ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 12. PHYLUM ARTHROPODS (Arthropoda)

2. Class Crustaceans (Branchiata or Crustacea)

Crustaceans have successfully colonized virtually all aquatic habitats, ranging from deep oceans and seas to tiny puddles. In fact, marine plankton consists of 90% crustaceans. Only a few species have adapted to terrestrial life (such as woodlice and land crabs), yet they are still restricted to damp, humid environments. Over 40,000 species have been described.

Carcinology (from Greek karkinos, meaning crab, and logos, study) is the branch of zoology dedicated to The Study of crustaceans.

Crustaceans vary in size from a few millimeters to 80 cm. Their body is divided into three tagmata (regions). The segments of each tagma may be distinctly separated or fused to varying degrees.

The HEAD bears 5 pairs of modified appendages: 2 pairs of antennae (antennae and antennules), which serve as Organs of Touch and chemical sense, and 3 pairs of mouthparts (mandibles, maxillae, and maxillipeds) that grind and filter food, directing it toward the Mouth. The head also features a pair of compound facet eyes, although some species possess only simple eyes. The thoracic region carries anywhere from 2 to 60 pairs of appendages, varying in shape according to their specific Functions.

Crustaceans breathe either through gills or across their entire body surface. Lateral gill covers, formed by the carapace, protect the gills from potential damage.

Typical representatives include crayfish, crabs, lobsters, spiny lobsters, shrimp, daphnia, and cyclops.

Crayfish inhabit rivers and lakes with relatively clean Water. Their coloration is determined by pigments in the integument, predominantly carotenoids. One of these is astaxanthin, which has a red color. When bound to Proteins, it forms brownish tones. When exposed to high temperatures, these complexes break down and astaxanthin is released, which is why crayfish turn red when cooked (giving rise to the saying "as red as a boiled crayfish").

Crayfish are benthic (bottom-dwelling) organisms; they hide in burrows and various shelters during the day, emerging at dusk and night to hunt. Crayfish tend to avoid sandy and rocky bottoms, preferring areas with muddy but non-flowing substrates.

Among the freshwater crayfish of Ukraine, the narrow-clawed and wide-clawed crayfish are the most common.

The head and Thorax of crayfish are immovably fused to form a cephalothorax, covered dorsally and laterally by a continuous shell called the carapace. A transverse groove on the carapace marks the boundary between the head and thorax.

The head of a crayfish points forward and terminates in a sharp spine, flanked by a pair of compound eyes on movable stalks. The head bears two pairs of antennae:

✵ the first pair — short, or antennules (forked) — function as organs of smell;

✵ the second pair — long, or antennae — function as organs of touch.

An Organ of Equilibrium, or statocyst (Figs. 40, 41), is located in the basal segment of the short antennae. It appears as a small pit lined with sensory hairs (setae). Because this pit opens directly to the external environment, tiny sand grains easily enter it, acting as "auditory stones" or "statoliths".

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Fig. 40. Diagram of a crayfish statocyst (after Dogel, 1981).

During the molting of the chitinous exoskeleton, the statoliths from the open pits are shed, and the animal captures a fresh supply of sand grains using its claws or by burying its head in the sand.

Fig. 41. Crayfish antennule with statocyst (after Dogel, 1981):

1 and 2 — forked antenna; 3 — basal segment; 4 — statocyst opening;

5 — sensory hairs; 6 — sand grains; 7 — nerve.

The mouth apparatus is located on the underside of the head and is surrounded by mouthparts (modified appendages) consisting of a pair of mandibles, two pairs of maxillae, and three pairs of maxillipeds.

The thorax consists of 8 segments bearing 8 pairs of thoracic appendages, of which the 3 anterior pairs are maxillipeds (involved in feeding), while the remaining 5 pairs are walking legs. In crayfish, the first pair of legs is equipped with claws.

The abdomen consists of six segments and an anal plate bearing the anus. Each segment carries a pair of biramous abdominal appendages (6 pairs), with the last pair being broadened.

