Protostomes Lecture Notes

Introduction to Protostomes

• Protostomes are a major group of animals. The name "Protostome" means "first mouth," indicating that the mouth develops before the anus during embryonic development.

  Foundation

• Five major animal lineages in the tree of life include:

  • Sponges (phylum Porifera)
  • Comb jellies (phylum Ctenophora)
  • Corals and jellyfish (phylum Cnidaria)
  • Protostomes and deuterostomes (two bilaterian lineages)

• Protostomes make up most animal species and include familiar animals like insects, worms, mollusks, and crustaceans.

Protostome Phylogeny

• Protostomes are a diverse group, including:
  • Arthropoda (insects, chelicerates, crustaceans, myriapods)
  • Nematoda (roundworms)
  • Mollusca (mollusks)
  • Platyhelminthes (flatworms)
  • Annelida (segmented worms)
• Non-protostome phyla include:
  • Chordata
  • Echinodermata
  • Cnidaria
  • Porifera

Ecological and Health Importance

• Protostomes inhabit nearly every habitat, including land, freshwater, and the ocean.
• Ecological roles:
  • Decomposers
  • Herbivores
  • Carnivores
• Impacts on human health, both positive and negative.

Protostomes and Humans

• Many protostomes are food sources (e.g., shrimp, snails).
• They provide ecosystem services like soil aeration by earthworms.
• Key research models include the fruit fly (Drosophila melanogaster) and the roundworm (Caenorhabditis elegans).
• Some harm crops or act as parasites.

Flatworms (Phylum: Platyhelminthes) and Human Diseases

• Schistosomes (Trematoda: Schistosoma spp.) cause schistosomiasis, affecting approximately 200 million people worldwide.
  • Transmission occurs through contact with freshwater contaminated with larval forms from snails.
• Tapeworms (Cestoda: Taenia spp., Diphyllobothrium spp.)
  • Acquired from undercooked pork, beef, or fish.
  • Can cause digestive problems or more severe effects if larvae invade tissues (e.g., neurocysticercosis).
• Liver flukes (Clonorchis sinensis, Fasciola hepatica)
  • Cause liver damage and bile duct inflammation.
  • Contracted from contaminated water or undercooked fish.

Roundworms (Phylum: Nematoda) and Human Diseases

• Hookworms (Ancylostoma, Necator)
  • Penetrate skin and migrate to intestines.
  • Cause anemia and malnutrition.
• Ascaris lumbricoides
  • Intestinal parasite causing ascaris infection.
  • Acquired by ingesting contaminated food or soil.
• Filarial worms (Wuchereria bancrofti, Brugia spp.)
  • Cause lymphatic filariasis (elephantiasis).
  • Spread by mosquito bites.

Leeches (Class: Hirudinea, Phylum: Annelida)

• Mostly non-pathogenic, but can cause infection at bite sites and rarely transmit pathogens.

Insect Vectors

• Arthropods like mosquitoes, ticks, and flies transmit diseases like malaria, dengue, Lyme disease, which are caused by protozoa, bacteria, or viruses.

Movement to Land

• Protostomes were among the first animals to move onto land.
• This occurred alongside the adaptive radiation of land plants.
• Land offered more sunlight, less competition, and new food sources.
• Adaptive radiation is the rapid evolution of many species from a common ancestor, filling different ecological roles.

Adaptations for Terrestrial Survival

• Protostomes evolved new traits to:
  • Exchange gases
  • Avoid drying out
  • Support their bodies without water
• Examples of solutions:
  • Roundworms and Earthworms:
  • High surface-area-to-volume ratio for gas exchange in moist environments.
• Arthropods and Mollusks:
  • Internal gills or respiratory structures reduce water loss.
• Insects:
  • Waxy layer on body to prevent water loss
  • Closeable spiracles (air openings).
• Desiccation-resistant eggs:
  • Thick membranes (insects) or shells (snails/slugs) help retain moisture.

Two Major Groups of Protostomes

• Based on DNA evidence, protostomes are divided into:
  • Lophotrochozoa (e.g., mollusks, flatworms, segmented worms)
  • Ecdysozoa (e.g., arthropods, roundworms)
• Key difference:
  • Lophotrochozoans grow incrementally.
  • Ecdysozoans grow by molting.

Flatworms (Phylum Platyhelminthes)

• Flat bodies (no coelom).
• Adaptation:
  • Provides large surface area for gas exchange.
  • Flat body plan allows nutrients and gases to diffuse efficiently to cells with minimal expenditure in complex internal structures.
• No circulatory or respiratory systems.
• Must live in moist environments due to dependence on surface diffusion.

Major Flatworm Groups

• Turbellaria – mostly free-living (non-parasitic).
• Cestoda – tapeworms (intestinal parasites).
• Trematoda – flukes (tissue/blood parasites).
• Monogenea – ectoparasites on fish.

Turbellaria

• Mostly aquatic (marine or freshwater).
• Eat small animals, protists, or detritus.
• Have a blind gut: one opening for food intake and waste expulsion.
• Mouth located mid-body on a tube called the pharynx.

Cestoda (Tapeworms)

• Internal parasites in vertebrates.
• Absorb nutrients across body surface (no gut).
• Humans infected by eating undercooked pork, beef, or fish.

Trematoda (Flukes)

• Can be endo- or ectoparasitic.
• Parasitize many animals, including humans.
• Schistosoma causes schistosomiasis.
  • Approximately 200 million people affected globally.
  • Causes blood in urine/stool, organ damage.

