Invertebrate Anatomy OnLine
Procambarus
©
Crayfish
27jun2006
Copyright 2001 by
Richard Fox
Lander University
Preface
This
is one of many exercises available from
Invertebrate Anatomy OnLine
,
an Internet laboratory manual for courses in Invertebrate
Zoology. Additional exercises can be accessed by clicking on the links to the left. A
glossary and chapters on supplies and laboratory techniques are also available. Terminology
and phylogeny used in these exercises correspond to usage in the Invertebrate Zoology textbook by
Ruppert, Fox, and Barnes (2004). Hyphenated figure callouts refer to figures in the
textbook. Callouts that are not hyphenated refer to figures embedded in the exercise. The
glossary includes terms from this textbook as well as the laboratory exercises.
Systematics
Arthropoda
P, Mandibulata, Crustacea
sP, Eucrustacea, Thoracopoda, Phyllopodomorpha, Ostraca, Malacostraca
C, Eumalacostraca, Caridoida, Decapoda
O, Dendrobranchiata
sO, Astacidea
iO, Astacoidea
SF, Cambaridae
F, (Fig 16-15, 19-67, 19-90)
Arthropoda
P
Arthropoda, by far the largest
and most diverse animal taxon, includes chelicerates, insects, myriapods, and crustaceans as well
as many extinct taxa such as Trilobitomorpha. The segmented body primitively bears a pair of
jointed appendages on each segment. The epidermis secretes a complex cuticular exoskeleton
which must be molted to permit increase in size. Extant arthropods exhibit regional
specialization in the structure and function of segments and appendages but the ancestor probably
had similar appendages on all segments. The body is typically divided into a head and trunk, of
which the trunk is often further divided into thorax and abdomen.
The gut consists of foregut,
midgut, and hindgut and extends the length of the body from anterior mouth to posterior
anus. Foregut and hindgut are epidermal invaginations, being derived from the embryonic
stomodeum and proctodeum respectively, and are lined by cuticle, as are all epidermal surfaces of
arthropods. The midgut is endodermal and is responsible for most enzyme secretion, hydrolysis,
and absorption.
The coelom is reduced to small
spaces associated with the gonads and kidney. The functional body cavity is a spacious
hemocoel divided by a horizontal diaphragm into a dorsal pericardial sinus and a much larger
perivisceral sinus. Sometimes there is a small ventral perineural sinus surrounding the
ventral nerve cord.
The hemal system includes a
dorsal, contractile, tubular, ostiate heart that pumps blood to the hemocoel. Excretory organs
vary with taxon and include Malpighian tubules, saccate nephridia, and
nephrocytes. Respiratory organs also vary with taxon and include many types of gills, book
lungs, and tracheae.
The nervous system consists of a
dorsal, anterior brain of two or three pairs of ganglia, circumenteric connectives, and a paired
ventral nerve cord with segmental ganglia and segmental peripheral nerves. Various degrees of
condensation and cephalization are found in different taxa.
Development is derived with
centrolecithal eggs and superficial cleavage. There is frequently a larva although development
is direct in many. Juveniles pass through a series of instars separated by molts until reaching the
adult size and reproductive condition. At this time molting and growth may cease or continue,
depending on taxon.
Mandibulata
Mandibulata is the sister
taxon of Chelicerata and in contrast has antennae on the first head segment, mandibles on the
third, and maxillae on the fourth. The brain is a syncerebrum with three pairs of ganglia
rather than the two of chelicerates. The ancestral mandibulate probably had biramous appendages and
a J-shaped gut, posterior-facing mouth, and a ventral food groove. The two highest level
mandibulate taxa are Crustacea and Tracheata.
Crustacea
sP
Crustacea is the sister
taxon of Tracheata and is different in having antennae on the second head segment resulting in a
total of 2 pairs, which is unique. The original crustacean appendages were biramous but
uniramous limbs are common in derived taxa. The original tagmata were head but this has been
replaced by head, thorax, and abdomen or cephalothorax and abdomen in many taxa. Excretion is via
one, sometimes two, pairs of saccate nephridia and respiration is accomplished by a wide variety of
gills, sometimes by the body surface. The nauplius is the earliest hatching stage and the naupliar
eye consists of three or four median ocelli.
