Lecture 11 - Protostome Animals (Lophotrochozoans & Ecdysozoans)

Introduction to Protostomes

Definition of Protostomes:
  - Protostomes are a major group of animals characterized by their embryonic development. The key features that distinguish them from deuterostomes include:
    1. The mouth develops before the anus during gastrulation.
    2. Isolated early embryonic cells cannot develop into a complete embryo, indicating a determinant cleavage.
    3. The formation of a coelom via the splitting of mesodermal blocks, known as schizocoelous development.

Water to Land Transition

Adaptations for Terrestrial Life:
  - Protostomes adapted to land with several key modifications similar to land plants:
    - Gas Exchange:
        - High surface-area-to-volume ratios in body structures like those of roundworms and earthworms facilitate gas exchange.
    - Preventing Desiccation:
        - Developments such as gills and other respiratory structures that are internal (not exposed to air) in arthropods and mollusks.
        - Insects developed waxy epidermal layers and eggs resistant to desiccation.
    - Body Support Adaptations:
        - Modifications in body structures to support life on land.

Lophotrochozoans

Definition:
  - A monophyletic group encompassing 13 phyla characterized by two distinct morphological traits:
    1. Lophophores:
        - Specialized structures used for suspension feeding.
    2. Trochophore Larvae:
        - Free-swimming larval stage capable of feeding.
Spiral Cleavage:
- Unique to lophotrochozoans, contrasting radial cleavage seen in deuterostomes.

Major Phyla within Lophotrochozoans:

Rotifera (2,100 species):
    - Tiny coelomate suspension feeders, found in marine, freshwater, and damp soils.
    - Characterized by a corona, a cluster of cilia at the anterior end that create a rotating motion for feeding and locomotion.
    - Mostly sexually reproducing with some species being parthenogenetic.
Platyhelminthes (Flatworms):
    - Acoelomate organisms with a flat body that lacks specialized gas exchange and circulation systems, enhancing material diffusion.
    - Found in moist, aquatic habitats. Four main classes include:
        - Turbellaria: (mostly free-living) with a blind digestive tract and capable of both sexual and asexual reproduction.
        - Cestoda: (tapeworms) endoparasitic, lacking a gut and mouth, absorbing nutrients through diffusion from their hosts, such as humans from undercooked pork, beef, or fish.
        - Trematoda: (flukes) endo-/ectoparasitic with complex lifecycles affecting vertebrates and invertebrates, causing diseases like schistosomiasis.
        - Monogenea: ectoparasites of hosts' skin or gills, harming fish.
Annelida (Segmented Worms): (16,800 species)
    - Coelomate with segmented body structures and complete digestive tracts. Asexual reproduction can occur via fragmentation or parthenogenesis.
    - Classes include:
        - Polychaeta: (usually free-living) exhibit diverse feeding strategies, most are sexually reproducing with external fertilization.
        - Oligochaeta: Earthworms that play crucial roles in soil aeration and organic matter cycling; hermaphroditic with internal fertilization.
        - Hirudinea: Leeches, primarily ectoparasitic, with some nonparasitic forms acting as predators.
Mollusca: (85,000 species)
    - General body plan includes a muscular foot for movement, visceral mass containing internal organs, and a mantle covering the visceral mass that secretes shell material.
    - Four major classes:
        - Polyplacophora (Chitons): Marine grazers with eight dorsal plates.
        - Bivalvia: Aquatic suspension feeders with two hinged shells, utilizing gills for filter feeding.
        - Gastropoda: Most diverse, with varying diets; exhibit unique adaptations like radula for feeding.
        - Cephalopoda: Highly evolved predators with advanced nervous systems; exhibit behaviors like color change and complex movement.

Ecdysozoans

Definition:
  - Ecdysozoans grow intermittently by a process known as molting (ecdysis), wherein they shed their exoskeletons or cuticles to allow for growth. This provides:
    - Protection from predators.
    - Space for muscle attachment.

Major Phyla within Ecdysozoans:

Nematoda (Roundworms):
- Unsegmented pseudocoelomates, present in diverse environments and exhibit wide adaptations, including parasitism in animals.
- Notable parasites include Toxocara canis, which infests dogs/cats and can affect humans.
Tardigrada (Water Bears): (1,045 species)
- Microscopic, segmented creatures with remarkable resilience, able to survive extreme conditions by entering a cryptobiotic state.
Onychophora (Velvet Worms): (210 species)
- Terrestrial and caterpillar-like, utilize slime secretion for ambushing prey.
Arthropoda: (1,120,000 species)
    - Characterized by jointed appendages, segmented bodies with prominent regions (tagmata), and a chitinous exoskeleton that undergoes periodic ecdysis.
    - Subphyla include:
        - Crustacea: Aquatic with diverse forms (lobsters, crabs, shrimp).
        - Hexapoda (Insects): Extensive diversity with adaptations such as flight.
        - Myriapoda (Centipedes and Millipedes): Body segmentation facilitating various ecological roles.
        - Chelicerata: Arachnids (scorpions, spiders) with specialized feeding adaptations (chelicerae).

Insect Diversity

Insect Anatomy:
  - Basic traits include:
    - 3 body segments: head, thorax, and abdomen.
    - 6 legs and usually 2 pairs of wings.
    - Unique respiratory systems integrating tracheae and spiracles.
Reproductive Strategies:
- Complete metamorphosis (holometabolism) observed in groups like butterflies and beetles.
- Incomplete metamorphosis (hemimetabolism) represented in grasshoppers and true bugs.
Examples of Orders:
  - Coleoptera (Beetles): Highest species diversity among insects, adapted biting and chewing mouthparts.
  - Diptera (Flies): One pair of wings, with the second pair reduced.
  - Hymenoptera (Bees/Wasps): Social structures and complex behaviors.
  - Lepidoptera (Butterflies/Moths): Characterized by scales on their wings and specialized feeding apparatus.