Invertebrate Anatomy OnLine
Caecidotea and Lirceus
Freshwater Isopods ©
Copyright 2000 by
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.
Mandibulata, Crustacea sP,
Eucrustacea, Thoracopoda, Phyllopodomorpha, Ostraca, Malacostraca C,
Eumalacostraca, Caridoida, Xenommacarida, Nomen nominandum, Neocarida,
Peracarida SO, Isopoda O,
Asellota sO, Asellidae F (Fig
16-15, 19-67, 19-90)
by far the largest and most diverse animal taxon, includes chelicerates,
insects, myriapods, and crustaceans as well as many extinct taxa such as
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.
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,
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.
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
organs also vary with taxon and include many types of gills, book lungs, and
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.
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.
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.
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
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.
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
compound eyes are stalked primitively although derived sessile eyes occur in
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. The female
gonopore is on the eighth thoracic segment and the male on the sixth.
is a large and ecologically important malacostracan taxon. It
includes the amphipods and isopods, each with well over 5000 species, as well as
several smaller taxa such as mysids, cumaceans, and tanaidaceans. Peracarids
have the characteristics of Malacostraca and are further defined by possession
of a ventral, thoracic marsupium, or brood pouch, in which the eggs are brooded. The
mandible bears a movable tooth, the lacinia mobilis, between the molar and
incisor. Development is direct and there is no larva. Most
peracarids are small, less than 2 cm and primarily inhabit marine and freshwater
habitats, although some are terrestrial.
is a large and diverse taxon with nine high level subtaxa. Isopods
are common in marine and freshwater habitats and there are important terrestrial
and parasitic suborders. It
includes the familiar terrestrial pill bugs, wood lice, and sea slaters, and
about 4000 less familiar marine species. Isopods
are usually dorsoventrally flattened and the gills and heart are abdominal. One
thoracic segment is fused with the head and there is, correspondingly, one pair
of maxillipeds and seven pairs of thoracic walking legs. The
gills and heart are abdominal.
is a large taxon of freshwater and marine isopods. The
uropods are terminal and biramous. The
posterior four abdominal segments and telson are fused to form the pleotelson. The
male, and sometimes female, first pereopods are subchelate. The third pleopods
are opercula protecting the remaining pleopods, which are gills. The first
pleopods are lost in females.
North American freshwater isopods belong to Asellidae of which there are two
important genera, Caecidotea (formerly Asellus)
and Lirceus. Asellids
are found in surface and subterranean waters with about 30 species reported from
North America. Specimens
for laboratory use can often be collected from local streams.
two genera, Lirceus and Caecidotea,
are quite similar, in fact they are difficult to tell apart, and this exercise
can be used with either. Because
of their small size this exercise is limited to external anatomy. The structures
described can be found by manipulating a living or preserved specimen with a minuten nadel and
fine forceps. The study
should be conducted on the stage of a dissecting microscope (10-20X) in an 8-cm
culture dish of 5 % ethanol, if living, or tapwater if preserved. Use
your judgment in selecting the appropriate magnification and be prepared to
change the magnification when necessary.
a male and ovigerous female isopods. Begin
with the male and examine it with magnification (Fig 1, 19-62). The basic
arthropod tagmata are
the head, thorax, and abdomen but most crustaceans, isopods included, have made
some adjustments in this basic pattern. Originally
the malacostracan head consisted of five segments, the thorax eight, and the
abdomen seven. In
isopods these have been rearranged into a cephalothorax of six segments, a
pereon of seven, and an abdomen of six. Thoracic segments are known as thoracomeres and
abdominal segments are pleomeres. Caecidotea is
strongly flattened, or dorsoventrally
depressed, a characteristic
of freshwater isopods and a useful distinction between them and freshwater
amphipods, which are laterally compressed.
Examine the anterior end of the dorsum of your specimen. In
isopods the first thoracic segment is fused with the head to form the cephalothorax,
or cephalon, of six segments (Fig 1, 2, 19-62). Superficially
the cephalothorax appears to be a single segment but in reality it is six
segments and it bears six pairs of appendages. The
six segments are fused and cannot be distinguished externally.
In surface-dwelling (epigean) species a pair of small sessile compound
eyes is present
dorsolaterally on the cephalothorax. These
are absent or reduced in hypogean (subterranean) species (Fig 2).
The cephalothorax bears six pairs of appendages belonging to
its six segments. Anteriormost
is the first antenna composed
of a proximal peduncle of
three long articles and a shortflagellum of
many very short articles (Fig 1, 2).
Next is the second
asellids the second antenna is much larger than the first. It
also is composed of a basal peduncle,
but with five articles, and a long, whiplike, multiarticulate flagellum.
