Ecdysozoans (Arthropoda)

Ecdysozoa Overview

• Defining Features:

  • Clade of protostomes characterized by molting (ecdysis).

  • Major phyla include Arthropoda and Nematoda.

• Key Evolutionary Adaptations:

  • Exoskeleton: Provides protection, support, and prevents desiccation. Composition mainly includes chitin and protein.

  • Segmented bodies: Enhances flexibility and specialization of body parts.

  • Jointed appendages: Facilitates diverse locomotion and various feeding strategies.

Phylum Arthropoda

• Diversity & Habitat:

  • Approximately 1.2 million described species, potentially millions more undescribed.

  • Found in various habitats: marine, freshwater, terrestrial, and aerial.

  • Important ecological roles include pollinators, decomposers, and predators.

• General Characteristics:

  • Bilaterally symmetrical and triploblastic.

  • Coelomate organisms often referred to as "jointed-foot."

  • Major advancements include a hardened exoskeleton, metamerism (segmentation), and an efficient nervous system.

External Morphological Features of Arthropods

1. Segmentation & Tagmata

• Arthropod bodies exhibit metameric segmentation, fused into distinct functional regions or tagmata:

  • Chelicerata: Prosoma (cephalothorax) & Opisthosoma (abdomen).

  • Crustacea: Cephalon (head), thorax, & abdomen.

  • Hexapoda (Insects): Head, thorax, & abdomen.

2. Exoskeleton

• Structure:

  • Composed largely of chitin, providing structural support.

• Functional Implications:

  • Acts as protection from predators and environmental stressors.

  • Prevents water loss, especially in terrestrial species.

  • Serves as a muscle attachment site:

  • Requires molting (ecdysis) for growth and is regulated by ecdysteroids.

3. Jointed Appendages

• Adaptations for various functions:

  • Uniramous: Single-branched, characteristic of Hexapoda.

  • Biramous: Two-branched, found in Crustacea and plays roles in locomotion and feeding.

Internal Morphological Features of Arthropods

1. Open Circulatory System

• Structure:

  • Hemolymph is pumped by a dorsal heart and flows through body cavities.

• Functional Implications:

  • Provides nutrient transport but lacks vessels that facilitate direct blood flow, using spaces between tissues instead.

2. Respiratory System

• Marine Arthropods: Primarily use gills for gas exchange.

• Terrestrial Arthropods (Hexapoda):

  • Tracheal system with spiracles, allowing air to diffuse directly to tissues.

  • Spiders (chelicerata) possess book lungs with stacked lamellae for increased surface area.

3. Nervous System

• Structure:

  • Comprises a ventral nerve cord and cerebral ganglia.

  • Highly developed sensory organs (compound eyes, antennae).

Classes within Phylum Arthropoda

Class Chelicerata

• Diversity: Includes spiders, scorpions, ticks, and horseshoe crabs (~70,000 species).

• Habitat: Mostly terrestrial, a few marine species.

• Characteristics:

  • Chelicerae adapted for feeding, with book lungs or tracheae for respiration.

  • Most are predators with two body regions (prosoma & opisthosoma).

Class Crustacea

• Diversity: Approximately 65,000 species (e.g., crabs, shrimp).

• Habitat: Largely marine, some freshwater, few terrestrial.

• Characteristics:

  • Three tagmata (cephalon, thorax, abdomen) and biramous appendages.

  • Nauplius larvae as a defining characteristic during development.

Class Hexapoda

• Diversity: ~5.5 million species, making it the largest group of animals on Earth.

• Habitat: Dominantly terrestrial and freshwater, few marine.

• Characteristics:

  • Body plan featuring three tagmata and six walking legs.

  • Exhibits variations in metamorphosis: hemimetabolous (incomplete) and holometabolous (complete).

Summary

• Ecdysozoans are remarkably successful, adapting through ecdysis, segmentation, and jointed appendages.

• Arthropods dominate ecosystems, both in species richness and ecological roles. Current research focuses on their evolution, behavior, and effects on ecosystems.