Protostomes 1 - Overview AND SPIRALIA/LOPHOTROCHOZOANS

RECAP: BILATERIA

  • Common Ancestor: All Bilateria share a common ancestor characterized by the following features:

    • Unique cell junctions.

    • Presence of collagen and proteoglycans in the extracellular matrix.

KEY TRAITS OF BILATERIA

  • Symmetry: Bilateral symmetry along an anterior-posterior axis.

  • Cell Layers: Triploblastic (three embryonic cell layers).

  • Nervous System: Centralized nervous system.

  • Digestive System: Generally possess a complete digestive tract with a mouth and anus:

    • Protozoa (Chapter 31): Contains organisms with protostome development (Blastopore develops into mouth).

    • Deuterostomes (Chapter 32): Contains organisms with deuterostome development (Blastopore develops into anus).

ANIMALS IN BILATERIA

  • Sponges: Eumetazoans, including classes such as Ctenophores, Demosponges, and Calcareous sponges.

  • Protozoa: Include Arrow worms and Lophotrochozoans.

  • Deuterostomes: Represented by Echinoderms and Hemichordates.

RECAP: PROTOSTOMES AND DEUTEROSTOMES

  • During gastrulation, a blastopore is formed:

    • Protostomes: “mouth first” development.

    • Deuterostomes: “mouth second” development.

GENERAL TRAITS OF PROTOSTOMES

  • Triploblastic: Same as Bilateria.

  • Blastopore: Becomes the mouth.

  • Derived Traits:

    • An anterior brain surrounding the digestive tract entrance.

    • A ventral nervous system consisting of paired or fused longitudinal nerve cords.

BODY CAVITY PRESENCE AND NOMENCLATURE

  • Types of Body Cavities:

    • Acoelomate: No fluid-filled cavity, mesenchyme is present.

    • Pseudocoelomate: Internal cavity contains fluid (pseudocoel), organs are suspended within.

    • Coelomate: Internal cavity full of fluid (coelom), lined with peritoneum covering the organs.

ANATOMICAL SIGNIFICANCE OF BODY CAVITIES

  • Major Difference: Presence or absence of body cavities differentiates protostome groups.

  • Molecular Phylogenetics: Indicates the protostome ancestor likely had a coelom, with some groups reverting to an acoelomate state.

SPECIFIC STRUCTURES IN ARTHROPODS

  • Body Cavity: Becomes a hemocoel or blood chamber, contributing to an open circulatory system.

CURRENT CLASSIFICATION OF PROTOSTOMES

  • Based on Genetic Data:

    • Main Clades:

    • Lophotrochozoa

    • Ecdysozoa

    • Position of Chaetognathes (Arrow worms) remains unclear in this classification scheme.

CHAETOGNATHES (ARROW WORMS)

  • Characteristics: Predatory, usually planktonic, marine organisms.

LOPHOTROCHOZOA AND ECDYSOZOANS

  • Lophotrochozoan Features:

    • Possess a free-living larval stage called the trocophore.

    • Some have a specialized feeding structure known as a lophophore.

  • Ecdysozoan Features:

    • Includes animals that molt, a process named ecdysis.

SPIRALIA: LOPHOTROCHOZOA

  • Named due to spiral cleavage (oblique division in relation to the polar axis), this trait is believed to be an ancestral condition for these animals.

ECDYSOZOA AND MOLTING

  • External Cuticle: Secreted by the epidermis; must be shed to allow growth (ecdysis).

  • Evolution of Molting: Evolved approximately 500 million years ago and is consistent across all ecdysozoans.

  • Vulnerability After Molting: Animals are exposed to predators immediately after shedding their old cuticle before the new one hardens.

  • Evolution of Alternatives: New mechanisms evolved for movement and gas exchange because the exoskeleton constrains these functions.

LOPHOTROCHOZOANS

  • Diversity: Includes many species with varied traits and traits lost across lineages.