Crayfish feed on worms, Mollusks, aquatic insect larvae, tadpoles, and plant matter. They readily consume charophyte Algae, which are rich in calcium. Crayfish act as natural scavengers in aquatic ecosystems by feeding on carrion. Food, ground by the mouthparts, enters the Pharynx and then passes into the Esophagus. From the esophagus, food enters The Stomach in portions, which consists of two sections (Fig. 42):

✵ the gastric mill, where food is ground

✵ the gastric mill, where food is ground down by means of chitinous plates; the walls of this section are significantly thickened due to well-developed Muscles that operate the chitinous plates;

✵ the cardiac/pyloric filter chamber, where food is filtered.

Only fine food particles pass into the midgut after filtration, where primary Digestion AND ABSORPTION take place. A large, bilobed digestive gland—the Liver (Fig. 42)—opens into the midgut, producing digestive juices that contain Enzymes that break down food into simpler compounds, which are then absorbed through the intestinal walls into the Blood.

The excretory System of the river crayfish is represented by a pair of green glands (named for their coloration), which open externally near the Base of the long antennae (Fig. 42).

Fig. 42. Internal Structure of the river crayfish:

1 — stomach muscles; 2 — green glands; 3 — stomach; 4 — Mandible muscles;

5 — liver; 6 — Testes; 7 — gills; 8 — Heart; 9 — intestines.

River crayfish breathe oxygen dissolved in water using gills (Fig. 42)—thin-walled outgrowths at the base of the thoracic legs. Externally, they are covered by a fold of the carapace that protects the gill cavity.

The Circulatory system is open. The Heart of the river crayfish (Fig. 42) is located on the DORSAL SIDE OF the body in the thorax and appears as a pentagonal sac with three pairs of ostia, through which hemolymph enters the heart from the body cavity. The blood of the river crayfish is colorless, whereas in other crustaceans it may be reddish due to dissolved respiratory pigments like Hemoglobin, or even bluish (e.g., in certain crabs) because it contains a specialized copper-containing substance that binds oxygen.

The Nervous system consists of a paired supraesophageal ganglion, a circumesophageal nerve ring, and a ventral nerve cord.

River crayfish are dioecious and exhibit external Fertilization. The female carries fertilized eggs (roe) on her abdominal appendages for 5–6 months. In spring, small juvenile crayfish hatch, closely resembling the adults (direct development). Young crayfish molt multiple times, grow, and reach sexual maturity in their third (males) or fourth (females) year of life. The lifespan of river crayfish is up to 20 years, during which they molt once or twice a year (in spring and autumn).

Marine crustaceans include lobsters, spiny lobsters, and crabs. Standing or slow-flowing freshwater bodies are inhabited by Daphnia and Cyclops.

Ecological role of crustaceans.

1. Cladocerans act as natural biofilters by purifying water of suspended organic particles and Bacteria.

2. Crayfish serve as a food source for predatory fish, waterfowl, and certain mammals.

3. Daphnia, Cyclops, and water hoglice are important prey for fish.

4. Crustaceans include fish parasites (carp lice), intermediate hosts of the broad tapeworm (Cyclops), and parasites of marine mammals.

Significance of crustaceans in human life.

Positive impact.

1. River crayfish, as well as lobsters, spiny lobsters, crabs, and shrimp, are of commercial importance.

2. Daphnia and cyclops are used to feed the juveniles of sturgeon, salmon, and other commercial fish species, as well as to produce dry food for aquarium fish.

Negative.

1. Copepods such as cyclops and diaptomus can serve as intermediate hosts for helminths.

2. Sessile barnacles cause significant economic damage to shipping by fouling the hulls of vessels.

Conservation of Crustaceans.

The population numbers of certain crustacean species have been declining sharply in recent years. Therefore, The Development of conservation measures is essential. This is particularly true for freshwater inhabitants, which face severe anthropogenic pressure, notably freshwater crayfish. The Kamchatka crab is also in need of protection.