Lecture Notes in Invertebrate Zoology

Content Module 3. Crustaceans. Tracheates. Insect Systematics and the Phylum Chelicerata

Lecture Topic 10. General Characteristics of the Phylum Arthropoda. Crustaceans. Structural Features of Lower Crustaceans

Origin. Common body structural features with Annelids. Specific Structural Features of Arthropods. Heteronomous segmentation and formation of jointed appendages. Exoskeleton and striated Muscles. Adaptations for widespread natural distribution. Division into subphyla. Branchiates as aquatic animals.

CHARACTERISTICS OF THE Class Crustacea. Body segmentation, Structure of appendages in relation to their Location and function. Nervous system and Sense Organs. Respiratory and circulatory systems. Body cavity—mixocoel. Digestive and excretory systems—STRUCTURE OF THE Stomach, metanephridia. Reproductive System, reproduction, and development. Molting. Structural features of lower crustaceans: subclasses Branchiopoda, Maxillopoda, and Ostracoda. Main representatives and their significance in fisheries as feed organisms and disease agents.

The class Branchiopoda inhabits A wide variety of continental Water bodies, including cave waters, temporary pools, saline lakes, and estuaries. All of them are free-living organisms. Over 800 species have been described. Order Branchiopoda.

The degree of HEAD cephalization varies among different representatives, ranging from a protocephalon to a fully segmented head; however, the head never fuses with the thoracic segments. The tagmata show relatively little differentiation from one another; the thoracic appendages are uniform in structure, leaf-like, often unsegmented, and serve swimming, filtering, and respiratory Functions. The Heart is shaped like a longitudinal vessel located on the dorsal side, extending from the posterior end of the abdomen almost to the head, and features up to 18 pairs of ostia. It lacks distinct chambers, but contains several metamerically arranged Valves that direct the hemolymph flow in only one direction—toward the head. The Nervous System is ladder-like, with widely spaced ventral nerve cords. Development involves metamorphosis: a nauplius hatches from the egg. Order Notostraca (tadpole shrimp). Notostraca inhabit shallow, standing freshwaters, particularly temporary ponds. Of the 15 known species, two are found in Ukraine. Body lengths do not exceed 4—5 cm. Order Cladocera (water fleas). These are predominantly freshwater, occasionally marine planktonic animals with body lengths up to 2 cm. About 400 species are known, occurring in freshwaters across Ukraine. Order Anostraca (fairy shrimp). Representatives of the order Anostraca live in various continental water bodies, including cave waters, temporary pools, saline lakes, and estuaries. Over 200 species have been described, 12 of which are found in Ukraine. The body length of anostracans does not exceed 2 cm.

Class Maxillopoda. Maxillopods are distributed worldwide. They inhabit both marine and fresh waters, living on the bottom and in the water Column at various depths, as well as in capillary water between sand grains. Alongside free-living forms, there are species with sessile and parasitic lifestyles. The body of maxillopods consists of a syncephalon, a Thorax, and an abdomen ending in a telson. The syncephalon comprises the head and the first thoracic segment, with the latter bearing maxillipeds. The antennules are uniramous and elongated; the antennae and mandibles are biramous, both pairs of maxillae are uniramous and multi-jointed; an unpaired labrum is present. The maxillipeds vary in structure (uniramous or biramous). The thorax consists of six, rarely four or five segments, the first of which is incorporated into the syncephalon. The thoracic legs are predominantly biramous and perform locomotor functions; they are adapted for swimming or generating water currents. The abdomen has a variable number of segments (3–5) and lacks appendages; the telson bears various articulated outgrowths ("caudal appendages"). In most species, only the unpaired (naupliar) eye is developed; the Circulatory system is highly reduced. A nauplius hatches from the egg.

Subclass Cirripedia comprises highly specialized sessile filter-feeding or parasitic forms. Barnacles inhabit exclusively marine environments, with over 1,000 extant species known. The fauna of the Black Sea contains no more than 10 species, and the Sea of Azov has only one. The size of cirripeds ranges from a few millimeters to 2—5 cm. The antennules and the entire anterior region of the head in barnacles are transformed into an attachment organ: in some species, a long fleshy stalk, in others, a flat broad disk. The antennae and compound eyes atrophy, and other head appendages become simplified, surrounding the oral opening located on the posterior part of the head. In some species, the head region disappears almost entirely (see below). The body of barnacles is enveloped in two Skin folds (mantle folds), similar to those of Mollusks.

