Practical Course in Zoology: Study Guide - T. A. Dauda 2014

Multicellular Animals
Arthropods
Subphylum Branchiopoda - Class Crustacea

Branchiopods (or gill-breathing Arthropods) are primarily aquatic arthropods. This suborder comprises a single Class, Crustacea. The total number of known species is at least 20,000. Crustaceans predominantly inhabit marine and various freshwater habitats, including subterranean waters.

Crustaceans are characterized by the simultaneous presence of two pairs of appendages: antennules and antennae. Unlike other arthropods, crustaceans alone retain the primitive biramous Structure of their limbs in most cases.

Respiration in crustaceans occurs via gills, which are specialized outgrowths of the limbs.

Until recently, all crustaceans were divided into two subclasses: lower crustaceans (Entomostraca) and higher crustaceans (Malacostraca). However, the Organization of lower crustaceans is so diverse and their phylogenetic pathways so varied that grouping them into a single subclass proved entirely artificial. The former Entomostraca subclass has now been split into several independent subclasses, whereas higher crustaceans (Malacostraca) form a natural group characterized by a uniform metameric composition and several other distinct features.

The class Crustacea is divided into five subclasses: Branchiopoda, Cephalocarida, Maxillopoda, Ostracoda, and Malacostraca. Each subclass comprises a varying number of orders. We will examine representatives of only three of these orders.

Order Cladocera (Water Fleas)

Numerous representatives of this order are Key Components of freshwater plankton. Under laboratory conditions, they are easily maintained in aquaria year-round. Branchiopods include tadpole shrimp, clam shrimp, and water fleas such as Daphnia and Bosmina. For morphological studies, *Daphnia pulex* (the common water flea) serves as an ideal model Organism.

Materials and Equipment. *Daphnia* is commonly found in small freshwater bodies. It is a tiny crustacean (1–2 mm in length). Under favorable conditions, they reproduce massively, imparting a reddish-brown hue to the water. They survive the winter well in aquaria at room Temperature given an adequate food supply. To maintain them, it is recommended to periodically add a small amount of bacterial broth, which serves as their primary food source. To obtain live material during the winter, the following technique can be employed: take a small sample of ice and silt from a pond where *Daphnia* and winter eggs are found in the summer, and transfer it to an aquarium in a warm room. Soon, *Daphnia* will hatch from the winter eggs and initiate parthenogenetic reproduction. In most cases, females hatch from these eggs, though altering external factors (such as temperature or illumination) can induce the appearance of males.

The Study of *Daphnia* anatomy is best performed on live specimens. If unavailable, preserved specimens may be used.

Required equipment: Microscope, dissecting microscope, hand lens, Glass slides and coverslips, test tubes, pipette, dissecting needles, wax for making wax "feet" on the slide, and a live culture of *Daphnia* in a small beaker.

Assignment. Observe the movement of live *Daphnia*. Prepare a temporary mount, examine, and sketch the anatomy of *Daphnia*.

Observations of Live Specimens. Place several *Daphnia* into a test tube filled with water and observe their movements using a hand lens or a dissecting microscope eyepiece. Their swimming pattern is jerky. The Organ of locomotion is the highly developed and branched second pair of antennae; *Daphnia* uses them like oars, and the strokes against the water propel the body upward and forward.

Preparing a Whole Mount. Place 2–3 *Daphnia* specimens (preferably smaller individuals) onto a glass slide using a pipette and cover with a coverslip. To avoid crushing the specimens, the coverslip should be propped up with wax "feet" beforehand. To do this, gently dab each of the four corners of the coverslip against a small piece of softened, hand-warmed wax so that a tiny speck remains on each corner. Then, lower the slide with the wax feet onto the slide, covering the drop of liquid containing the *Daphnia*.

Fig. 80 Internal anatomy of *Daphnia*:

1 — antennules; 2 — biramous antennae; 3 — simple eye; 4 — compound eye; 5 — excretory gland; 6 — Heart; 7 — thoracic limbs; 8 — abdomen; 9 — brood pouch.

Examining the Mount. Examine and sketch *Daphnia* under a dissecting microscope and at low magnification under a compound microscope. Typically, the collected material will consist exclusively of females; males appear only in the autumn.

When studying the anatomy of *Daphnia*, dim the microscope's field of view. Observations should be made on a specimen lying on its side. Note that the HEAD of *Daphnia* terminates in a beak-like helmet. Locate the branched antennae, the small antennules, the unpaired compound eye (which moves thanks to clearly visible Muscles), and the small, simple nauplius eye (which is not always visible) on the head (Fig. 80). The rest of the body is enclosed in a transparent bivalve carapace, open ventrally and fused with the body along the dorsal side. At the posterior end, the carapace extends into a spine.

