Ecdysozoa

BIOL 108 Winter 2026

Topic 23: Ecdysozoa

  • Clade Bilateria encompasses three major groups:
    • Lophotrochozoa
    • Ecdysozoa
    • Deuterostomia
  • Ecdysozoans:
    • Most species-rich animal group.
    • Defined by molecular phylogeny composed of 8 distinct phyla.
    • Monophyletic group with uncertain relationships among the included phyla.
  • Two prominent phyla within Ecdysozoa:
    • Nematodes:
    • Most numerous animals on Earth.
    • Arthropods:
    • Most biodiverse animals.

Cuticle Functions

  • Ecdysozoan cuticle:
    • Acts as a barrier against desiccation.
    • Protects body from mechanical injuries.
    • Provides structural support through a tough exoskeleton, which shapes the body and acts as a rigid framework for muscle contraction.
  • Nature of the cuticle:
    • Non-living outer layer; does not grow or modify after initial formation.
    • Ecdysozoans undergo molting (shedding of cuticle) to grow in a stepwise manner.
  • Cuticle composition by phylum:
    • Arthropods: Chitin, a long-chain nitrogenous polysaccharide.
    • Nematodes: Crosslinked collagen and glycoproteins.

Structure of Ecdysozoan Cuticle

  • The cuticle is structured in three distinct layers:
    1. Endocuticle:
    2. Exocuticle:
    3. Epicuticle:
    • Thin, waxy, water-resistant outer layer without chitin.
  • Ducts:
    • Fine ducts from epidermis reach the top of the cuticle.
  • Moulting process:
    • Moulting allows growth, changes between developmental stages (e.g. larval to juvenile), regeneration of damaged tissues and limbs, and removal of external parasites.

Moulting in Ecdysozoans

  • Sclerotization:
    • Cuticle in arthropods may be sclerotized (hardened) via cross-linking chitin polymers with proteins, resulting in a rigid structure (exocuticle).
    • The endocuticle remains unsclerotized, allowing flexibility.
    • Sclerites (hardened plates) serve as protective armor or functional appendages.
    • Crustaceans exhibit limited sclerotization with a significant degree of biomineralization, primarily using calcite and Mg calcite to harden the exoskeleton.
  • Flexible components:
    • Unsclerotized portions of the cuticle allow for movement due to being made of unmodified chitin that is flexible and translucent.

Moulting Process Steps

  1. Apolysis:
    • Old cuticle separates from underlying epidermis, and the ecdysozoan becomes inactive.
  2. Secretion of moulting fluid:
    • Inactive enzymes secreted into the apolysis separation.
    • Epidermis grows through division, leading to increased surface area and folds.
  3. Proto-epicuticle formation:
    • A new thin protective layer (proto-epicuticle) is deposited on the epidermis.
  4. Activation of enzymes:
    • Chitinases and peptidases digest and reabsorb the old endocuticle for reuse in new cuticle formation; the exocuticle remains undigested.
    • New undifferentiated procuticle layer is deposited under the proto-epicuticle.
  5. Reabsorption of moulting fluid:
  6. Deposition of new epicuticle:
    • Completion of the new epicuticle before shedding occurs.
  7. Ecdysis:
    • The old exocuticle and epicuticle are physically shed, assisted by ingestion of air or water; the new cuticle appears wrinkled before expanding.
    • Expansion reveals a larger, freshly moulted ecdysozoan.
  8. Sclerotization:
    • The outer layer of the new procuticle undergoes sclerotization via secretions, completing cuticle development.
    • Duration for sclerotization can range from several minutes to many days.

Ecdysozoa Phylum Nematoda

  • Nematodes:
    • Also known as roundworms or threadworms.
    • They are the most numerous group of animals and have adaptations allowing them to inhabit diverse environments; crucial for various ecological and biological functions.
    • Size varies from 0.3 mm to over 8 meters, most species being less than 5 mm, with approximately 25,000 named species and possibly up to 100 times more.
  • Morphology:
    • Simple body plan—resulting from secondary simplification of a previously complex design.
    • Characteristics include:
      • Lack of segmentation, appendages, or eyes.
      • Cylindrical shape with tapered ends.
    • Nematodes possess a complete digestive tract but lack a circulatory system; they contain a hemocoel acting as their body cavity instead of a true coelom.

Nematode Movement and Reproduction

  • Movement:
    • Achieved through side-to-side contractions of longitudinal muscles in the body wall.
  • Reproduction:
    • Primarily sexual, with most species displaying distinct male and female morphologies, while some are hermaphrodites.
    • Internal fertilization and direct development occur.
  • Model organism:
    • Caenorhabditis elegans:
      • Used extensively in developmental biology due to its well-characterized biology.
      • Embryonic development exemplifies protostome development; for instance, male C. elegans has 1031 cells, and removal of one impacts adult development significantly.

Ecological Roles of Nematodes

  • Free-living nematodes:
    • Found in soil and freshwater/marine sediments; crucial for decomposition and nutrient cycling.
    • Feed on microorganisms, decaying organic matter, and other small organisms, assuming roles as predators or microbivores.
  • Parasitic nematodes:
    • Plant-parasitic nematodes implicated in substantial agricultural losses (~10% of annual crop yields).
    • Infect both foliar and root tissues; specialized structures called stylets enable penetration into plant cells to extract nutrients.
    • Example: Root-knot nematodes infecting various plants.

Nematode Parasites of Humans

  • Human hosts:
    • Approximately 50 nematode species can infect humans, including pinworms and hookworms, causing serious health issues.
    • Trichinosis:
    • Humans contract this condition from consuming undercooked meat (primarily pork) harboring juvenile Trichinella spiralis worms in muscle tissues of carnivorous and omnivorous hosts.
    • Nematodes also contribute to significant tropical diseases, such as lymphatic filariasis (elephantiasis) caused by filarial worms, which disrupt the lymphatic system and are transmitted by mosquitoes.

Specific Nematode-Related Diseases

  • Guinea Worm Disease:
    • Caused by Dracunculus medinensis.
    • Efforts for eradication show promise, demonstrated by significantly reduced cases worldwide (only 15 reported in 2024).