Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 33.4 Ecdysozoans Are the Most Species-Rich Animal Group
Ecdysozoa is a clade of protostome animals that grow by molting their external covering—a process called ecdysis. Though defined largely by molecular data, this group contains some of the most abundant and diverse animal phyla, including nematodes and arthropods, which dominate most ecosystems on Earth.
What Defines Ecdysozoans?
- Shed their cuticle through ecdysis to grow
- Comprise 8 phyla; most studied: nematodes and arthropods
Nematodes (Phylum Nematoda)
- Also called roundworms; found in soil, aquatic habitats, plants, and animals
- Covered in a collagen-based cuticle, shed during growth
- Have a complete digestive tract but no circulatory system
- Rely on fluid in the hemocoel to distribute nutrients
- Move by thrashing using longitudinal muscles
Ecological and Medical Roles:
- Free-living nematodes decompose matter and cycle nutrients
- Caenorhabditis elegans: model organism for developmental biology
- Parasitic nematodes:
- Trichinella spiralis causes trichinosis
- Some alter host gene expression or promote blood vessel growth
Arthropods (Phylum Arthropoda)
- Most diverse animal phylum; includes insects, crustaceans, arachnids
- Key features:
- Segmented body
- Jointed appendages
- Chitin-based exoskeleton
- Exoskeleton must be shed to grow
- Origin dates to Cambrian explosion (~535 MYA)
- Early forms like trilobites had unspecialized segments
Body Systems and Adaptations
- Open circulatory system with hemolymph
- Gas exchange varies:
- Gills (aquatic)
- Tracheal system (insects), book lungs (spiders)
- Sensory organs: eyes, antennae, mechanoreceptors
- Silk in spiders used for webs, escape, ballooning, etc.
- Specialized appendages for feeding, movement, defense, reproduction
Major Arthropod Lineages
- Chelicerates: spiders, scorpions, ticks, mites
- Use chelicerae to feed; lack antennae
- Book lungs for gas exchange
- Myriapods: millipedes and centipedes
- Millipedes: herbivores; 2 pairs of legs per segment
- Centipedes: carnivores; 1 pair per segment; venomous claws
- Pancrustaceans: crustaceans + insects
- Crustaceans: shrimp, crabs, barnacles, etc.
- Two pairs of antennae; use gills
- Insects (Hexapoda): over 1 million species
- Flight evolved ~359–252 MYA
- Metamorphosis:
- Incomplete: nymphs resemble adults
- Complete: larva → pupa → adult
- Crucial for pollination, decomposition, food chains, crop health
- Vectors of disease: malaria, sleeping sickness
- Crustaceans: shrimp, crabs, barnacles, etc.
In a Nutshell
Ecdysozoans grow by molting and include two of the most ecologically dominant animal groups—nematodes and arthropods. While nematodes thrive as recyclers and parasites, arthropods rule in species number, body plan diversity, and ecological impact. Insects, a branch of arthropods, have transformed Earth’s ecosystems through flight, metamorphosis, and plant-insect coevolution.