The remaining appendages are mouthparts and
are all located in an oral cone on the ventral surface of the cephalothorax. Turn
your specimen over and find the large cluster of mouthparts under the head. This
is the oral cone.
The maxillipeds lie
on the posterior and ventral surface of the cone and can be seen without
dissection (Fig 2, 6). They
cover the remaining mouthparts. The maxillipeds are the appendages of the first
thoracic segment which, you remember, is fused with the head and its appendages
have become accessory mouthparts.
Hidden from view by the maxillipeds are three pairs of true
mouthparts belonging to the posterior three head segments. These are the
mandibles on the third head segment, the first maxillae on the fourth, and the
second maxillae on the fifth and last head segment. The two mandibles lie to the
right and left of the mouth on the ventral surface of the head. The
maxillae are posterior to the mouth between the mandibles and the maxilliped. Lift
the maxilliped with a minuten
nadel to see the mouthparts
beneath it in the oral cone.
middle region of the isopod body comprises seven segments and is the pereon. The
pereon consists of those thoracic segments not fused with the head to form the
cephalothorax. These segments, although thoracomeres are known by
the more specific name of pereomeres, or pereonites (Fig
2). In the case of
isopods and amphipods, only one of the eight thoracomeres is part of the
cephalothorax, leaving seven free thoracomeres (pereonites) to form the pereon. These
segments are easy to see and count in dorsal view (Fig 1, 2). Look
at the dorsum and count the pereonites.
Figure 1. Dorsal view of Caecidotea (Redrawn
from Williams (1972). Isopod63L.gif
Pereonites are good examples of typical arthropod segments.
Each is covered by an exoskeletal
ring composed of dorsal and
ventral sclerites connected laterally by the flexible articular membranes (Fig
dorsal sclerite is the heavily sclerotized, arched tergite (Fig
1, 2). The
seven tergites of the pereon are easy to see and count. The ventral portion of
each segmental exoskeletal ring is the sternite (Fig
is also sclerotized but less so than the tergite. The
flexible lateral articulations are the pleural membranes.
Figure 2. Lateral view of the subterranean isopod Caecidotea
kendeighi (female) (Redrawn from
Lewis and Bowman, 1981). Isopod76L.gif
Each tergite is fused laterally with the coxa of its leg. This
lateral addition creates an eave overhanging the edge of the segment. Turn
your specimen over again and observe that the legs, or pereopods (Fig
2), are attached to the lateral wings of the tergite. Those
wings are, in fact, the coxae.
With the exception of the first pair in males, the pereopods
are all similar to each other, hence the name "isopod" (= same foot). The
first pereopod of males (and sometimes of females) is a prehensile organ known
as a gnathopod (Fig 2, 3, 19-62A).
Look at one of the pereopods, whichever one you can see best
with higher magnification. Malacostracan
thoracic appendages consist of seven articles which are, from proximal to
distal, the coxa, basis,
ischium, merus, carpus, propodus, and dactyl (Fig
with the distal dactyl and work your way back to the articulation with the body. You
should come up one short. There
are only six apparent articles in an isopod leg. This
is because the lateral eaves of the tergites mentioned above are, in fact, the
coxae which have fused with the tergites so they no longer appear to be part of
Look more closely at the first male pereopod, the gnathopod (Fig
propodus is enlarged and forms a "palm" against which the dactyl, acting like a
finger, closes to form a pincer for grasping. This
arrangement is said to be subchelate. The
pincers of decapod crustaceans, such as crayfish and crabs, have chelate pincers
in which the propodus forms an immovable finger, not a palm, to oppose the
dactyl, which is a movable finger. A
limb with no prehensile mechanism is said to be simple.
Look at the ventral surface of the pereon. It,
like the dorsum, is covered by sclerotized plates but these are sternites. They
are flexible and not as heavily sclerotized as the tergites.
Figure 3. The
gnathopod (first pereopod) of a male Caecidotea. Redrawn
from Williams (1972). Isopod64L.gif
the specimen back over and look at the dorsum of the abdomen, or pleon. Its
segments are pleomeres. The first two pleomeres are small and inconspicuous
remaining four are fused with each other and with the telson to form a single
large pleotelson, in
which segmentation is not apparent (Fig 1, 2, 19-62A). The abdominal appendages
consist of five anterior pairs of pleopods and a pair of posterior uropods.
Turn the specimen back over and look at the ventral pleon. Five
pairs of appendages, known as pleopods,
are here but they are not immediately apparent for they do not look like legs
and they are stacked under each other. The first two pleopods are small and are
highly modified in males. The
second pleopods are external genitalia for the transfer of sperm to the female. The
fourth and fifth pleopods are gills and the third is an operculum to protect the
gills (Fig 6). The
gills are located in the branchial chamber under the pleotelson and covered by
Figure 4. Anterior pleopods of a male Caecidotea.