Eucrustacea
Eucrustacea includes all
Recent crustaceans except the remipedes. The taxon is characterized by a primary tagmosis
consisting of heat, thorax, and abdomen although the derived condition of cephalothorax and abdomen
is more common. Eight is the maximum number of thoracic segments.
Thoracopoda
In the ancestral
thoracopod the thoracic appendages were turgor appendages used for suspension feeding in
conjunction with a ventral food groove. Such appendages and feeding persist in several Recent taxa
but have been modified in many others.
Phyllopodomorpha
The compound eyes are
stalked primitively although derived sessile eyes occur in many taxa.
Malacostraca
C
Malacostraca includes most
of the large and familiar crustaceans such as crabs, shrimps, lobsters, crayfish, isopods, and
amphipods. Primitively the trunk consists of 15 segments, eight in the thorax and seven in the
abdomen but in most Recent species the abdomen has only six segments (Fig 19-19). The female
gonopore is on the eighth thoracic segment and the male on the sixth.
Decapoda
O
The largest and most
familiar crustaceans belong to Decapoda. The 10,000 species of crabs, shrimps, crayfishes,
lobsters, and their relatives are decapods. The first three segments of the decapod thorax are
fused with the head to form a cephalothorax and their appendages are maxillipeds. The remaining
five pairs of thoracic appendages bear simple or chelate walking legs. The resulting ten legs
accounts for the name “decapod”. A large carapace extends posteriorly from the head and is
fused dorsally with all eight thoracic segments. Laterally the overhang of the carapace
encloses the branchial chamber with the gills. The most primitive decapods (shrimps, lobsters,
and crayfishes) have well developed abdomens whereas the most derived taxa (true crabs in
Brachyura) have reduced, almost vestigial, abdomens (Fig 19-24).
Laboratory Specimens
Astacidea
iO includes the clawed lobsters and the freshwater crayfishes. This exercise can be used
with any crayfish or clawed lobster. Preserved crayfishes are provided by biological supply
companies without indication of identity but most North American species belong to
Procambarus (115 taxa),
Cambarus (82 taxa), or
Orconectes (76 taxa).
Pacifastacus has eight species, restricted to Pacific coast watersheds and Missouri River
headwaters. The study should be conducted on the stage of a dissecting microscope. Living
specimens should be anesthetized with chloroform-saturated water.
External Anatomy
Place a living or preserved crayfish in a dissecting pan of appropriate size and take it to your
bench for study. Crayfish have bodies similar to that of the presumed ancestral
crustacean. Such a body is essentially shrimplike in that it is elongate and nearly
cylindrical in cross section. The abdomen is well developed, its segmentation is readily
apparent, and appendages are present on every segment. Note the
bilateral symmetry of the animal and find anterior, posterior, dorsal, ventral,
right, and left.
Exoskeleton
The body is covered by a hard, jointed, nonliving cuticle, or
exoskeleton secreted by the underlying epidermis. In general, the body wall
consists of little more than the exoskeleton and the epidermis beneath it because an animal with a
rigid exoskeleton has little need for any additional body wall. The original body wall muscles
have become specialized individual muscles and no longer form continuous layers in the body wall as
they do in the wormlike ancestors of the arthropods. Continuous layers of circular and
longitudinal muscles would be inefficient under a solid and immovable exoskeleton. The
exoskeleton provides strength, structural support, and protection so connective tissue is not
needed. The body cavity is a hemocoel, not a coelom, so there is no peritoneum. Only the
epidermis is essential and must be retained because it secretes the exoskeleton.
The exoskeleton is molted periodically to allow the animal to increase in size. A complicated
musculature, derived from the circular and longitudinal muscles of the ancestors, originates and
inserts on the inner surfaces of the exoskeleton and moves its many parts. The exoskeleton
between adjacent regions is thin and flexible to permit motion. A complicated "endoskeleton"
composed of internal processes, called apodemes, extends internally from the inner surface of the
exoskeleton. The gills and the anterior and posterior regions of the gut are covered with
epidermis and a thin exoskeleton.
Many parts of the exoskeleton bear small, articulated, movable bristles called
setae. These can be seen over most of the body. Thicker articulated
processes from the exoskeleton are
spines. Simple outgrowths of the exoskeleton that are not articulated are
usually called
teeth.