  • Examples of Groups:

    • Bryozoans

    • Entoprocts

    • Flatworms

    • Rotifers and relatives

    • Gastrotrichs

    • Ribbon worms

    • Brachiopods

    • Phoronids

    • Annelids

    • Mollusks

SPECIFIC GROUPS IN LOPHOTROCHOZOA
PLATYHELMINTHES (FLATWORMS)

  • Species Count: Approximately 30,000 species.

  • Body Plan: Acoelomate, dorsoventrally flattened; lacks a gas transport system, resulting in cells being in proximity to the surface.

  • Habitat: Includes both free-living and parasitic forms.

ROTIFERA (ROTIFERS)

  • Species Count: Approximately 1,800 species, predominantly freshwater.

  • Size: Ranges from 50-500 µm.

  • Adaptations:

    • Specialized internal organs and a complete digestive tract.

    • Resilience to desiccation and ionizing radiation due to an effective cellular antioxidant system.

    • Bdelloid Rotifers: Exclusively female, eggs develop through parthenogenesis; exist without sexual reproduction for millions of years.

BRYOZOANS (MOSS ANIMALS)

  • Species Count: 6,500 living species.

  • Habitat: All aquatic, mostly marine, and mostly sessile.

  • Colony Structure: Composed of many small zooids, with organic or calcareous exoskeleton structures.

  • Feeding Structure: Lophophore; skeletal material often composed of calcium carbonate.

NEMERTEA (RIBBON WORMS)

  • Feeding Mechanism: Possess a muscular proboscis.

  • Reproduction: Dioecious (separate male and female).

  • Development: Trocophore larvae present.

  • Digestive System: Complete digestive tract unlike flatworms.

BRACHIOPODS

  • Description: Solitary marine animals with two-part shells, connected by ligaments.

  • Similarity: Resembles bivalve mollusks but evolved independently.

  • Life Cycle: Gametes released into the water for fertilization; larvae are planktonic.

  • Historical Note: Once highly abundant in the Paleozoic and Mesozoic eras; currently, only about 450 species survive.

ANNELIDA (SEGMENTED WORMS)

  • Species Count: Approximately 19,000 species.

  • Habitat: Restricted to aquatic or moist environments.

  • Body Plan: Clearly segmented with isolated coeloms in each segment; involves a trocophore larval stage.

MOLLUSCA

  • Diversity: Includes the largest known invertebrates, such as the colossal squid (nearly 500 kg) and diminutive land snails.

  • Body Structure: Highly variable across classes with many possessing calcareous shells.

  • Trocophore Larvae: Present in multiple subclasses.

INTRODUCTION TO ECDYSOZOA

  • Key Groups:

    • Arrow Worms

    • Nematoda (roundworms)

    • Nematomorpha (horsehair worms)

    • Tardigrada (water bears)

    • Onychophorans (velvet worms)

    • Arthropods

NEMATODES

  • Abundance: Extremely abundant; approximately 25,000 species described, found universally.

  • Size: Range from microscopic to species reaching 9 meters in length, such as parasites in whale placentas.

  • Habitat: Scavengers in soil, freshwater, and marine sediments.

NEMATOMORPHA (HORSEHAIR WORMS)

  • Behavioral Adaptations: Certain larvae manipulate the behavior of their hosts (e.g., causing crickets to jump into water).

  • Life Cycle: Free-living adults emerge from the host's body after manipulation.

TARDIGRADA (WATER BEARS)

  • Characteristics: Small-bodied, paired appendages, can live in extreme conditions, often found in marine sands and moist environments.

ONYCOPHORANS (VELVET WORMS)

  • Description: Approximately 200 terrestrial species, possessing paired, unstructured appendages.

ARTHROPODS

  • Diversity: The most abundant and diverse group of animals characterized by segmentation, jointed appendages, and a rigid exoskeleton, which supports movement and prevents water loss.

REVIEW OF LEARNING OUTCOMES

  • Reiteration of learning outcomes from the beginning of the lecture for emphasis and clarification.

READING ASSIGNMENT

  • Textbook Reference: Sadava, Chapter 30 and 30.1