Invertebrate Anatomy OnLine
Triops longicaudatus
©
Tadpole Shrimp
19jun2006
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, Phyllopoda, Notostraca
O, Triopsidae
F (Fig 16-15, 19-18, 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.
Phyllopoda
Phyllopoda consists of about 800 species in four higher taxa; the “large phyllopodans” consisting
of Notostraca, Laevicaudata, and Spinicaudata and Cladocera, which are the “small phyllopodans”.
Trunk appendages are phyllopods and a large carapace encloses much or all of the body. Large
phyllopodans typically inhabit relictual habits where fishes are absent but Cladocerans show no
such restrictions. Tagmata are a head, thorax, and reduced abdomen. The abdomen lacks
appendages but has a posterior caudal furca on the telson. A ventral food groove is usually present
and employed in feeding. A so-called dorsal organ is present on the dorsal midline of the posterior
head.
Notostraca
O
Notostracans are tadpole
shrimps, of which only 10 species are known worldwide. They inhabit quiet, fishless, usually
temporary, freshwaters where they crawl over the bottom or swim in the water. They use the
anterior trunk appendages for both types of locomotion as well as for feeding. Tadpole shrimps
are deposit feeders and predators. They are sometimes abundant in rice
fields. Notostracans differ from anostracans primarily in having a carapace (noto=back,
ostrac=shell), sessile compound eyes, and appendages posterior to the genital segments. The trunk
is composed of about 40 segments and is divided into a large thorax and a small abdomen.
Laboratory Specimens
Viable tadpole shrimp eggs are
available from Ward's Natural Science Co. (see Supplies chapter). This company collects
detritus, including eggs, from the bottom of temporary ponds in Utah and ships it under the name
"living fossils". The eggs are easily hatched and the shrimp can be reared to maturity in the
laboratory. It is thus possible to see living tadpole shrimps in any laboratory, an
opportunity that few biologists, especially those living in the eastern United States, would ever
have. The eggs provided are those of
Triops longicaudatus.
All notostracans are similar and
this exercise can be used for any species. All North American species are western (or boreal)
and belong to the genera
Triops (=
Apus) or
Lepidurus. The exercise emphasizes external anatomy. The internal organs
resemble those of anostracans such as brine shrimp. As usual, living material is preferable to
preserved but either is acceptable, especially for study of external anatomy.
Behavior
Examine a living tadpole shrimp
in an 8-cm culture dish of pondwater. Place the dish on the stage of your dissecting
microscope, with the substage light off, and watch it swim. Notostracans, like most aquatic
animals but unlike anostracans, exhibit a
dorsal light response, swimming with the dorsum facing toward the light
source. In nature the normal swimming posture is right side up (with the dorsum up). In
laboratory situations individuals can be induced to swim upside down (with the dorsum down), if a
light is placed beneath them.
>1a. On the stage of the dissecting microscope watch a tadpole shrimp swim in a
glass dish with overhead (incident) illumination and note the nature of the light response. It
is dorsal or ventral? Which surface is usually up? Turn the substage lamp on and observe the
response. Does the orientation of the animal change? Now which surface is usually up?
<
External Anatomy
Tagmata
Study the external anatomy of
your specimen. If it is alive place a drop of chloroform in the dish and wait it to become
inactive.
Look first at the dorsal
surface. The
body consists of a
head and
trunk and is mostly covered by the large, dorsal
carapace (Fig 1, 19-13). Little of the body is visible dorsally. Turn
the animal over and look at the ventral surface.
Figure 1. Dorsal view of a tadpole shrimp,
Triops longicaudatus, reared from sediments from a temporary pond in Utah.
Notostraca1L.gif
The
head is typical of crustaceans and is composed of five fused segments but there is
a tendency to reduction or loss of head appendages. The long trunk is not distinctly divided
into thorax and abdomen. Most of the trunk segments bear appendages.
As you study the animal try to
decide where you think the thorax stops and the abdomen begins. The issue is disputed. The
first 11 trunk segments each bear a pair of appendages. These are followed by a region of
fused segments each of which bears up to six pairs of appendages. Finally the trunk ends with
a region of segments with no appendages. Some biologists consider the
thorax to be the two regions with appendages and the
abdomen to be the region without appendages. Another interpretation is that
the region of fused segments is part of the abdomen.
