BIOL 108 Topic 23 Ecdysozoa PDF

Topic 23: Ecdysozoa Clade Bilateria contains Lophotrochozoa, Ecdysozoa, and Deuterostomia. Ecdysozoans are the most speciesrich animal group. BIOL 108 Winter 2024 © 2024 Neil Harris − Two prominent ecdysozoan phyla:  Nematodes are the most numerous animals.  Arthropods are the most biodiverse anim...

Topic 23: Ecdysozoa Clade Bilateria contains Lophotrochozoa, Ecdysozoa, and Deuterostomia. Ecdysozoans are the most speciesrich animal group. BIOL 108 Winter 2024 © 2024 Neil Harris − Two prominent ecdysozoan phyla:  Nematodes are the most numerous animals.  Arthropods are the most biodiverse animals. − Clade Ecdysozoa is defined by molecular phylogeny and consists of 8 phyla.  Ecdysozoa is a monophyletic group, although the relationships between phyla within the clade are uncertain. Fig 33.2 1 Chitin (WC) Ecdysozoan cuticle Ecdysozoans are covered by a cuticle that is periodically moulted. − Ecdysozoan shared, derived trait. The cuticle is a tough, non-living, outer layer of organic material deposited outside the epidermis. − The epidermal cells secrete the cuticle. − The cuticle is composed primarily of chitin, a long-chain nitrogenous polysaccharide. BIOL 108 Winter 2024 © 2024 Neil Harris Cuticle functions: − Acts as a barrier against desiccation and mechanical injury. − Provides structural support, forming a tough external skeleton (exoskeleton) that shapes the body and offers a framework for muscle attachment. The cuticle is a non-living, outer layer with no capacity for growth or modification. − Ecdysozoans shed the entire cuticle to grow through a process called ecdysis (=moulting). 2 Molting cicada (WC) Ecdysozoan cuticle BIOL 108 Winter 2024 © 2024 Neil Harris The cuticle consists of three layers: − Endocuticle (endo = within) Procuticle (chitin) − Exocuticle (exo = outside) − Epicuticle (epi = upon): thin, waxy, water-resistant outer layer (no chitin). − Fine ducts run from the epidermis to the top of the cuticle. exocuticle endocuticle cuticle procuticle epicuticle Although the cuticle composition varies among ecdysozoans, the process of ecdysis is similar. − Ecdysis allows ecdysozoans to grow incrementally and facilitates changes between developmental stages, such as metamorphosis, e.g. larval to juvenile, and growth of wings. − Ecdysis allows damaged tissue and missing limbs to regenerate and removes external parasites. epidermis 3 Ecdysozoan moulting (ecdysis) BIOL 108 Winter 2024 © 2024 Neil Harris WC WC The cuticle is often sclerotized (scler = hard) in arthropods. − Sclerotization cross-links the chitin polymers with proteins to create a hardened and rigid cuticle. − Only specific regions of the cuticle are sclerotized. Dorsal sclerites and unsclerotized lateral cuticle of a centipede (above) and spider (below)  Called sclerites when shaped like plates. − Sclerotized cuticles are used as armour plates and hardened appendages that function as tools or wings. The cuticle between sclerites remains unsclerotized − The unsclerotized cuticle contains unmodified chitin that is translucent, pliable, and flexible, allowing for movement. bugguide.net Cockroach posterior sclerites https://edis.ifas.ufl.edu/in915 4 Ecdysozoan moulting (ecdysis) exocuticle endocuticle cuticle epicuticle BIOL 108 Winter 2024 © 2024 Neil Harris epidermis The moulting process begins internally before external signs of cuticle shedding appear: 1. The ecdysozoan becomes inactive and the old cuticle separates from the underlying epidermis. − Apolysis (apo = away from, lys = loosening) 2. Inactive enzymatic moulting fluid is secreted into the separation space, and the epidermis grows (cell proliferation and enlargement), increasing the epidermal surface area (folded). 5 Ecdysozoan moulting (ecdysis) exocuticle endocuticle cuticle epicuticle BIOL 108 Winter 2024 © 2024 Neil Harris epidermis 3. A new proto-epicuticle, a thin protective layer, is deposited on the surface of the epidermis. 4. Moulting fluid enzymes (chitinases and peptidases) are activated: − Old endocuticle digested: digested materials absorbed by the epidermis and re-used to build the new cuticle. − Sclerotized exocuticle is not digested. − A new undifferentiated procuticle layer is deposited beneath the proto-epicuticle, which protects the newly deposited procuticle from being digested. 5. Moulting fluid reabsorbed. 6 Ecdysozoan moulting (ecdysis) exocuticle endocuticle cuticle epicuticle BIOL 108 Winter 2024 © 2024 Neil Harris epidermis 6. The deposition of the new epicuticle is completed. 7. The old exocuticle/epicuticle is moulted. − The ecdysozoan may swallow air or water to help split the old cuticle. Newly molted and incompletely sclerotized beetle − The new cuticle expands (newly moulted ecdysozoan is larger). − The surface area of the new cuticle > old cuticle (new cuticle initially wrinkled). 8. The outer layer of the new procuticle is sclerotized by secretions via cuticle ducts to form the exocuticle above the unsclerotized endocuticle. − Sclerotization can take between several minutes to several days to complete. 7 Ecdysozoa Phylum Nematoda BIOL 108 Winter 2024 © 2024 Neil Harris Nematodes: roundworms or threadworms (nemato = thread). − Nematodes are the most numerous animals. − They are found in most aquatic habitats, soil, plant tissues, and animal body fluids and tissues. − Display a wide size range from 0.3 mm to over 8 meters, with most species being