Annelids – Segmented Worms (Phylum Annelida)

• Key features:
  • Coelom present.
  • Segmented body.
  • Complete digestive tract (mouth to anus).
• More complex than flatworms.

Annelid Groups

• Polychaeta (“many bristles”) – mostly marine.
• Oligochaeta (“few bristles”) – includes earthworms.
• Hirudinea – leeches.

Polychaeta

• Mostly marine worms.
• Many have:
  • Parapodia – paddle-like appendages.
  • Chaetae – bristles used for movement or anchoring.
• Some are mobile predators, others live in tubes.

Oligochaeta (Earthworms)

• Mostly terrestrial or freshwater.
• Deposit feeders — ingest soil and extract nutrients.
• Important ecosystem engineers:
  • Aerate soil with tunnels.
  • Add organic matter via feces.

Hirudinea (Leeches)

• Most are ectoparasites.
• Attach to fish, humans, or other animals.
• Suck blood or body fluids.
• Some are predators or scavengers.
• Medicinal leeches exist.

Medical Use of Leeches (Hirudotherapy)

• Leeches (especially Hirudo medicinalis) have been used for centuries in medicine.
• In modern medicine, they are used to:
  • Restore blood flow in delicate surgical areas (reattached fingers, ears, or skin grafts).
  • Relieve venous congestion by drawing blood from areas where blood can't drain properly.
• How it works:
  • Leeches secrete anticoagulants (e.g., hirudin) to prevent blood from clotting.
  • Their feeding improves circulation and promotes healing.
• Hirudotherapy is FDA-approved and used in microsurgery and reconstructive surgery.

Mollusks (Phylum Mollusca)

• Key body parts:
  • Foot – for movement.
  • Visceral mass – contains organs and gills.
  • Mantle – secretes the shell, encloses body cavity.
• Includes: snails, slugs, clams, mussels, squids, and octopuses.

The Muscular Foot

• Main function: movement.
• Snails/chitons: crawl using foot as a muscular hydrostat.
• Bivalves: use foot to dig.
• Cephalopods: foot modified into tentacles for grasping and crawling.

The Visceral Mass

• Contains internal organs and gills.
• In most mollusks, the coelom is reduced.
• Instead, organs sit in a cavity called the hemocoel.
• Fluid bathes organs directly (open circulatory system).
• Radula:
  • Tongue-like feeding structure with sharp teeth.
  • Used to scrape or cut food.
  • Lost in bivalves, which are filter feeders.

The Mantle

• Secretes the shell in many mollusks.
• Mantle functions vary:
  • Snails: forms a lung in some species.
  • Bivalves: forms siphons to draw in water.
  • Cephalopods: forms a siphon for jet propulsion.

Mollusk Diversity

• Four major groups (lineages):
  • Chitons – shell of overlapping plates.
  • Bivalves – clams, mussels (two shells).
  • Gastropods – snails and slugs.
  • Cephalopods – squid, octopuses (tentacles, advanced brains).

Ecdysozoans

• Ecdysozoa = protostomes that grow by molting.
• Shedding a cuticle (soft outer layer) or exoskeleton.
• After molting, body expands before new covering hardens.
• Growth by molting is called ecdysis
  • Hormone involved = ecdysone

Why Molting Matters

• The cuticle/exoskeleton:
  • Protects from predators and environment.
  • Supports muscles for movement.
• But during molting, animals are vulnerable and soft-bodied.

Roundworms (Phylum Nematoda)

• Unsegmented worms with:
  • Pseudocoelom (not fully lined with mesoderm).
  • Elastic cuticle (must molt to grow).
  • No appendages.
• Body plan: tube within a tube.
• No specialized circulatory or respiratory systems.
  • Rely on diffusion for gas exchange and nutrient transport.
• Extremely diverse diets: Bacteria, fungi, detritus, small animals.
• Some are free-living, others parasitic (humans, plants, animals).

Roundworms and Disease

1. Ascaris lumbricoides
   • Causes: Ascariasis
   • Found in: Contaminated food or soil
   • Symptoms: Stomach pain, malnutrition, blockage of intestines
   • One of the most common human parasites
2. Hookworms (Ancylostoma, Necator)
   • Enter through: Bare feet (from soil)
   • Feed on: Blood from the intestinal wall
   • Symptoms: Fatigue, anemia, weakness
3. Filarial Worms (Wuchereria bancrofti, etc.)
   • Spread by: Mosquito bites
   • Live in: Lymphatic system
   • Cause: Elephantiasis (severe swelling of limbs/genitals)
4. Strongyloides stercoralis
   • Found in: Contaminated soil
   • Can cause: Long-term infections
   • Dangerous in: People with weak immune systems
5. Trichinella spiralis
   • Spread by: Eating undercooked pork
   • Live in: Muscles
   • Cause: Muscle pain, fever, swelling

Ecdysozoan Growth

• Ecdysozoans grow intermittently by molting.
  • Shedding of the soft cuticle or hard exoskeleton.
• Once animal molts:
  • Fluid causes body to expand.
  • New, larger cuticle or exoskeleton forms.
• Ecdysozoans undergo a succession of molts as they grow.
  • Sometimes dramatic morphological transformations occur during these molts.
• Cuticle and exoskeleton:
  • Protect these animals from predators.
  • Provide an effective structure for muscle attachment.
• During molting, animal’s soft body is exposed and vulnerable.
  • Hormone ecdysone (Important in regulation of molting cycle)

Tardigrades and Velvet Worms

• Tardigrades (