Lecture 24 - Ecdysozoans and Deuterostomes

Announcements and Reminders

  • SPOT Evaluations: Available on Canvas until December 10.

    • Evaluations are voluntary and anonymous.

  • We're in the final stretch of the course.

    • Office hours available for clarifications, or students can set up appointments.

  • RRQs 16-20: Due by 11:59 PM on Wednesday, December 10.

    • Students should avoid last-minute submissions and discuss old exams beforehand.

  • Final Exam: Scheduled for Monday, December 15 from 10:15 AM to 12:15 PM.

    • Exam will cover 85-90% new material and 10-15% cumulative.

  • BMAC Students: Must reserve seats as soon as possible!

  • Module 7: Due by 11:59 PM on Monday, December 1.

    • Access information via the Biology Student Resources Canvas site.

    • Complete one activity and participate in one discussion thread.

Lecture Overview

  • Today's Topic: Ecdysozoans and Deuterostomes, Part 1.

    • Bilateria:

    • Ecdysozoa:

      • Nematodes

      • Arthropods

    • Deuterostomes:

      • Echinoderms

      • Hemichordates

      • Chordates:

      • Cephalochordates

      • Urochordates

      • Vertebrates

        • Jawless fishes

        • Jawed vertebrates: gnathostomes

Ecdysozoans

Taxonomy and Characteristics

  • Ecdysozoa: Sister clade to Lophotrochozoa (together known as "protostomes").

    • Contains approximately 8 phyla; notably contains Arthropoda with ~1,000,000 described species (~75% of all described animal species).

  • Cuticle: An extracellular layer covering all ecdysozoans, providing protection and skeletal support.

    • To grow, they must molt (shed the cuticle), termed ecdysis.

Molting Process

  • Before molting, ecdysozoans prepare a new cuticle underneath the old one to minimize exposure time.

  • Morphological variations:

    • Some have thin, flexible cuticles facilitating gas exchange but limiting to damp habitats.

    • Others (like arthropods) have thick, strong cuticles forming robust exoskeletons.

  • Arthropods adapted jointed appendages to move effectively within their rigid exoskeletons.

Examples of Ecdysozoans

Nematodes

  • Nematodes: ~25,000 species; unitary and pseudocoelomate.

    • Habitats: Abundant in freshwater, marine, soil, and as parasites in other organisms.

    • Size: Generally small, with some parasitic species reaching meters in length.

    • Diet: Feed on host body fluids or bacteria; possess a complete gut.

    • Model Organism: Caenorhabditis elegans (959 somatic cells, extensively studied in developmental biology).

Parasitic Nematodes

  • Estimated nearly one million undiscovered species.

    • Plant Parasites: Affect agriculture by feeding on roots and inducing root galls.

    • Animal Parasites: Cause significant human diseases (elephantiasis, filariasis, anisakiasis).

Potential Benefits of Parasitic Nematodes

  • Trichurus suis (whipworm): While parasitic in pigs, may alleviate Crohn's disease and colitis symptoms; FDA has classified it as an Investigational New Drug for human trials.

Other Phyla of Ecdysozoans

  • Nematomorpha: ~350 species (bonus phylum, not on exam).

    • Parasites of arthropods in early stages, emerging as adults in freshwater.

  • Tardigrades: ~800 species, known as “water bears.”

    • Characteristics: Unitary, coelomate, segmented, small (<0.5 mm), and all aquatic.

    • Feeding: Mostly on plant cells; some are predatory.

    • Cryptobiosis: Can survive extreme conditions; a bonus phylum (not on exam).

  • Onychophorans: ~110 species (velvet worms).

    • Characteristics: Unitary, coelomate, segmented, terrestrial, predatory species using slime to immobilize prey.

    • Bonus phylum (not on exam).

Arthropods

  • Arthropods: ~1 million species; unitary, coelomate, segmented creatures with rigid cuticles.

    • Features include protection against predators, skeletal support, and prevention of desiccation.

    • Jointed appendages facilitate various functions: locomotion, feeding, gas exchange, etc.

