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Anatomy of Triops by Richard Fox of Lander University, Greenwood, South Carolina, USA.

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.

Viable tadpole shrimp eggs are available from a wide range of Suppliers, including Ward's Natural Science Co. .  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, ever get.  The eggs usually provided are those of Triops longicaudatus. though Triops cancriformis is available from Triops.cc

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 page 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.

Examine a living tadpole shrimp in an 8-cm culture dish of pondwater.  Place the dish on the stage of your 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.

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

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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.)

For this page 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.

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.

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.

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 (1st phyllopod) of Triops longicaudatus.

1st Thoracopod

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.

Figure 3.  The first thoracopod (1st phyllopod) of Triops longicaudatus.

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.

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.

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 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.

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.

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).   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.

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.


Supplies

YEAST/CONGO RED SUSPENSION
Paramecium and rotifer food for visualizing feeding.  Boil 10g dry yeast with 0.1g Congo red in 30 ml water for 10 min.  Cool and store refrigerated until needed.  The suspension can be kept frozen indefinitely.

YEAST SUSPENSION
Daily Artemia food for maintaining young Tapole Shrimp cultures. 1 package baker's yeast suspended in 100 ml fresh water.


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-

A review of the Notostraca.  Bull. Brit. Mus. Nat. Hist. Zool 3(1):1-57.

Pennak RW.  1978.  Fresh-water Invertebrates of  the  United States 2nd ed.  Wiley, New York.  803pp.

Ruppert EE, Fox RS, Barnes RB.  2004. Invertebrate Zoology, A functional evolutionary approach, 7th 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.