The body of *Daphnia* is laterally compressed; both eyes (right and left) have fused into a single, complex unpaired eye equipped with a ganglion.

Examine the thoracic and Abdominal Regions visible through the transparent shell. The thoracic region bears five pairs of limbs, at the bases of which lie oval, sac-like swellings—the gill appendages. The limbs are densely fringed with fine setae. The thoracic legs remain in constant motion throughout the animal's life, facilitating both respiration and feeding. Observe the movement of these limbs. Bacteria, Algae, Protozoa, and fine organic particles suspended in the water are filtered out by the setae on the legs and gathered into a food bolus, which is then directed toward the Mouth. The abdomen lacks appendages; it curls ventrally and terminates in a caudal fork (furca). Two long setae extend dorsally from the abdomen. Examine the intestine with its paired hepatic caeca, visible through the abdominal wall. In a live *Daphnia*, the pulsating heart is clearly visible on the dorsal side just above the midgut. Blood Vessels are absent, and hemolymph circulates freely through the spaces between Organs. Flanking the gut are the Gonads (Ovaries or Testes).

Just below the Base of the antennae, locate the loop-shaped maxillary gland, which serves as the excretory organ of *Daphnia*.

On the DORSAL SIDE OF the female, between the body and the carapace, lies the brood chamber, where embryonic development takes place. Occasionally, eggs or developing juvenile *Daphnia* can be found inside the brood chamber.

Learn to distinguish males from females: males are smaller in size and lack a brood chamber.

Make a detailed drawing of the anatomy of *Daphnia*.

Order Copepoda

Copepods are predominantly planktonic crustaceans, much like cladocerans. Unlike the latter, however, copepods are abundant constituents of marine plankton. Common Examples include Cyclops, Diaptomus, and Calanus. Parasitic species are also known to occur.

Representatives of the genus Cyclops are used as the primary study organisms.

Cyclops (Cyclops sp.).

Materials and Equipment. Various species of the genus Cyclops inhabit the freshwater bodies of our country. All species within this genus are small, typically ranging from 1 to 5 mm in length. The species most frequently encountered in shallow water bodies include Cyclops puscus, C. albidus, C. strenuus, and others.

Specimens can be collected using a plankton net or simply by dipping water from a pond. Copepods thrive and reproduce readily in aquaria, particularly when fed with Ciliates.

Observations are conducted on living material. Under the microscope, both the external Morphology and internal organization are clearly visible.

Required supplies include: a microscope; a hand lens; glass slides and cover slips; a pipette; filter paper; a test tube; and a culture of live copepods in a water-filled container.

Task. Observe the movement of live copepods. Prepare a slide, examine, and sketch The structure of Cyclops.

Observations of Living Specimens. Using a pipette, transfer several crustaceans from the culture container into a test tube filled with water. Observe their locomotion using a hand lens.

Preparation of a Whole Mount. Draw up 2–3 specimens with a pipette, place them on a glass slide, and cover with a cover slip (positioning them so that one specimen lies with its dorsal side up and the other with its ventral side up). Remove excess water using a strip of filter paper (to restrict the movement of the copepod).

Examination of the Slide. Examine and sketch the slide under low magnification of the microscope (Fig. 81).

Fig. 81. Cyclops (Cyclops sp.):

1 — eye; 2 — antennules; 3 — cephalothorax;

4 — free thoracic segments; 5 — abdomen; 6 — egg sacs; 7 — caudal rami; 8 — gut.

Note that the body is segmented and consists of a cephalothorax and an abdomen terminating in caudal rami (furca). The rami bear feathered setae. A carapace is absent.

Locate the median (unpaired) eye.

The cephalothorax consists of five segments; the abdomen consists of four segments in the female and five in the male. Examine the anterior end of the cephalothorax, where two pairs of antennae are located: the first pair—the antennules—is significantly more developed than the second. The antennae are short and uniramous. The thoracic region bears four pairs of biramous appendages adapted for swimming (the 5th pair is vestigial), which gives the order its name. Gills are absent; respiration occurs across the entire body surface. The abdomen lacks any appendages other than the caudal rami.

Examine the intestine, which is visible through the translucent body wall. The mouth opening, situated on the ventral side of the head, is surrounded by mouthparts—jaws and maxillipeds—while the anus is located at the posterior end of the abdomen between the Branches of the caudal rami. A heart is absent. In the anterior third of the cephalothorax, an unpaired gonad (Testis in the male, Ovary in the female) is occasionally visible.

Find a female carrying fused, fertilized eggs within two egg sacs suspended laterally on the abdomen. These eggs hatch into copepod larvae known as nauplii.

Make a drawing of the slide. Recall which parasitic worms utilize Cyclops as an intermediate host.