Tagmata
The
body is composed of a linear series of
segments, or somites. The malacostracan body consists of 19 segments but you
cannot see or count them all. Each one, however, bears a pair of jointed appendages, which
are visible and countable. In the ancestral crustacean all segments were identical,
or nearly so (homonomous), as were their appendages. In derived crustaceans the segments and
their appendages are specialized for various purposes and for the most part no longer resemble each
other closely (heteronomous).
Groups of adjacent segments and their appendages tend to have similar functions and together
accomplish certain specialized tasks. This results in a regionalization of the body into
tagmata. In crustaceans there were originally three tagmata; the head,
thorax, and abdomen. The crustacean head is always composed of five segments but the thorax
and abdomen are variable. Within Malacostraca there is no variability in segment number and
there are always eight segments in the thorax and (almost) always six in the abdomen.
Cephalothorax
It is common in crustaceans for the head to fuse with some anterior thoracic segments to form a new
tagma, the cephalothorax. The appendages of these thoracic segments are modified to serve as
mouthparts and are known as maxillipeds. The remaining unaffected thoracic segments form
another new tagma, the pereon. Such secondary tagmosis occurs in Decapoda where the cephalothorax
consists of the five head segments and the first three thoracic segments and bears five pairs of
head appendages and three pairs of maxillipeds. The remaining five thoracic segments are the
pereon and their appendages are pereopods. Most authors use different rules for decapods and
refer to the head and entire thorax as the cephalothorax even though only three appendages are
fused with the head. That custom is not followed here.
In Malacostraca the cephalothorax and most of the pereon is covered dorsally and laterally, but not
ventrally, by a double sheet of exoskeleton called the
carapace (Fig 1, 19-19). Although you cannot tell it by looking at the
animal, the carapace is an outgrowth of the exoskeleton of the last head segment. It grows
posteriorly to cover and protect the thoracic segments. It is a fold of the body wall and as
such consists of two complete layers of the wall (Fig 8). The outer wall of the carapace is
sclerotized and covered by a thick exoskeleton and is hard and strong but the inner wall has only a
thin exoskeleton and is transparent and flexible.
The segments of the cephalothorax are fused together and cannot be told apart but those of the
pereon remain independent and are not fused together, even though dorsally and laterally they
appear to be. The carapace that covers them is not segmented but it, remember, is not part of
the pereon. Under the carapace, the pereon is segmented. The segmentation of the thorax
is apparent ventrally where it is not covered by carapace. No such segmentation can be seen in
any view of the cephalothorax. A conspicuous transverse dorsal groove divides the carapace
into an anterior 1/3 and posterior 2/3. This is the
cervical groove and it marks the boundary between head and thorax.
Figure 1. A dorsal view of an American lobster,
Homarus americanus. Adapted from Herrick, 1909.
Anteriorly the
carapace bears a conspicuous anterior, median, pointed process called the
rostrum. The
orbits are a pair of semicircular notches, or sinuses, in the carapace lateral to
the base of the rostrum. Each orbit contains an
eyestalk with a
compound eye at its distal end. The black, multifaceted
cornea of the eye covers almost of the entire circumference of the
stalk.
Pereon
The thorax is composed of eight segments, called
thoracomeres, and as we have seen, all eight are hidden beneath the
carapace. Each thoracic segment bears a pair of appendages, or
thoracopods. The anterior thorax, consisting of three segments, is fused with
the head to form the cephalothorax. The posterior five segments remain independent of each
other and of the head. The posterior thorax, composed of these five segments, is the
pereon and its segments are
pereomeres. The pereon is not part of the cephalothorax even though it is
covered by the carapace. Do not confuse the carapace with the cephalothorax. They are not
the same thing. The carapace is a fold of the body wall which, in decapods, covers the
cephalothorax and pereon.
Abdomen
The
abdomen of primitive decapods is well developed with clearly visible segments and
powerful longitudinal muscles. It is this abdominal musculature that is primarily responsible
for the culinary popularity of shrimp, lobsters, and crayfish. The abdomen is also known as
the
pleon and its segments are
pleomeres. Count the abdominal segments. There are six of them, all are
clearly visible, with none fused with another or with the thorax. The posterior end of the
body is not a segment. It is the
telson. If you counted it as a segment, you came up with the wrong
number. The
anus is located on the ventral side of the telson.