Carapace
Note that a carapace is present
but there is no cephalothorax. No thoracic segment is fused with the head so there is no
cephalothorax. Carapace and cephalothorax are not the same and should not be confused,
although they often are.
The crustacean carapace is a
posterior fold of the body wall of the segment of the second maxilla, which is the posterior edge
of the head. It overhangs the body, to greater or lesser extent, and may be attached to
it. In Notostraca, the carapace covers all of the thorax but is not attached to it at any
point.
Head
Look at the dorsal surface
again. The head bears a pair of dorsal
compound eyes (Fig 1, 19-13) that lie close to each other near the
midline. The compound eyes are sessile, not stalked as are those of anostracans. In
addition, there is a
naupliar eye on the anterior midline. The compound eyes are on the dorsal
surface of the head but the naupliar eye is deep within the head. All the eyes are easily seen
through the integument of the head.
A distinct transverse groove,
the
mandibular groove, marks the division between the anterior three head segments and
the posterior two (Fig 1). A second transverse groove, the
cervical groove, just posterior to the first, marks the division between the head
and thorax.
Head Appendages
Look at the ventral surface of
the head. A lenslike
window on the ventral midline of the head admits light to the ventrally aimed
naupliar eye.
The
first antennae are small, short, slender filaments on the ventral surface of the
head, at about the level of the eyes. The second antennae are similar and located lateral to
the first. They are vestigial and inconspicuous. They are absent in some species but are
present in
Triops longicaudatus.
The large, well-developed
mandibles oppose each other across the ventral midline. Their opposing median
surfaces bear strong brownish-yellow teeth. In living, unanesthetized specimens you can watch
the teeth move apart then close together as the animal periodically opens and closes the mandibles.
Of the usual crustacean head appendages, only the mandibles are well developed.
A transparent, unpaired, median
labrum arises from the body wall between the bases of the antennae and extends
posteriorly to cover the mouth and ventral ends of the mandibles.
The first and second maxillae
lie posterior to the mandibles. They are small but bear distinct setae. The second
maxillae are larger than the first. The nephridiopores are located on the second
maxillae. (The second maxillae are absent in some species.)
Trunk
In this exercise the trunk is
considered to consist of a thorax of appendage-bearing segments and abdomen of segments without
appendages. The anterior thorax consists of 11 segments and each bears a pair of appendages, called
thoracopods. The segments of the posterior thorax are incompletely separated
to form
rings. Each ring may consist of as many as six fused segments and
consequently may bear up to six pairs of appendages. There may be up to 70 pairs of appendages
on the entire thorax. The genital segments are located between the two regions of the
thorax.
The posterior few rings of the
trunk are the abdomen do not bear appendages. The
telson is the posterior end of the trunk. It bears a
caudal furca consisting of two long, multiarticulate, whiplike rami (Fig 1,
19-13). The anus lies on the telson between the bases of the two rami.
Trunk Appendages
Most of the thoracic appendages,
or thoracopods, resemble each other but the first 11 pairs are best developed. There is a
slight tendency to regional specialization and the first thoracopod is unlike the remaining
pairs. It has a sensory function, replacing the reduced antennae in that role, whereas the
remaining anterior thoracic appendages (2-10) are the major locomotory, feeding, and respiratory
limbs.
The 11th appendages of females
form brood pouches. The many appendages posterior to the 11th move the spent feeding and
respiratory current away from the body and are also respiratory.
Most of the thoracopods are
flat, leaflike
phyllopods derived from and resembling the ancestral biramous crustacean
appendage. The first thoracopod, however, is not a phyllopod. As is true of anostracans,
it is difficult to draw exact homologies between the parts of the notostracan limb and that of the
ancestral limb. The names used here reflect possible homologies but these are by no means
certain and are questioned by some crustacean specialists.
Begin with the second thoracopod
skipping the unusual, antenniform first thoracopod for the time being. Examine this appendage
while it still on the animal with high power of the dissecting microscope. (If
instructed to do so, remove this appendage and make a wetmount of it for examination with the
compound microscope.)