The endopod of pleopod 2 is the intromittent organ. A, Pleopod 1. B,
Pleopod 2. Redrawn from Williams (1972). Isopod65L.gif
Move the posteriormost pair of pereopods out of the way so
you can see the anterior end of the ventral abdomen. This
will reveal a pair of tiny digitiform penes (which
are not segmental appendages) and pleopods
1 and 2 (Fig 4). The
pleopods are folded posteriorly so that the penes lie on top of pleopod 1, which
lies on top of pleopod 2, which lies on top of the large operculum (pleopod 3)
and so on.
Figure 5. Pleopod
3 of Caecidotea. The exopod
is the operculum. The endopod is hidden under the exopod. Redrawn
from Williams (1972). Isopod66L.gif
Pleopod 1 is small, two-segmented, and uniramous (Fig 4A). Use
the minuten nadel to
lift pleopod 1 so you can see pleopod 2. The
second pleopod is biramous and its endopod is the intromittent
organ (Fig 4B). The tip of
this organ is shaped to fit into a seminal receptacle on the female and its
shape is species specific. This
is a mechanical isolating mechanism. Pleopod
1 is absent in females and pleopod 2 is a small vestige (Fig 6, 19-62B).
6. Ventral view of an ovigerous female Caecidotea
eggs are indicated under the oostegites. Redrawn from van Name, 1936).
The external genitalia lie atop the two third pleopods. Pleopod
3 is biramous with a large exopod and smaller endopod (Fig 5, 6). The
exopods are the large plate-like opercula that
protect the remaining two pairs of pleopods. Lift
the operculum of and look beneath it. The
small endopod is
Under the operculum are the oval white, page-like endopods
and exopods of pleopods 4 and 5. These
four rami look like the pages of a book and are the gills. They
are housed in the concavity of the ventral pleotelson, a space known as the branchial
The appendages of the sixth abdominal segment are the
biramous uropods which
are conspicuous features of the posterior margin of the pleotelson, visible both
dorsally and ventrally (Fig 1, 2, 6). Each
uropod consists of a basal protopod from
which arise two branches, a lateral exopod and
a medial endopod (Fig
1, 2). The anus is
at the ventral tip of the pleotelson (Fig 6).
a female specimen in reproductive condition, with eggs in the marsupium. Place
it in a dish of tapwater, if preserved, or, or 5 % ethanol, if alive, and
examine it with the dissecting microscope. Review
the anatomy you learned on the male specimen and then turn your attention to the
When reproductive, females develop large thin flexible oostegites,
or brood plates, on the medial border of some thoracic appendages (Fig 6,
19-62B, 19-52). These
transparent plates overlap each other and enclose a ventral brood pouch, or marsupium,
in which the embryos are brooded. Use
your minuten nadel to
lift the edge of one of the oostegites to demonstrate its presence and
are difficult to see if you don’t lift them. The
young may be eggs still or they may be more advanced and recognizable as
juvenile isopods. Eggs
and embryos are ventilated by movements of the oostegites.
at the ventral pereon. The anterior pleopods of males and females differ and in
females there are no external genitalia. The first pleopods are absent and the
second are vestigial in females (Fig 6) whereas in males the second are
intromittent organs. The exopods of the two third
pleopods are, as in the
male, the opercula for
the branchial chamber (Fig
6). The rami of the fourth and fifth pleopods are the gills in
the branchial chamber.
AP, Thorp JW . 1991. Crustacea:
Introduction and Peracarida in Thorp
JW, Covich AP (eds). Ecology
and classification of North American freshwater invertebrates. Academic
Press, San Diego. 911pp.
JJ, Bowman TE. 1981.
The subterranean asellids (Caecidotea) of Illinois (Crustacea: Isopoda:
Asellidae). Smithsonian Contr. Zoology 335: 1-66.
RW. 1989. Freshwater
invertebrates of the United States, 3ed. Wiley.
EE, Fox RS, Barnes RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
JW. Covich AP. 1991. Ecology
and classification of North American freshwater invertebrates. Academic
Press, San Diego. 911pp.
Name WG. 1936. The American
land and fresh-water isopod Crustacea. Bull. American Mus. Nat. Hist. 71:1-535.
WD. 1972. Freshwater
isopods (Asellidae) of North America. Biota
of freshwater ecosystems Identification manual 7: 1-45. US
Environmental Protection Agency,
Project 18050 ELD.
Living or preserved freshwater isopods, male and female
8-cm culture dish
5 % non-denatured ethanol in lake water