The exoskeleton of the abdominal segments of the crayfish approximates the typical ancestral
condition. Primitively, each body segment is enclosed in four articulated exoskeletal plates,
or
sclerites, that form a complete ring around the segment. Dorsally is the
tergite, ventrally the
sternite, and laterally there are two
pleurites, one on each side.
In astacideans (lobsters and crayfish) the tergite and pleurites are fused together to form a hard
arch of exoskeleton covering the dorsal and lateral aspects of the segment. On segments 2-6
the pleurites extend ventrally past the body as side plates, or
epimera, which together form a shallow ventral channel below the body of the
abdomen.
The sternites cover most of the ventral surface of the abdomen but the pleurites cover its lateral
parts. For the most part sternites are thinner and more flexible than tergites and
pleurites. They are transparent and, in living specimens, the abdominal musculature and nerve
cord can be seen through them. The posterior margin of each sternite, however, is thick and
heavy and forms a reinforcing arch across the venter from one pleurite to the other. The
appendages articulate with the pleurites at the ends of this sternal arch.
Appendages
Decapod appendages are easiest to study by beginning at the posterior end and working
forward. As you do this, keep in mind that they are numbered in the opposite direction, from
anterior to posterior.
Before beginning your study of crayfish appendages it might be a good idea to review the morphology
of a typical crustacean appendage. Arthropod appendages are paired, with one pair per
segment. Each appendage is a linear series of articles joined by flexible
articulations. Appendages may be biramous with two branches, or uniramous, with only one
branch.
A
biramous appendage has a basal article attached by its proximal end to the
body. From its distal end arise two
rami, or branches (Fig 2, 19-3B, 19-4A). The basal article is the
protopod. Often the protopod is divided into two articles, the coxa and
basis. The two rami are an outer, or lateral,
exopod and an inner, or medial,
endopod. The two rami may be composed of any number of articles depending on
what they are specialized to accomplish. They may be similar to each other or
different. If only one ramus is present and the appendage is said to be
uniramous (Fig 3). Sometimes additional branches of the protopod or rami are
present. Any additional branch on the lateral side of the appendage is an
exite and any extra medial branch is an
endite. Finally, an exite on the base of the appendage is given the special
name of
epipod.
Abdominal Appendages
Uropods
Study the abdominal appendages of your specimen but do not remove them. Each of the six abdominal
segments bears a pair of appendages. Most of these are
biramous. The last (posteriormost) pair of abdominal appendages, located on
abdominal segment 6, are
uropods (Fig 3, 19.2B).
Figure 2. A lobster (Homarus americanus) uropod. Redrawn from Herrick (1909).
The uropods have a relatively small protopod and two large, flat rami. The
exopod is biarticulate (has two articles). The distal border of each ramus
bears a fringe of setae. Spread the rami of the two uropods apart and array them beside the
telson (the telson is neither an appendage nor a segment). The four rami plus
the telson make up the
tail fan, which functions as a large paddle. With the fan deployed, flexure
of the powerful abdominal muscles moves the fan rapidly forward under the body and results in the
generation of a forward jet of water that propels the animal backwards.
Pleopods
The remaining five pairs of abdominal appendages are
pleopods 1-5 (counting from anterior to posterior). Pairs 2-5 are biramous
and are similar to each other. They are narrow and whiplike, although not very long (Fig 19-12B).
The pleopods of females are better developed than those of males and are used to carry the eggs,
which are glued to the fringe of setae around the rami.
The first pleopods of males are uniramous and are modified to serve as intromittent organs to
transfer spermatozoa to the female. In males the first pleopods are uniramous and are referred
to as
gonopods. Adult male crayfish can be either first form or second
form. The gonopods of first form males are sclerotized and hard and suitable for
intromission. Those of second form males are unsclerotized and soft and cannot transfer sperm
to the female. The first form gonopods have a species specific shape that fits like a key into a
seminal receptacle, the annulus ventralis, with a corresponding shape, like a lock, on the female
venter. If you have a male determine if it is first form or second form.