Thoracopod 2
The central part of the
appendage is the
protopod (Fig 2) whose proximal end is attached to the body. On the lateral
surface of the protopod are two exites. (Any process from the lateral border of a crustacean
limb is an exite and any process from the medial border is an endite.) The proximal process is
the gill. It is teardrop-shaped and does not have setae. The much larger, setose, distal
exite is the
exopod.
On the medial edge of the
protopod there are several endites. The distal endite is the
endopod. It is stiff, sharp and blade-shaped. The remaining
endites resemble the endopod but are smaller. The proximal endite is strong
and armed with spines on its medial margin. It is a
gnathobase. The two (right and left) gnathobases of each pair of appendages
are close to each other and face each other across the midline. The remaining endites are
farther from the midline. The two rows of gnathobases form the right and left sides of the
conspicuous midventral
food groove.
Figure 2. The second thoracopod (1
st phyllopod) of
Triops longicaudatus. Notostraca3L.gif
Thoracopod 1
The first thoracopod is modified
to function as a sensory structure. It has the same parts as other thoracopods but they differ in
morphology and function. Its
protopod is narrow. A
gill and
exopod are present and resemble those of the phyllopods. The
endopod is reduced to a small, almost seta-less, distal process. The four
endites are long, multiarticulate flagella that look and function like antennae
(i.e. antenniform). The distal one is longest and the proximal one is quite short. The
gnathobase is like those of the other trunk appendages. As the animal moves over the
substratum the antenniform flagella come in contact with it and with potential prey. When such
an object is detected by these flagella the shrimp leaps onto it and covers it with the
carapace.
The 11th pair of trunk
appendages form brood pouches in females. The protopod, gill, and exopods contribute to the
pouch. The protopod forms a cup for which the exopod is the cover. These limbs are not
modified in the male.
Feeding
The feeding method of
notostracans is similar to that proposed for the ancestral crustacean. The anterior phyllopods
(2-10) stir sediments and swirl muddy water and particles up into the wide, midventral food
groove. Motion of the gnathobases moves food anteriorly in the food groove. The motion of
the spiny gnathobases can be seen in living specimens viewed from the ventral surface.
The large flat exopods are
primarily responsible for stirring and lifting sediments. Fine silt particles and water escape
laterally but coarse particles, including food, remain in the ventral food groove. Here they
are torn into small pieces by the sharp bladelike endopods and moved anteriorly to the mandibles
and mouth by the gnathobases. The mouth faces posteriorly to receive food arriving in the food
groove.
Figure 3. The first thoracopod of
Triops longicaudatus. Notostraca2L.gif
Particulate food includes small
insect larvae, oligochaete worms, and tadpoles. Notostracans may also engage in suspension
feeding while swimming. For this they use the setae of the endites.
>1b. Feeding is easily observed in living notostracans. To observe
predation place a tadpole shrimp in a small dish with some brine shrimp smaller than the
tadpole. Small oligochaetes such as
Tubifex can also be used. Observe the shrimp with the dissecting microscope. If
you are patient you should eventually see the tadpole shrimp discover a prey animal and leap upon
it. Continue watching as the powerful mandibles tear the prey into small pieces which are then
swallowed.
<
>1c. Suspension feeding is also an important feeding mode and can be
demonstrated by placing a little yeast/Congo red suspension in a dish with a tadpole
shrimp. Instructions for preparation of the stained yeast will be found in the Supplies
chapter.
Watch the shrimp continuously if
you wish or set it aside and return to it in about 30 minutes. The anterior end of the gut
(stomach) will quickly turn bright red as stained yeast cells accumulate there. Soon the
entire gut will be red. Pigment will eventually appear in the branched digestive ceca in the
head. This is the best way to see digestive system. Return to this preparation when you
study the gut.
<
Internal Anatomy
Most internal features are
difficult to see from the outside. The heart is a long, dorsal tube in the anterior 11 trunk
segments. It has a pair of ostia in each of these segments. Hemoglobin is sometimes
present in the blood and the animal may be pink as a result.
The excretory/osmoregulatory
organs are the paired
maxillary glands (= saccate nephridia) in the segment of the second maxilla (Fig
1). The long looped ducts of these glands can be seen in the carapace (Fig 1, 19-13). The
role of the maxillary glands is primarily osmoregulatory. Nitrogen, in the form of ammonia, is
lost by diffusion across the gill surfaces.
