Lecture Notes on Invertebrate Zoology

Content Module 3. Crustaceans. Tracheates. Insect Systematics and Subphylum Chelicerata

Lecture Topic 11. Crustaceans. Structural Features of Higher Crustaceans

The phylum with the greatest diversity. Over 1.5 million species are known. Arthropods have adapted to a wide range of living conditions: they inhabit seas and oceans, the Water Column, the bottom, soil, fresh and saline water bodies, and land. The size and body shape of crustaceans vary significantly. They range from tiny

planktonic forms, up to 1 mm in length, to large benthic lobsters and crabs reaching up to 80 cm long, while the Japanese spider crab, for instance, has a leg span of 1.5-2 m. Sessile forms with a calcareous shell and parasitic crustaceans of animals and humans are heavily modified in appearance. The body consists of HEAD, thoracic, and Abdominal Regions. Body segments bear a pair of biramous appendages. Originally, crustacean appendages are multifunctional, performing several roles: locomotion, Respiration, and assisting in feeding. However, in most crustaceans, a morphofunctional differentiation of appendages is observed. Appendages consist of individual podomeres movably connected by joints. The head of crustaceans is composed of the prostomium (acron) and four segments. The thoracic and abdominal regions of crustaceans can consist of a variable number of segments (from 5-8 up to 50). In some crustaceans, thoracic appendages are multifunctional, as in branchiopods, performing swimming, respiratory, and filter-feeding Functions; in others, a division of labor is observed. For example, in the crayfish, out of the eight pairs of thoracic legs, the first three pairs are biramous maxillipeds (used for holding and filtering food), and the next three pairs are uniramous walking and simultaneously grasping legs with a claw at the end. At the same time, all thoracic legs of the crayfish bear gills at their base and additionally perform a respiratory function. The abdomen consists of several segments and a telson, and is usually devoid of appendages. Nevertheless, most higher crustaceans possess biramous appendages on the abdomen. In shrimp, they serve a swimming function, while in isopods, they act as respiratory Organs.

The chitinous integument of many crustaceans is impregnated with calcium carbonate, providing high rigidity. Soft membranes ensuring mobility are located between fused body regions or segments, as well as between the podomeres of legs and appendages. When segments fuse into regions, a common dorsal chitinous carapace is formed. The cuticle serves as an exoskeleton to which Muscles attach. The musculature consists of separate Muscle bundles with striated Structure. The body cavity is a mixocoel, circulating hemolymph, which functions simultaneously as both coelomic fluid and Blood.

The Digestive System consists of three sections: the ectodermal foregut, entodermal midgut, and ectodermal hindgut. Each of these sections differentiates distinctively depending on the feeding type. The excretory system of arthropods is represented by modified coelomoducts — coxal glands, or specialized organs that evolved within the phylum, namely Malpighian tubules. The Circulatory system is open and partially reduced. Arthropod respiratory organs are quite diverse: ranging from gills in aquatic animals to book Lungs and tracheae in terrestrial ones. The Nervous system is similar in structure to that of Annelids, consisting of a supraesophageal ganglion, circumesophageal connectives, and a ventral nerve cord. Sense Organs are well-developed in most arthropods (Touch, chemical sense, equilibrium, Vision). Arthropods reproduce exclusively sexually. Most are dioecious, though hermaphrodites also occur. Sexual Dimorphism is frequently observed. In males, antennae are sometimes modified into grasping organs. Females of certain crustaceans are often easily recognized by attached egg sacs. In male crayfish, the 1st and 2nd pairs of abdominal legs function as copulatory organs. Gonads are paired, sometimes partially fused. Genital ducts and openings are paired. In male crayfish, 1-2 pairs of abdominal legs are transformed into copulatory tubes. During the breeding season, they fill with sperm flowing from the male's genital openings. Fertilization is internal. In most Representatives of the order, development occurs with metamorphosis. For some shrimp, nauplius and metanauplius larvae are characteristic. In many forms of crabs, a zoea larva hatches from the egg, whereas in crayfish and some freshwater higher crustaceans, the entire embryonic development takes place within the egg, hatching into fully formed miniature juveniles.