Using your own specimen if you have a first form male or a borrowed one from another student,
identify the specimen to genus using the sculpturing of the 1
st pleopod. In the genus
Procambarus the male first pleopod usually terminates in more than two processes. In
the genus
Cambarus, the first form male first pleopod has two or less terminal processes and they
are bent at right angles to the shaft of the pleopod. In the genus
Orconectes the first form male first pleopod has two or fewer processes and they are not
bent. They arise at the end of the shaft or from its posterior side and are parallel to the
shaft or slightly curved.
Pereon and Pereopods
Each thoracic segment bears a pair of appendages but just as there are two distinctly different
regions of the thorax, there are two distinctly different types of thoracic appendages. The
appendages of the anterior three thoracic segments are
maxillipeds, are part of the cephalothorax, and function as auxiliary
mouthparts. The appendages of the posterior five thoracic segments (pereomeres) are
pereopods and function as walking legs or pincers. They are part of the
pereon.
The five segments of the pereon bear a total of 10 appendages which accounts for the name Decapoda
(= 10 feet). All decapods have five free thoracic segments and five pairs of
pereopods. The ten appendages are usually referred to loosely as "walking legs" whether or not
they are used for walking. All pereopods lack the exopod and thus are uniramous. The
endopod is long and narrow (Fig 3). This shape of ramus is referred to as
stenopodous in contrast to a broad, flat, leaflike phyllopod such as the uropod.
" Lift the ventral edge of the carapace and
note that it is attached dorsally to the thorax but is free laterally. With strong scissors
cut away the unattached lateral portion of the
left side of the carapace without cutting into the attached portion. Be
careful that you do not cut into the body and do not damage the numerous structures in the space
below the carapace. This space is the
branchial chamber and it contains the
gills. The gills, which are outgrowths (epipodites) of the thoracic
appendages, will be studied later. They are feathery, white, filamentous processes. Keep
them moist so they do not dry out. Removal of the carapace exposes the entire length of the
pereopods and makes it easier to study them. The lateral wings of the carapace are the
branchiostegites (Fig 8). The branchiostegites enclose the branchial
chamber. You just removed the left branchiostegite.
Look at the middle walking leg. It is pereopod 3 (Fig 3, 19-20). The typical malacostracan
thoracopod is composed of seven articles. The two proximal articles are the subdivided
protopod and the distal five are the five articles of the endopod. The seven articles are, in
order from proximal to distal; coxa, basis, ischium, merus, carpus, propodus, and dactyl. Find
the seven articles of pereopod 5. The proximal article is the
coxa. It is wide and short and articulates with the sternite of the third
pereomere. Distally it articulates with a short, narrow
basis. The basis joins with the
ischium along an oblique articulation.
Notice that the ischium appears to be composed of two articles in that it has an oblique groove
encircling it near its articulation with the basis. This groove marks the location of the
fracture plane where the crayfish can deliberately autotomize (auto = self, tome =
cut) its limb. This plane is specialized for this function and the animal can loose its limb,
at this plane only, without trauma or significant blood loss. It usually replaces the limb
with subsequent molts.
The ischium articulates with a long narrow
merus. Next there is a short
carpus followed by a long
propodus. The final article is a sharp, pointed
dactyl, or nail.
Figure 3. Pereopod 3 of the lobster,
Homarus americanus. Adapted from Herrick, 1909.
Pereopods 1-3 resemble each other in that the propodus and dactyl form a prehensile, or grasping,
pincer. The propodus bears a long, fingerlike, distal process against which the dactyl opens
and closes. The dactyl is a
movable finger and the propodal process is an
immovable finger. Such a pincer is known as a
chela and appendages bearing one is said to be
chelate. Pereopods 4 and five do not have chelae and are "
simple". The small chelae of pereopods 2 and 3 are used to transfer food to
the mouth.
Gills are associated with all thoracopods except maxilliped 1. Most appendages have more than
one gill and they may be attached to the pleurite, coxa, or the articulating membrane between the
pleurite and coxa. The gills will be considered later.
