Echinodermata Lecture Notes

Echinodermata Overview

  • Phylum Echinodermata
  • Superphylum Deuterostomia

General Characteristics of Echinoderms

  • Examples: Sea stars, sea urchins, sea cucumbers, sea feathers, etc.
  • Habitat: Exclusively marine species.
  • Tissue Structure: Triploblastic, eucoelomate (true coelom).
  • Developmental Characteristics:
    • Classified as deuterostomes, meaning that the first opening in embryonic development becomes the anus, while the mouth is formed secondarily.
    • First appearance of an endoskeleton in the kingdom Animalia.

Morphology of Echinoderms

  • Endoskeleton:
    • Located underneath the epidermis, composed of calcareous ossicles formed from mesoderm tissue.
    • Ossicles can be rigidly attached or can have a hinged connection.
  • Symmetry:
    • Adults exhibit radial symmetry.
    • Larvae exhibit bilateral symmetry.
    • Cephalization (development of a head) is only present in larval states.
  • Body Plan:
    • Each arm or section contains digestive glands, gonads, nerve cord, and tube feet.
    • Digestive system is complete with mouth on ventral side and anus on dorsal side.

Embryological Development and Symmetry

  • Echinoderms are the most ancestral of all deuterostomes.
  • Ancestors were likely sessile, filter-feeding animals. Expectation for symmetry:
    • Larval body plan: Bilateral Symmetry
    • Adult body plan: Radial Symmetry.
  • Hox Genes control the transition from bilateral to radial symmetry:
    • Reflects the complexity of body plans and adaptations to different lifestyles (e.g., suspension feeding).

Water Vascular System

  • Description: A central coelomic cavity that transports seawater within the organism.
  • Components:
    • Madreporite (sieve plate): Water enters through this structure.
    • Stone Canal: Connects the madreporite to the ring canal.
    • Ring Canal: Circulates water around the central part of the body.
    • Radial Canals: Extend from the ring canal into each arm.
    • Ampullae: Connected to radial canals, controlling tube feet movement.
    • Contraction of ampullae extends tube feet; release retracts tube feet.
  • Functional Roles:
    • Movement coordination.
    • Gas exchange through tube feet (oxygen enters and carbon dioxide exits).

Digestive and Excretory Systems

  • Digestive Process:
    • Mouth on ventral side leads to cardiac stomach, which then continues to pyloric stomach for further digestion and distribution of nutrients.
    • Each arm contains digestive glands to assist in nutrient breakdown.
  • Waste Management:
    • Metabolic waste is expelled via the water vascular system out through the tube feet.
    • No osmoregulation is necessary as body fluid composition resembles seawater.
    • Larvae are equipped with metanephridia excretory systems.

Reproductive Strategies

  • Reproduction:
    • Echinoderms are generally dioecious, resulting in high genetic diversity.
    • External fertilization occurs through broadcast spawning.
    • Most produce bilateral larvae.
    • Some classes, like Crinoidea, Asteroidea, Ophiuridea, can also reproduce through fragmentation.

Classification of Echinoderms

  • Five Major Classes of Echinodermata:
    • Class Asteroidea (Sea Stars)
    • Characteristics: Five thick arms extending from a central body; can be predators/scavengers; use tube feet for movement and feeding.
    • Class Ophiuroidea (Brittle Stars)
    • Characteristics: Five long, thin arms; much harder endoskeleton; possess greater flexibility due to hinged ossicles; feeding through mucus and arms.
    • Class Echinoidea (Sea Urchins and Sand Dollars)
    • Characteristics: Spherical shape; ossicle lattice forms a test; utilize Aristotle’s Lantern for feeding; tube feet for movement and burrowing.
    • Class Crinoidea (Sea Lilies and Sea Feathers)
    • Characteristics: Filter feeders with upward-facing mouths; most are sessile, some can be motile; arms have feather-like modifications.
    • Class Holothuroidea (Sea Cucumbers)
    • Characteristics: Secondarily bilaterally symmetrical; elongated bodies; deposit feeders; have specialized tube feet and a unique respiratory tree.

Unique Features of Echinoderm Classes

  • Class Crinoidea:
    • Oldest surviving group of echinoderms, predominantly sessile; arms modified for increased surface area.
  • Class Asteroidea:
    • Exhibit predatory behavior; ability to evert stomach for external digestion; radial symmetry with cooperation in movement.
  • Class Ophiuroidea:
    • Possess complex sensory systems for light detection due to their predatory nature; movement is characterized by jerky motions.
  • Class Echinoidea:
    • Sea urchins use movable ossicles for feeding; sand dollars employ tube feet for burrowing.
  • Class Holothuroidea:
    • Possess reduced endoskeleton; unique respiratory process involving water intake through the anus.