Lecture 12 - PowerPoint

Lecture Overview Topics

Arthropod Evolutionary Relationships

General Arthropod Characteristics

Phylogeny of Arthropods

Chelicerates: Transition to Land

• Adaptations: Chelicerates, including spiders and scorpions, developed critical adaptations for life on land. These include:

  • Stronger Legs: Enhanced limb strength enables them to support their weight and navigate terrestrial environments.

  • Respiratory Modifications: They possess advanced respiratory systems to minimize water loss while maximizing oxygen intake, using structures like book lungs.

Importance of Chelicerates

• They play crucial ecological roles, such as:

  • Pest Control: By preying on agricultural pests, they contribute to maintaining the ecological balance and promoting biodiversity.

  • Nutrient Cycling: Their interactions within ecosystems aid in the breakdown of organic matter, returning essential nutrients to the soil.

General Characteristics

• Chelicerates possess specialized mouthparts called chelicerae for grasping and feeding.

• Their bodies consist of two primary segments:

  • Cephalothorax: A fused head and thorax that houses vital organs.

  • Abdomen: Contains digestive and reproductive systems.

• Typically, adult chelicerates have four pairs of walking legs adapted for various locomotor functions, including climbing, burrowing, and swimming.

Taxonomy of Chelicerates

• Class Arachnida: Includes a diverse range of species such as:

  • Spiders: Diverse feeding strategies with many being web-weavers or hunters.

  • Scorpions: Identified by their large pincers and venomous stinger, providing them with defense against predators.

  • Ticks: Parasitic in nature, feeding on the blood of mammals and birds, often posing health risks by transmitting diseases.

  • Mites: Occupying various ecological roles from decomposers to agricultural pests.

• Class Pycnogonida: Known as sea spiders, these creatures have extended legs relative to their small bodies, adapted for a benthic lifestyle, primarily feeding on soft-bodied animals like sea anemones.

Crustaceans

Importance of Crustaceans

• They are vital for aquatic ecosystems. Examples include:

  • Krill: Form the foundation of many marine food webs, serving as prey for larger species, including whales and fish.

  • Scavengers: Help in resource recycling within the ecosystem, contributing to the overall health of aquatic systems.

General Characteristics

• Crustaceans exhibit a segmented body plan with:

  • Cephalothorax: Combining the head and thorax, which houses essential organs.

  • Abdomen: Typically used for locomotion and reproduction.

• Their exoskeleton is highly flexible, allowing them to occupy diverse ecological niches.

• Appendages vary for specialized functions:

  • Feeding: Maxillae and mandibles are modified for efficient food acquisition.

  • Locomotion: Specialized appendages such as pleopods enhance their swimming abilities.

Taxonomy of Crustaceans

• Class Branchiopoda: Includes small species like brine shrimp, which have leaf-like appendages for respiration and locomotion, often found in temporary freshwater habitats.

• Class Maxillopoda: Encompasses tiny organisms like copepods and barnacles; they play critical roles in both marine and freshwater ecosystems.

• Class Malacostraca: Comprises larger and more morphologically diverse crustaceans such as crabs, lobsters, and shrimp, known for their economic significance and varied life histories.

Arthropod Evolution Characteristics

Bilateral Symmetry

• Arthropods exhibit bilateral symmetry, contributing to efficient movement and organization of sensory organs, thus enhancing their survival capabilities in diverse environments.

Protostome Classification

• In protostomes, the developmental process results in the mouth forming first and the anus second, defining their embryonic pathways and body system arrangements.

Coelomate Structure

• A true coelom acts as a hydrostatic skeleton, providing support and facilitating mobility, while allowing room for complex organ development.

Segmentation

• Segmentation enables arthropods to evolve specialized segments for various functions, supporting their extensive morphological diversity and ecological roles.

Jointed Limbs

• The presence of jointed limbs allows for modifications suited for different environments, whether for walking, swimming, or grasping, enhancing their ecological adaptability.

General Characteristics of Arthropods

Key Features

• Complex Brain with Sensory Organs: The central nervous system integrates various sensory inputs, enabling rapid reactions to environmental changes, crucial for survival.

Feeding Strategies

• Arthropods display a range of feeding strategies:

  • Herbivorous: Feeding on plant material.

  • Predaceous: Consuming other animals.

  • Detritivorous: Breaking down decomposing organic matter, showcasing their adaptability to different environments.

Reproductive Strategies

• Aquatic species typically utilize external fertilization, synchronizing reproduction with environmental cues.

• Terrestrial species tend to favor internal fertilization, enhancing reproductive success in variable habitats. Additionally, many species engage in complex mating rituals and exhibit parental care behaviors.

Oviparous

• Most arthropods lay eggs, though select species demonstrate viviparity, where live birth occurs, illustrating a diversity of reproductive strategies.

Open Circulatory System

• In arthropods, hemolymph performs functions similar to blood and participates in locomotion and nutrient transportation, ensuring efficient distribution throughout the body.

Gas Exchange

• Arthropods possess varied adaptations for effective gas exchange:

  • Aquatic species primarily use gills for oxygen extraction.

  • Terrestrial species have adapted structures like tracheae and book lungs to facilitate efficient respiration in different oxygen availability conditions.

Exoskeleton

• The exoskeleton, composed mostly of chitin and proteins, provides strength and flexibility. Regular moulting (ecdysis) is essential for growth, albeit presenting vulnerability during this energetically demanding process.