  • Evolution: Early arthropods (e.g., trilobites) had similar body segments, while derived groups exhibit structural variation for specialized functions.

Circulatory System

  • Arthropods possess a partially open circulatory system (hemolymph).

    • Blood flows in both vessels and open spaces; adaptations for gas exchange include gills and lungs.

Subgroups of Arthropods

Chelicerates

  • Three main clades: Chelicerates, Myriapods, Pancrustaceans.

    • Characterized by two body regions (cephalothorax, abdomen) with jointed appendages.

  • Pycnogonids (sea spiders): Marine, carnivorous, exhibit male parental care.

  • Horseshoe crabs: Predatory, important marine scavengers.

Myriapods

  • Two body regions: head and trunk.

    • Centipedes: One pair of legs per body segment, predators with venomous claws.

    • Millipedes: Two pairs of legs per apparent segment, primarily scavengers/herbivores.

Pancrustaceans

  • Diverse group including crustaceans (shrimp, lobsters, crabs) and insects.

    • Crustaceans: Typically exhibit three body regions with paired appendages and a unique nauplius larval stage.

    • Insects: Three body regions (head, thorax, abdomen); unique gas exchange system using spiracles and tracheae.

Insect Diversity and Lifecycles

  • Flight Evolution: Evolved around 350 million years ago, driving diversification alongside angiosperms, suggesting coevolution of insects and flowering plants.

  • Metamorphosis: Insects exhibit two types:

    • Incomplete Metamorphosis: Larval stages resemble adults (e.g., grasshoppers).

    • Complete Metamorphosis: Larvae differ significantly from adults (e.g., butterflies).

Importance of Insects

  • Crucial as pollinators for many plants and pests that impact agricultural crops.

  • Serve as vectors for human diseases (e.g., malaria, Chagas' disease).

Deuterostomes

Overview

  • Deuterostomes: Sister-taxon to protostomes.

    • Embryonic characteristics: Radial cleavage, blastopore forming the anus, mesoderm from archenteron outpocketings.

    • Major taxa: Echinoderms, Hemichordates, Chordates; all coelomate.

Chordate Characteristics

  • Defined by four traits at some life stage:

    1. Notochord

    2. Dorsal hollow nerve cord

    3. Pharyngeal slits

    4. Muscular post-anal tail.

Phylogenetic Relationships

  • Chordates include invertebrate groups (cephalochordates and urochordates) and vertebrates.

Chordate Examples

Cephalochordates

  • Marine, known as lancelets, small (<5 cm); burrow in sediment, filter feeding.

Urochordates

  • Marine, contain key chordate traits in larval stages; primarily suspension feeders.

    • Groups include Ascidians (sea squirts), Larvaceans, and Thaliaceans (salps).

Reminder on Chordate Phylogeny

  • Position in the evolutionary tree from ancestral deuterostomes through various vertebrate adaptations.

Vertebrates

  • Originated ~530 million years ago.

    • Key features include a jointed dorsal vertebrate column, cranium, and duplication of Hox genes.

    • Cyclostomes (hagfishes and lampreys) split from gnathostomes before jaws evolved; hagfishes play critical scavenger roles in marine ecosystems.

Essential Vocabulary

  • Pentaradial symmetry, water vascular system, chordates and key traits.

  • Ecdysozoa, cuticle, ecdysis/molting, nematode, arthropod, open circulatory system, and associated anatomical features.

Study Questions

  1. Members of what two animal phyla can undergo cryptobiosis?

  2. How does a rigid cuticle affect arthropod locomotion and gas exchange?

  3. Discuss the causes of recent insect diversification.

  4. Describe the biology of echinoderms and hemichordates.

  5. Identify the sister taxon to vertebrates.

  6. List the four key traits in chordates during their lifecycle.

  7. Summarize vertebrate taxa covered in this lecture.

  8. Explain the feeding behaviors of hagfishes and lampreys and whether they use jaws.

  9. Discuss the reasons ecdysozoans undergo ecdysis?