The mouth opens between the two
mandibles on the ventral surface of the head. A short, vertical esophagus connects it with the
stomach in the head. Two digestive ceca have branches extending into the carapace. The
intestine extends posteriorly through the trunk to join a short rectum which opens at the
anus. The intestine is easily seen.
>1d. If you have living specimens and have not already done so, place some
yeast/Congo red suspension in the dish with a shrimp. After 15-30 minutes examine the animal
with the dissecting microscope. The gut, now filled with red pigment, is easily seen. If
necessary, and if there are plenty of specimens, add a drop of chloroform to the water in the dish
to stop the motion of the animal. This may kill the specimen so don't do it if living animals
are in short supply.
<
The paired gonads extend almost
the entire length of the trunk on either side of the gut. They open via gonopores on the 11th
pair of thoracopods.
Parthenogenesis is common and
males may be rare. Females produce thin-shelled summer eggs or thick-shelled resting eggs
which survive freezing and desiccation. Eggs hatch as nauplii or metanauplii.
References
Kaestner, A.
1970. Invertebrate zoology, Crustacea, vol
III. Wiley Interscience, New York. 523pp.
Lankester ER
. 1881. Observations and reflections on the appendages and
on the nervous system of
Apus cancriformis. Quart. J. Micros. Sci. 21:343-
Linder F. Contributions to the morphology and taxonomy of the Branchjiopoda
Notostraca, with special reference to the North American species. Proc. US Nat. Mus.
102(3291):1-69, pls 1-7.
Longhurst
AR
. 1955. A review of the
Notostraca. Bull. Brit. Mus. Nat. Hist. Zool 3(1):1-57.
Martin JW. 1992. Branchiopoda, pp25-224 in Harrison FW, Humes AG (eds.)
Microscopical anatomy of invertebrates, vol 9 Crustacea. Wiley, New York. 652pp.
Pennak RW. 1978. Fresh-water Invertebrates of the United
States 2
nd ed. Wiley, New York. 803pp.
Ruppert EE, Fox RS, Barnes RB.
2004. Invertebrate Zoology, A functional evolutionary approach, 7
th ed. Brooks Cole Thomson, Belmont CA. 963 pp.
Tasch P. 1969. Branchiopoda, in R. C. Moore (ed) Treatise on
Invertebrate Paleontology, pt R: Arthropoda 4(1). Geological Soc. America, Boulder.
Supplies
Dissecting microscope
Compound microscope
Living or preserved
Triops
8-cm culture dish
Chloroform
Yeast/Congo red suspension
Lab Prep
The teaching staff should hatch
eggs ("living fossils") purchased from Ward's Natural Science Co. and provide the class with living
(or perhaps preserved if specimens have been saved from classes in previous years) adults and
juvenile stages to study. Usually only a few tadpole shrimp hatch from each vial of detritus
and several vials will be required to provide an entire class with living specimens. Fairy
shrimp will also be present. The soil is collected from the bottom of temporary ponds in
Utah.
It may be desirable to preserve
the specimens used each year for use in subsequent years in the event that insufficient living
animals are available. Some instructors, especially those with large classes, may want to use
preserved material for study of anatomy but provide a few living specimens for behavioral
observations. Material to be preserved should first be fixed in 5% formalin overnight, washed
thoroughly in freshwater, and then stored in 80% ethanol or 40% isopropanol.
To hatch the eggs empty the
contents of the vial as received from Ward's into a large fingerbowl of chlorine-free
freshwater. The eggs hatch quickly (24 hours) and grow rapidly. Tadpole shrimps are
carnivorous and will eat any other small soft-bodied animals in the dish, including each other and
the fairy shrimp that are also present. Newly hatched nauplii begin to disappear almost as
soon as they hatch when they fall prey to their slightly larger siblings. To maximize the
production of shrimp, each nauplius should be removed to its own small culture dish as soon as it
appears. The fairy shrimp developing in the culture can be studied using the
Artemia exercise in this collection
.
Tadpole shrimp can be fed a
yeast suspension and/or
Artemia larvae and juveniles. Avoid use of formalin- or soap-contaminated glassware
or instruments when rearing larvae.