Pereopods 4 and 5 are almost identical to each other and differ from 1-3 in being simple, rather
than chelate. The coxa of pereopod 4 has a large membranous, leaflike epipod that is absent
from 5. This epipod extends vertically between the gills. Similar epipods are present on
the pereopods 1-3 and maxilliped 3. The first pereopods are much larger than any other
appendage. They are chelate and, because of the striking size of their chelae, are referred to
as
chelipeds and rarely as walking legs. The usual seven articles are present and the
chelae, as expected, are formed of the propodus and dactyl.
Notice the variety of articulations in the joints of the chelipeds. Flex and extend each joint
to see what kinds of motion its articulation allows. Each joint has an axis on which its two
articles rotate with respect to each other. Determine the axis of rotation for each of the six
joints of the cheliped.
The female gonopores are the external openings of the oviducts. They are located on the medial
side of the coxa of pereopod 3 (= thoracomere 6). The male gonopores are the external openings
of the vasa differentia from the testes and are found at the tip of the two short genital papillae
(= penes) on the medial surface of the coxa of pereopod 5 (= thoracomere 8). The position of
the male and female gonopores on these segments is constant throughout Malacostraca.
A conspicuous oval
annulus ventralis (= seminal receptacle) is located on the ventral surface of the
female pereon between the coxae of pereopods 4 and 5. It bears a deep median cleft, known as achink. The male gonopod is inserted into the chink where it deposits sperm into
the recess. If your specimen is a male, find a female crayfish and look at the
annulus.
In the genus
Procambarus the annulus ventralis is surrounded by flexible cuticle and is freely movable
(but may be partly hidden by an overhang of the preceding sternite). The annulus ventralis of
Orconectes is inflexibly fused to the sternum immediately anterior to it. In
Cambarus the annulus ventralis is not uniformly sclerotized and, even though fused with
the sternite, is capable of slight hingelike motion between the anterior and more heavily
sclerotized posterior portion. Study a female crayfish and see if you can identify it to genus
using these features of the annulus ventralis.
During copulation the male turns the female over so their ventral surfaces face each
other. The male uses his chelipeds to hold the female in position. The male gonopods
(pleopod 1) are held together and inserted into the annulus ventralis of the female. The male
genital papillae deliver sperm to the base of the gonopods. The sperm travel along grooves in
the gonopods to the female annulus where they are stored, sometimes for weeks or months before
being used for fertilization. Immediately before shedding eggs the female secretes a
glue-like glair onto the ventral surface of the abdomen and its pleopods. She then releases
sperm from the annulus onto the glair. Next she releases eggs from her gonopores on
thoracomere 6 onto the glair-covered pleopods. The eggs are fertilized and stick to the
pleopods where they are then brooded until they hatch into miniature crayfish which remain
associated with the mother for a time.
Cephalothorax
Maxillipeds
The anterior three pairs of thoracic appendages are maxillipeds. Unlike the pereopods, the
maxillipeds are biramous. The
third maxillipeds are on the third thoracomere and are immediately anterior to the
chelipeds. Each is large and intermediate in shape and size between the heavy, robust legs of
the pereon and the delicate mouthparts of the head. Each has a large, stenopodous
endopod and a small filamentous
exopod (Fig 4, Fig 19-2A). The protopod is divided into a coxa and a basis,
as it is in all thoracic appendages. The small exopod arises from the distolateral corner of
the basis. One function of the third maxilliped is to protect the more delicate appendages
anterior to it.
Figure 4. Maxilliped 3 of the lobster
Homarus. Redrawn from Herrick (1909).
Hold the third maxillipeds aside and look at the next appendage. It is the
second
maxilliped. It too is biramous but is much smaller that the third. Its
exopod is longer than its endopod.
The
first maxilliped is the appendage of the first thoracomere. Its exopod
resembles those of the other maxillipeds and is long and narrow. Its endopod is short and
inconspicuous. There are two large wide, thin endites that cup over the bulge of the
mandible. The long posterior epipod extends posteriorly into the branchial
chamber.
The remaining five pairs of appendages are those of the five head segments. The posterior
three are mouthparts whereas the anterior two are antennae with a sensory function.
