Animal Diversity: Protostomes (Arthropods) & Deuterostomes

Arthropods and Deuterostomes

Introduction to Arthropods

• Definition: The last group of protostomes to be discussed. Invertebrates characterized by an exoskeleton and jointed legs.

• Etymology: "Arthro" means jointed, and "pod" means foot, hence "jointed feet."

• Phylogenetic Lineage: Belong to the Ichthysozoan lineage, following nematodes.

Arthropod Morphology and Physiology

• Appendages: Possess jointed appendages (e.g., legs, wings, mouthparts), defined as growths from the body with a joint (like an elbow).

  • Composed of chitin, a hard, thick carbohydrate, which also forms their exoskeleton.

• Body Segmentation: Have a segmented body, typically fused into three main sections:

  • Head

  • Thorax

  • Abdomen

  • Some species have a fused head and thorax, known as a cephalothorax.

• Exoskeleton of Chitin:

  • Benefits: Provides a mechanism for muscle attachment and offers protection.

  • Drawbacks: Can be vulnerable, particularly during molting. Limits maximum body size.

• Water Loss Protection: Terrestrial arthropods have a waxy layer on their exoskeleton to prevent dehydration.

• Respiration: Varies with habitat and species:

  • Terrestrial: Possess a tracheae, a network of tubes. Previously thought to be passive for air diffusion, more recent discoveries indicate active compression and relaxation, similar to mammalian lungs.

  • Aquatic: Utilize gills.

  • Spiders and Horseshoe Crabs: Use specialized book lungs.

Arthropod Size Limits and Molting

• Size Limitation: The exoskeleton restricts growth because its strength decreases as the diameter increases. As body size (and thus surface area) increases, the body cannot withstand the volume and would collapse (e.g., an ant scaled up 4 times would collapse).

• Molting (Ecdysis): All arthropods shed their rigid outer exoskeleton to grow.

  • Process: Shedding allows for significant growth.

  • Additional Benefits:

    • Sheds unwanted external elements like parasites (e.g., barnacles on crabs).

    • Allows for the regeneration of lost legs.

Arthropod Subphyla

There are four subphyla within the phylum Arthropoda:

• 1. Chelicerata:

  • Examples: Scorpions, spiders, mites.

  • Appendages: First set of appendages are modified into pincers or fangs.

  • Feeding: Primarily predatory, but mites exhibit diverse feeding strategies (herbaceous, predatory, parasitic).

• 2. Crustacea:

  • Dominant Habitat: Primarily dominant marine arthropods.

  • Examples: Crabs, shrimps, barnacles.

  • Terrestrial Exceptions: A few terrestrial crustaceans exist, such as sow bugs and pill bugs (also known as roly pollies).

  • Morphology: Each segment of the thorax and abdomen typically features an appendage, which can be branched (e.g., barnacles extending branched legs for filter feeding).

  • Sensory Organs: First appendages are antennae, used as feelers and for other sensory functions.

• 3. Myriapoda:

  • Etymology: Means "lots of legs and many segments."

  • Examples: Millipedes, centipedes.

• 4. Hexapoda:

  • Characteristics: Represents the classic insects. Possess three body sections and six legs.

  • Diversity: The largest subphylum within Arthropoda.

  • Examples: Dragonflies, bees, wasps, butterflies, moths, fleas, fruit flies, termites.

Arthropod Diversity and Evolutionary Success

• Global Dominance: Arthropods constitute approximately 85\% of all described animal species.

  • Insects account for 73\% of all described animals.

  • Other arthropods make up 12\%.

  • Chordates are 4\% and other phyla are 11\%
    of the described animal species.

• Species Numbers:

  • Vertebrate species: ~38,000

  • Mammal species: ~4,500

  • Insect species: ~900,000

  • Estimated undescribed insect species: Ranges from 2 million to 30 million.

• Reasons for Evolutionary Success:

  • 1. Small Size: Allows for more complex habitats and greater subdivision, creating more ecological niches and enabling more species to coexist in microhabitats with less competition.

  • 2. Flight: Provides enhanced movement capabilities, allowing for rapid colonization of new areas or plants.

  • 3. Complete Metamorphosis: A change in body plan during development (e.g., caterpillar to butterfly).

    • Stages: Typically involves egg, larva (also called pupa), and adult stages, where the animal looks distinctly different at each stage.

    • Specialization: Immature stages can specialize in feeding and growth, while adults can specialize in dispersal and reproduction.

    • Incomplete Metamorphosis: Juvenile versions resemble miniature adults, without drastically different body plans.

    • Cultural Reference: Franz Kafka's "The Metamorphosis" describes an abrupt, dramatic change in humans.

Introduction to Deuterostomes

• Etymology: From Greek, meaning "mouth second," referring to the anus developing first, followed by the mouth.

• Major Groups: Composed of two major phyla: Echinodermata and Chordata.

Echinodermata

• Etymology: Named for their spiny skin; "echinos" means spiny, and "dermos" means skin.

• Species: Comprises about 7,000 described living species.

• Examples: Sea stars, sea urchins, sand dollars, sea cucumbers.

• Habitat: Exclusively marine species.

• Morphology:

  • Adults: Exhibit pentaradial symmetry, with rays or arms arranged in groups of five.

  • Larvae: Typically bilaterally symmetrical.

  • Endoskeleton: Made of interlocking calcium carbonate plates (essentially calcium), covered by an epidermis.

• Water Vascular System: A unique physiological system consisting of a central ring and radial canals along the arms.

