LAB 9 ECHINODERMS

LAB 9 ECHINODERMS

Introduction

• Overview of Lab 9 focusing on Phylum Echinodermata, sister group to Chordata.
• Echinoderm diversity: includes sea stars, sea urchins, sand dollars, brittle stars, sea cucumbers.
• Approx. 6,500 species in Phylum Echinodermata.
• Importance of echinoderms in learning about marine biology.

Phylum Echinodermata

• Phylum name derived from Greek: Echino (spine) and dermata (skin).
• Echinoderms are free-living marine animals.
• Undergo deuterostome embryological development (similar to Chordates).

Unique Characteristics

1. Water-Vascular System (WVS): Derived from coelomic cavity, includes:
   • Fluid-filled canals
   • Tubular extensions (tube feet)
   • Functions: locomotion, respiration, excretion, chemoreception, food collection.
   • Connected to seawater via madreporite.
   • Circulatory medium includes coelomic fluid and seawater.
2. Radial Symmetry:
   • Adult echinoderms display secondary pentamerous (five-point) radial symmetry, while larvae exhibit bilateral symmetry.
   • Evolutionary link to bilateral ancestors before adopting a sessile feeding lifestyle.
3. No Cephalization:
   • Body surfaces designated as oral (mouth) and aboral (anus).
4. Internal Skeleton:
   • Composed of calcareous ossicles, some with spiny projections.
5. Tube Feet:
   • More than 2,000 tube feet in some species used for various functions.

Class Stelleroidea

• Includes armed echinoderms, such as sea stars and brittle stars.
• Body structure: a central disc with five or more radial arms.

Subclass Asteroidea

• Contains sea stars (e.g., Asterias sp.).
• Morphology:
  • Star-shaped with flattened central disc and arms with ambulacral grooves.
  • Tube feet located in ambulacral grooves, crucial for locomotion and food handling.

Asterias sp. (Sea Star)

• Role as a keystone species in ecosystems, controlling populations of mussels.
• Distinction of aboral and oral surfaces through the presence of ambulacral grooves.
• Internal features: gonads and digestive tracts in arms.
• Aboral surface with spines, madreporite, and pedicellariae (moveable structures)

Subclass Ophiuroidea

• Includes brittle stars and serpent stars.
• Key traits:
  • Distinct central disc, long, flexible arms.
  • No ambulacral grooves, tube feet are less used in locomotion.
• Gorgonocephalus sp. (Basket star):
  • Highly branched arms, capable of capturing small crustaceans.

Class Echinoidea

• Comprises sea urchins and sand dollars.
• Distinct features:
  • Circular or oval body without arms.
  • Fused ossicles form a rigid test, with long spines in regular echinoids and flat bodies in irregular echinoids.

Echinus (Sea Urchin)

• Characteristics: radial symmetry with ambulacral and interambulacral zones.
• Aristotle’s Lantern present, used for feeding by scraping surfaces.

Dendraster & Mellita (Sand Dollars)

• Flattened body with secondary bilateral symmetry.
• Adaptations for burrowing, including specialized tube feet for feeding and respiration.

Class Holothuroidea

• Known as sea cucumbers, over 1,500 species.
• Unique features:
  • Worm-like body, tube feet modified into tentacles at anterior end.
  • Internal respiratory structures (respiratory trees) connected to cloaca.
  • Defense through evisceration.

Cucumaria sp. (Sea Cucumber)

• Distinctive morphology: worm-like body, tube feet in five ambulacral zones.

Class Crinoidea

• Oldest extant echinoderm class, includes sea lilies and feather stars.
• Feeding Adaptations:
  • Suspension feeding via long arms, oral surface is the upper side.
  • Unique mouth location adapted for capturing food from water currents.

Antedon sp. (Feather Star)

• Notable for numerous arms and circled tube feet, used for feeding.

Echinoderm Comparison Chart

Taxonomic Flow Chart

• Phylum Echinodermata
  • Class Stelleroidea
    • Subclass Asteroidea: Asterias sp. (Sea Star)
    • Subclass Ophiuroidea: Gorgonocephalus sp. (Basket Star)
  • Class Echinoidea
    • Regular: Echinus sp. (Sea Urchin)
    • Irregular: Dendraster sp. (Pacific Sand Dollar)
  • Class Holothuroidea
    • Cucumaria sp. (Sea Cucumber)
  • Class Crinoidea
    • Antedon sp. (Feather Star)