echinoderms

Introduction to Phylum Echinodermata and Learning Outcomes

  • ## Learning Objectives

  • By the conclusion of this lecture, students should be able to:     - Identify the members of Phylum Echinodermata.     - Describe the basic biological characteristics and key evolutionary innovations of this group.     - Explain the concept of embryonic stem cells and their adaptive importance for Deuterostomes.     - Discuss the significance of embryonic stem cells in medical research.

  • ## Evolutionary Context and Phylogenetic Placement

  • Animals evolved from an ancestral "protist" lineage.

  • Key evolutionary transitions in the animal kingdom include:     

    • - Symmetry and Tissues: Sponges (no true tissues, no symmetry) vs. Cnidarians (tissues like endoderm and ectoderm, radial symmetry).     

    • - Bilaterians: Organisms with bilateral symmetry and the development of the mesoderm (triploblastic).     

    • - Protostomy vs. Deuterostomy:         

      • - Protostomes include Flatworms, Mollusks, Annelids, Roundworms, and Arthropods.         

      • - Deuterostomes include Echinoderms and Chordates.     

    • - Segmentation: Observed in Annelids, Arthropods, and Chordates.     

    • - Body Cavities: Transition from Acoelomate (e.g., Flatworms) to Coelomates (e.g., Mollusks, Annelids, Echinoderms, Chordates).

  • Estimated Species Diversity:     - Arthropods: >1,000,000 species.     - Mollusks: ∼110,000\sim110,000 species.     - Annelids: ∼12,000\sim12,000 species.     - Flatworms: ∼20,000\sim20,000 species.     - Roundworms: ∼20,000\sim20,000 species.     - Chordates: ∼60,000\sim60,000 species.     - Cnidarians: ∼10,000\sim10,000 species.     - Echinoderms: ∼7,000\sim7,000 species.

Embryonic Development and Stem Cells

  • ## Protostomes vs. Deuterostomes

  • The distinction between these two groups is based on the fate of early embryonic cells.

  • Protostomes:     

    • - Embryonic cells have a determinate fate.     

    • - The developmental path of each cell is fixed early on; if cells are separated, the embryo typically dies or development is terminally disrupted.

  • Deuterostomes (Echinoderms and Chordates):     

    • - Embryonic cells have an indeterminate fate.     

    • - Early embryo cells are identical, undifferentiated, and pluripotent (embryonic stem cells).     

    • - Pluripotency means these cells have the potential to become any of the many different types of cells in the body.

  • ## Adaptive and Medical Importance

  • Adaptive Importance: If a cell becomes separated during early development in deuterostomes, it has the capacity to form an entirely new, complete organism. This is the biological basis for the formation of monozygotic (identical) twins.

  • Medical Importance: Because these cells are pluripotent, they are highly valuable in medical research for potential regenerative therapies and understanding cell differentiation.

General Characteristics of Echinodermata

  • ## Habitats and Scope

  • Echinoderms are the only animal phylum that is exclusively marine.

  • They are found in oceans worldwide but are never found in freshwater (e.g., Lake Superior, Mississippi River) or terrestrial environments (e.g., Mojave Desert).

  • ## Morphology and Symmetry

  • Larval Period: Echinoderm larvae exhibit bilateral symmetry.

  • Adult Phase: Adults undergo metamorphosis to become secondarily radial.

  • Pentaradial Symmetry: Specifically, adults often show five-part radial symmetry, meaning there are five different ways to divide the organism to get two symmetrical halves.

  • Centralization: Consistent with radial symmetry, echinoderms lack a centralized nervous system (no brain) and have no distinct head.

  • ## Skeletal System and Body Texture

  • Endoskeleton: Formed by hard plates of calcium carbonate (CaCO3CaCO_3) held together by connective tissue.

  • Mutable Connective Tissue: Echinoderms possess a unique ability where their body texture can change from tough/hard to rubbery/soft (and vice versa) in a matter of seconds.     - This unique trait is being studied in biotechnology for the development of "smart materials."

