5. On one side of your crab, gently remove all the gills at their base. Use the charts in your
folder to find the Third Maxilliped appendage. While looking at the gill chamber (with
removed gills) move the third maxilliped back and forth.
What do you observe in the gill chamber?
5a. How is this engineering design beneficial to respiration?
6. The gills are contained in a chamber separate from the other internal organs. They are
covered with a very fragile clear membrane that may break off in preserved specimens when
the carapace is removed. This membrane functions to isolate the gills and their incurrent
water from the rest of the organs in the body cavity.
Why do you think this is important to the health of the animal?
OBSERVATIONS AND ANALYSES
1. Name and briefly describe one advantage and one disadvantage of an exoskeleton?
A-
D-
2. What body parts does the crab use to ingest food?
3. How is the crab adapted for different types of locomotion?
4. How does the crab sense its environment? What are some of the different structures
involved?