Maxillae
The
second maxilla is the appendage of the fifth and posteriormost head segment and it
lies immediately anterior to the first maxilliped. It generates the water current that pumps
water out of the front of the branchial chamber. Its basal portion bears four flat, narrow
endites, a slender endopod, and a long flat
gill bailer, whose movements generate the respiratory current through the
branchial chamber. The gill bailer, also known as the scaphognathite, is composed of the
exopod and epipod (Fig 5). The bailer lies beside the carapace and extends anterior to and
posterior to the basal part of the second maxilla. The large thin trough-shaped epipod of the
first maxilliped extends back toward the branchial chamber. It functions in concert with the
gill bailer of the second maxilla. The bailer lies in and beats in the trough formed by the
epipod of the first maxilliped.
Figure 5. Maxilla 2 of the lobster,
Homarus. After Herrick (1909).
The
first maxillae are small and more delicate than the second. They are the
smallest of the mouthparts and lie curved tightly against the smooth, hard surface of the
mandible. Each has two broad endites and a narrow, larger, endopod. There is no
exopod.
Mandibles
The
mandibles are the most anterior of the mouthparts. They are heavily calcified
and equipped with powerful muscles. There is a large basal portion which bears a
cutting edge on a medial lobe. A three articled
palp arches over the cutting edge. The mandible has partial responsibility
for shearing small pieces of food from larger ones. It can rotate only slightly on its
axis.
A single, large, fleshy
labrum, or upper lip, attaches to the anterior body wall just dorsal to the
mandibles and fills much of the space behind the cutting lobes. The labrum is a fold of the
body wall and is not an appendage.
Antennae
The remaining two pairs of appendages are both sensory antennae (Fig 1, 19-2A). The biramous
second antennae are by far the larger of the two pairs (Fig 7, 19-2B). Each
arises by a biarticulate protopod consisting of a proximal coxa and a distal basis. The short,
wide exopod is called the
antennal scale. It arises from the basis. The endopod, which also arises
from the basis, has a short thick basal
peduncle of three articles and a very long narrow, whiplike
flagellum of many articles. The lower surface of the coxa bears a small circular
tubercle with an opening in its center. This is the
nephridiopore, the external opening of the kidney.
Figure 6. Mandible of the lobster,
Homarus. Redrawn from Herrick (1909).
The
first antennae (= antennules), are situated below the eyestalks. They are
much smaller than the second antennae. Each has a triarticulate basal stalk from which arise
two slender multiarticulate
flagella. There is a statocyst in the basal article of each first
antenna.
The two
eyestalks are also on the anterior head. Each bears a conspicuous
compound eye at its distal end.
Respiratory System
The respiratory apparatus of decapod crustaceans consists of numerous gills, a branchial chamber to
house them, and a water pump to generate a respiratory current over them. The gill bailer of the
second maxilla is the pump. The number of gills in crayfishes is 17-18 pairs and lobsters have
20 pairs. The gills are epipods attached to the coxa or the adjacent articulating membrane or
pleurite of most thoracopods.
The pale, feathery
gills are housed in the
branchial chamber between the lateral carapace and the body (Fig 8,
19-3A). You opened the left branchial chamber when you removed the left carapace and the gills
on that side are exposed to view. The right chamber should still be intact and covered by the
branchiostegites of the carapace. Note that the branchial chamber is outside the body
and is filled with water, even though it is under the carapace and appears to be internal.
The gills extend vertically into the branchial chamber from their attachments on or near the coxae
of the thoracopods (Fig 19-38B). Look closely and see that the gills of successive appendages
are separated from each other by the long, membranous epipods of those appendages. The epipods
form the boundaries of water channels that extend vertically from the free, unattached ventral edge
of the carapace upwards to the attached dorsal edge (Fig 19-38B). Notice that the coxae of
each pair of adjacent pereopods are shaped so that together they form V-shaped
inhalant channels that lead into one of the vertical channels in the branchial
chamber. There are five such inhalant channels.
Figure 7. Antenna 2 of the lobster,
Homarus. The ventral surface and nephridiopore are shown in the inset. Redrawn
from Herrick (1909).