The mantle of non-parasitic forms secretes calcareous plates outwardly, forming a protective shell. The soft body is covered by the mantle; specifically, the thoracic region in members of the order Thoracica possesses six segments with biramous appendages, in members of the order Acrothoracica — four segments with uniramous appendages, while Rhizocephala lose segmentation and appendages altogether. The abdomen and its appendages are reduced. Cirripeds are generally hermaphroditic, but some of them (such as *Alcippe*) are gonochoristic, with males being extremely small and residing within the mantle cavity of the female. Even more interesting are other Cirripedia (family Scalpellidae, certain species of the genus *Balanus*), in which dwarf "complementary" males occur alongside hermaphroditic individuals. Finally, there are species whose normal individuals can function as either females or hermaphrodites, both of which host dwarf males. The environmental cues determining whether a larva develops into a female, a male, or a hermaphrodite remain unknown. A typical nauplius hatches from the barnacle egg. After several molts, it develops into a metanauplius stage, which is followed by a stage unique to cirripeds — the cyprid larva. It possesses a bivalve cuticular shell enclosing the entire body, four-segmented antennae equipped with adhesive disks and cement glands, simplified mandibles and maxillae, and six pairs of thoracic biramous swimming legs. The presence of the bivalve shell makes the larva resemble ostracods of the genus *Cypris*, hence its name. The cyprid larva swims using its thoracic legs for some time and then attaches to a suitable substrate via its antennules. Its subsequent development proceeds differently among various taxonomic orders.

Copepods inhabit all types of freshwater and marine water bodies. Regarding their feeding habits, they can be filter feeders, predators, or herbivores; there are planktonic forms as well as benthic dwellers. Nearly 9,000 species have been described. Over 20% of them lead a parasitic lifestyle. The body ranges from 0.1 to 32 mm in length, consisting of a syncephalon, a five-segmented thorax, and a three- to four-segmented abdomen ending in a telson with a furca. A naupliar eye is present on the head. The antennules are elongated, sometimes longer than the body, and function in swimming and sensation (chemoreception, Touch); they are uniramous. The antennae are shortened and typically biramous. The mandibles feature masticatory Teeth. In some marine species, these teeth are capped with silicon dioxide crowns, increasing their durability. There are two pairs of maxillae and a pair of maxillipeds. In filter-feeding forms, the antennae and mouthparts generate water currents and act as a filtering apparatus. The five pairs of thoracic legs are biramous and swimming; in males of some species, the last pair is modified to attach spermatophores to the female genital pores (which are located on the first abdominal segment in both sexes). The furca often consists of several branches covered with long setae, which enhances floating in water. The Respiratory system is absent; a heart is present only in certain representatives as a small vesicle. Copepods are gonochoristic. Sexual Dimorphism is quite pronounced: males are smaller than females, and there are structural differences in the antennules and posterior thoracic legs; both sexes differ particularly sharply in parasitic species. Parthenogenesis is known in some freshwater forms. Fertilization is spermatophore-mediated. Branchiurans are a small group (about 130 species) of fish ectoparasites; 3 species have been found in Ukraine. The body is dorsoventrally flattened and divided into anterior and posterior regions. The anterior region is formed by the syncephalon, covered by a carapace bearing a pair of compound eyes and 1—3 naupliar ocelli dorsally. The antennules and antennae are shortened. The antennules bear a hook-like appendage for attaching to the fish. The mandibles are modified into a piercing-sucking proboscis; the first maxillae are transformed into a fixing organ — a sucker; the second maxillae are uniramous and also serve for attachment; a pair of maxillipeds is present. The posterior half of the body consists of the thorax and abdomen; all its segments are movably connected. The thorax comprises 4 segments, each bearing a pair of biramous swimming appendages; the abdomen consists of two limbless segments and a telson. The body size of branchiurans does not exceed 10 mm. Circulatory and respiratory systems are absent; the midgut possesses numerous blind caeca that fill with the host's Blood. Branchiurans are gonochoristic; sexual dimorphism is not prominent.