Biological Systems of Echinoderms

  • ## The Water Vascular System

  • This system is a hallmark of the phylum and serves as a "jack-of-all-trades."

  • Functions:     

    • - Movement: Powers the tube feet.     

    • - Feeding: Assists in capturing or handling prey/food.     

    • - Circulation: Transports nutrients and gases.     

    • - Respiration: Facilitates gas exchange.

  • Note: The water vascular system does NOT act as the nervous system.

  • Tube Feet: These are extensions of the water vascular system that protrude through holes in the endoskeleton. They are used for movement, feeding, and sensory perception.

  • ## Nervous System

  • Organisms feature one central nerve ring with radiating branches.

  • They possess sensory cells and light-sensitive eyespots distributed across the body.

  • ## Digestive System

  • Echinoderms have a complete digestive system (anus and mouth).

  • Advantages of a Complete Digestive System:     

    • - Food can be digested extracellularly.     

    • - Organisms can eat, digest, and defecate simultaneously.     

    • - Different sections of the system can specialize in specific functions: food intake, digestion, or excretion.     

    • - Waste and food are kept separate, ensuring that only waste is expelled.

  • ## Reproduction and Regeneration

  • Reproduction: Typically have separate sexes with external fertilization (e.g., starfish releasing sperm into the water). Fertilized eggs develop into free-swimming larvae.

  • Regeneration: Many species can regenerate lost body parts. Some use "autotomy" (dropping body parts) as a defense mechanism when under attack.

  • Asexual Reproduction: Some species can reproduce asexually by splitting their bodies.

Diversity and Classification within Echinodermata

  • ## Class Crinoidea (Sea Lilies and Feather Stars)

  • Considered the "O.G." (ancestral) echinoderms.

  • Once thought to be extinct, they are filter feeders.

  • Sea lilies are often found in deep water (e.g., 1200 ft deep).

  • ## Class Asteroidea (Sea Stars)

  • Primarily predators.

  • Characteristics include a central disc with five (or multiples of five) arms.

  • Example: Sunflower starfish.

  • ## Class Ophiuroidea (Brittle Stars)

  • Scavengers (detritivores) or predators of small animals.

  • Possess slender arms used to "row" over the substrate.

  • Known for easily losing arms as a defense (hence "brittle").

  • Example: Basket star.

  • ## Class Echinoidea (Sea Urchins and Sand Dollars)

  • Mostly herbivores that graze on algae.

  • They lack arms but have five rows of tube feet.

  • Sea urchins feature prominent spines protruding from their skeleton.

  • ## Class Holothuroidea (Sea Cucumbers)

  • Scavengers or detritivores feeding on small items on the seafloor.

  • Their skeletal plates are reduced and scattered, giving them a softer body.

  • They have retractable tentacles around the mouth, which are modified tube feet.

Questions & Discussion

  • Q1: Which of the following is NOT true about the first cells of a seastar or human embryo?     - Response: E. "If they get separated, this results in the death of the embryo." This is false because deuterostomes have indeterminate fate; separation leads to identical twins, not death.

  • Q2: What do you and an adult seastar have in common besides being deuterostomes?     - Response: A. An endoskeleton. (Note: Adults seastars are radial, lack a brain, and have a water vascular system rather than a closed circulatory system with a heart like humans).

  • Q3: Which characteristic is consistent with adult echinoderms being radial?     - Response: B. Lack of centralization in the nervous system (no brain).

  • Q4: One thing the water vascular system does NOT help with?     - Response: E. Acting as a nervous system.

  • Q5: Which is NOT an advantage of a complete digestive system?     - Response: A. "Food can be digested extracellularly." While true that they digest extracellularly, this is also possible in some incomplete systems; the other options (simultaneous processes and specialization) are the specific advantages of a complete system.

  • Q6: Where could you hope to see an echinoderm?     - Response: B. Gulf of Mexico. Because they are exclusively marine, they won't be in freshwater lakes, rivers, swamps, or deserts.

  • Q7: Recap: Which is NOT a characteristic of echinoderms?     - Response: D. Acoelomates. Echinoderms are coelomates.