  • Functionality: Filled with water, allowing for circulation of gases, nutrients, and waste throughout the body.

  • Tube Feet: Leads to the development of tube feet, which are manipulated by water pressure for movement and other functions.

• Feeding: Diverse feeding strategies:

  • Predaceous: Sea stars, for example, prey on bivalves by attaching to both shells, exerting constant suction until the bivalve's muscles exhaust. They then evert their mouth parts and stomach into the shell crack to consume the contents.

  • Grazers: Sea urchins.

  • Filter Feeders: Sea cucumbers.

Chordata

• Diversity: An extremely diverse group encompassing fish, amphibians, reptiles, and mammals, along with two smaller groups: tunicates and lancelets.

• Defining Features (present at least during part of life):

  • Dorsal Hollow Nerve Cord: A hollow cord containing nerves (e.g., the neural tube in a lancelet).

  • Notochord: A flexible rod that supports the nerve cord.

• Tunicates: Appear sponge-like as adults, but their larvae possess the defining chordate features.

• Lancelets: Invertebrate chordates that retain the defining features throughout life.

• Vertebrates: A subgroup within Chordata.

  • Notochord Replacement: In vertebrates, the notochord disappears early in development and is replaced by the vertebral column (vertebrae), which surrounds and protects the nerve cord.

  • General Body Plan: Dorsal nervous system (referring to the back, like a spine), internal skeleton, organs suspended in a coelom, and a well-developed circulatory system with a heart.

Evolution of Vertebrates

• First Vertebrates: Likely mudsuckers that ingested mud for organic material.

• Osmoregulation: Possessed the ability to control solute levels (e.g., salts) in their cells, allowing them to exploit estuaries (brackish interfaces between ocean and freshwater).

Groups of Modern Fishes

• 1. Jawless Fishes:

  • Examples: Hagfish and lampreys.

• 2. Cartilaginous Fishes:

  • Skeleton Composition: Skeletons made of cartilage.

  • Examples: Sharks and rays.

• 3. Ray-Finned (Bony) Fishes:

  • Skeleton Composition: Cartilage is replaced by a bony skeleton.

  • Diversity: The most diverse group of fish.

  • Examples: Seahorses, clownfish (e.g., Finding Nemo), lionfish, pipefish.

  • Swim Bladder Evolution: Evolved lung-like sacs for respiration, which were modified into a swim bladder.

    • Advantage: Provides buoyancy, allowing them to stay upright or motionless at particular depths, increasing mobility.

Amphibians

• First Terrestrial Vertebrates: Examples include frogs, toads, and salamanders.

• Frog vs. Toad: No biological distinction; differences are primarily ecological.

• Habitat: Mostly confined to moist environments.

• Respiration: Breathe with lungs and across their moist skin, highlighting the skin's vital role in respiration.

• Reproduction: Typically occurs in water with external fertilization (sperm must swim to the egg).

  • Eggs: Not waterproof, generally laid in water, though some species (e.g., certain tree frogs) lay eggs in close proximity to water, such as on leaves overhanging a pool.

Reptiles

• Adaptation to Dry Land: Better adapted to dry land environments.

• Key Features:

  • Skin covered with scales.

  • Internal fertilization, reducing reliance on water for reproduction.

  • Amniotic egg with a waterproof shell.

• Monophyletic Group: Represents a monophyletic group, meaning it includes a common ancestor and all its descendants.

• Birds as Reptiles: Birds are classified within the reptile phylogeny; their scales are modified into feathers.

• Examples: Includes various lizards, snakes, crocodiles, alligators, and birds.

• Intelligence: Display remarkable cognitive abilities (e.g., crows using traffic to crack nuts at pedestrian crossings; parrots like Alex demonstrating language comprehension, object description, and counting).

Mammals

• Defining Features:

  • Hair: Present in all mammals, chemically indistinguishable from "fur" (both are made of keratin); humans have less hair compared to many non-human mammals.

  • Mammary Glands: Produce milk to nurse young.

• Species Diversity: Roughly 4,500 described species, relatively low compared to insects, but exhibit vast morphological and ecological diversity.

• Size Range:

  • Smallest: ~2 grams (e.g., pygmy shrew, less than the weight of a penny).

  • Largest: ~160,000 kilograms (e.g., Antarctic blue whale, exceeding the weight of two 18-wheelers).

• Locomotion: Some mammals possess flight capabilities (e.g., bats).

• Environmental Impact: Mammals, particularly human species, have had the greatest impact on Earth.

• Mammalian Groups: There are three commonly distinguished groups:

  • 1. Monotremes:

    • Reproduction: Egg-laying mammals.

    • Legs: Possess reptilian-like splayed legs.

    • Examples: Platypus, echidna.

  • 2. Marsupials:

    • Reproduction: Give birth to small, undeveloped young.

    • Nursing: Nurse their young in a ventral pouch.

    • Examples: Kangaroo, wombat, koal (tiger koal), koala, Tasmanian devil, possums.

  • 3. Eutherian Mammals (Placental Mammals):

    • Diversity: The most diverse group, comprising over 4,000 of the ~4,500 mammalian species.

    • Placenta: Possess a placenta, an organ that forms at the embryo's contact with the mother's uterine lining, providing nutrition to the embryo.

    • Examples: Humans, bats, meerkats, whales, tigers, lemurs, other primates.

• Monophyletic Status: The classification of mammals as a monophyletic group is a key concept to consider.