Dorsally the several vertical channels converge on an oblique,
exhalant channel that runs anteriorly along the dorsal margin of the branchial
chamber (Fig 19-38B). The floor of the anterior half of this channel is made of the epipod of
maxilliped 1 which separates the channel from the gills. The roof and walls are formed by the
branchiostegite of the carapace and body. The gill bailer of maxilla 2 lies in the anterior
end of the exhalent canal in the epipod of maxilliped 1. Undulations of the bailer generate
the negative pressure that draws water in the inhalant canals, vertically over the gills, and then
anteriorly, to exit lateral to the mouthparts.
All decapod gills are associated
with thoracopods but differ in the exact location of their attachment. Podobranchs (podo =
foot, branch = gill) arise on the lateral surface of the coxa of the thoracopod (Fig
8). Arthrobranchs (arthro = joint) arise on the thin articulating membrane between the coxa
and the pleurite of the body wall. Pleurobranchs (pleuro = side) are attached to the pleurite
dorsal to the limb articulation but are usually absent in crayfishes.
Look at pereopod 4. Like
most crayfish thoracopods it has one podobranch and two arthrobranchs (one anterior and one
posterior). Pereopods 1-4 and maxilliped 3 each have one podobranch and two
arthrobranchs. Pereopod 5 may or may not have a pleurobranch, depending on
taxon. Maxilliped 2 has one small podobranch and an arthrobranch. Maxilliped 1 has no
gills but has the important epipod that encloses the exhalant water channel and the gill bailer.
Some crayfishes have a pleurobranch on pereopod 5 and some do not. Crayfishes thus have a total of
17-18 pairs of gills whereas lobsters, with several pleurobranchs, have 20.
Figure 8. Cross section of the branchial chamber and gills of a generalized
decapod.
" Snip the end from one of the gills, place
it in a 6-cm culture dish of water and examine it with the dissecting microscope.
Crayfishes and lobsters have
filamentous (= trichobranchiate) gills in which the respiratory surface consists of numerous long
filaments radiating from a central axis, rather like a bottlebrush (Fig
19-37C,D).
Look at the cut surface of the
gill axis. Here you will see two blood channels, cut in cross-section, that extend the length
of the gill (Fig 19-37C). One is the afferent channel that takes unoxygenated blood into the
gill and the other is the efferent vessel that drains oxygenated blood away from the
gill. Similarly, each filament is partitioned into two channels by a longitudinal
septum. One channel is afferent, the other efferent.
Additional Exercises
Watch the video "America's crayfish, Crawling in troubled waters", produced by Virginia Tech and
the U.S. Fish and Wildlife Service.
Internal Anatomy
Pending completion of this
section, the description of the internal anatomy of
Homarus americanus (American lobster) in this series (
Invertebrate Anatomy OnLine
)can be substituted
.
References
Bullough WS.
1958. Practical Invertebrate Anatomy 2
nd ed. MacMillan, New York. 483p.
Herrick FH. 1909. Natural History of the American Lobster. Bull.
Bur. Fish. 26:150-408, pls 33-47.
Hobbs
HH. 1991. Decapoda,
in Thorp JW, Covich AP (eds). Ecology and classification of North American freshwater
invertebrates. Academic Press, San Diego.
Huxley TH. 1880. The Crayfish, An Introduction to the Study of
Zoology. Appleton, New York. 371p. (Reprinted 1973, M.I.T. Press, Cambridge.)
Lochhead JH. 1950. Crayfishes (and
Homarus) in F. A. Brown (ed) Selected Invertebrate Types. Wiley, New
York. pp422-447.
Pennak, R.W. 1989. Freshwater invertebrates of the United States,
3ed. Wiley.
Ruppert EE, Fox RS, Barnes RB.
2004. Invertebrate Zoology, A functional evolutionary approach, 7
th ed. Brooks Cole Thomson, Belmont CA. 963 pp.
Snodgrass RE.
1952. A Textbook of Arthropod Anatomy. Cornell Univ.
Press, Ithaca. 363 p. (reprinted 1971 by Hafer Publishing, New York) (crayfish pp
142-179).
Supplies
Dissecting pan
Living or preserved crayfish
Dissecting set
Chloroform-saturated water for living
specimens.
Video projection equipment
The video, “Crawling in troubled waters” and a
spectacular crayfish poster are available from Earthwave Productions Inc,
www.earthwave.org, 817.443.0258 or Virginia Cooperative
Extension,
monteh@vt.edu, 540.231.6192