Lecture 15: Echinoderms

Definition: Deuterostomes are a clade of triploblastic animals characterized by their mode of development where the mouth forms secondarily to the anus.
Key Features:
    - Triploblastic organization.
    - Radial cleavage during the early stages of embryonic development.
    - Inclusion in the larger clade called Ambulacraria, which includes Echinodermata (such as sea stars) and Hemichordata.

Deuterostomes: These animals develop the anus first during embryogenesis.
Protostomes: In contrast, the mouth forms from the first opening in the embryo.
Classification Confusion: Some organisms previously grouped with deuterostomes were reconsidered, leading to distinctions within ecdysozoa and spiralia clades.

Definition: A phylum consisting of marine animals featuring radial symmetry and an endoskeleton formed from calcareous plates.
Importance: Echinoderms are significant as they can act as keystone species, which are organisms critical for maintaining the structure of ecological communities.

Asteroid (Sea Stars)
- Radial symmetry, typically with five arms.
- Predatory behavior: evert their stomachs to engulf prey, typically bivalves.
Ophiuroidea (Brittle Stars)
- Differentiated primarily by their long, slender arms.
- Organisms can be predatory or suspension feeders.
Crinoidea (Sea Lilies)
- Characterized by feathery arms, resembling flowers.
Echinoidea (Sea Urchins and Sand Dollars)
- Have globular or flattened bodies with spines.
- Utilize Aristotle's lantern for feeding.
Holothuroidea (Sea Cucumbers)
- Soft-bodied echinoderms with a leathery texture, demonstrating secondary bilateral symmetry.

Ambulacraria includes both echinoderms and hemichordates, while Ectodermyzoa includes groups such as arthropods and nematodes, highlighting the evolutionary complexities.

Pentaradial Symmetry: Echinoderms typically exhibit a five-part symmetry, although their larval forms display bilateral symmetry.
Endoskeleton: Comprised of ossicles, providing structural support contrasting with the exoskeleton of arthropods made of chitin.
Water Vascular System: A unique hydraulic system critical for locomotion and feeding.
    - Structure includes:
        - Sieve Plate: Entrance to the system for seawater.
        - Central Ring Canal: Distributes water to radial canals leading to tube feet.
        - Ampulla: Muscular sac regulating tube foot movement.
        - Tube Feet: Extend and retract for movement and feeding.
Digestive System: Complete digestive tract with two stomachs:
- Cardiac Stomach: Muscular portion for digestion.
- Pyloric Stomach: Receiving food, less muscular than the cardiac stomach.
Reproductive System: Most echinoderms exhibit dioecious reproduction, with external fertilization producing planktonic larvae, which serve as a food source for other organisms.

Predatory Behavior: Many echinoderms prey on other marine invertebrates, notably by opening bivalve shells and externally digesting their prey.
Suspension Feeding: Used by brittle stars and others for capturing small food particles suspended in the water.

Lack centralized nervous systems or cephalization; instead, they have a diffuse nerve net.
Possess photoreceptors on their arms, allowing light detection but not image formation.

Echinoderms contribute significantly to marine biodiversity and are essential for ecological research due to their roles as predators and prey in marine ecosystems.