lecture 5 10/27 (NEMATODA AND ANTHROPODA)
Today’s Focus:
Exploring two major and incredibly diverse groups of animals: Nematodes (roundworms) and Arthropods (insects, crustaceans, spiders, etc.). These phyla represent a vast majority of animal life on Earth.
Introduction to Trilobites: fascinating extinct marine arthropods, exemplified by fossils passed around in class.
Trilobite info:
Approximately 20,000 described species, showcasing a remarkable evolutionary radiation over millions of years.
Extinct around 250 million years ago during the Permian-Triassic extinction event, one of the most severe mass extinctions in Earth's history.
Fossil locations:
Larger fossil specimen from I-35, south of Norman, Oklahoma, indicating the presence of ancient marine environments in that region.
Other smaller, well-preserved examples found near railroad tracks near Keystone Lake, offering insights into their distribution.
Estimated age of these specific trilobite fossils: 300 million years, placing them in the Carboniferous period.
Common Characteristics of Nematodes and Arthropods
Cuticle Layer:
A tough, flexible, yet rigid outer layer that provides both physical protection and structural support for these animals. It is non-cellular and secreted by the epidermis.
Necessity for growth: Because the cuticle is rigid and does not grow with the animal, it must be periodically shed to allow for an increase in body size.
Growth method: Ecdysis (molting) – the process of shedding the old cuticle. This is a metabolically demanding and vulnerable stage for the animal.
Contrast with clams: Unlike nematodes and arthropods, clams (Mollusca) grow their shell continually by adding new material to the shell edges, reflecting a different evolutionary strategy for growth and protection.
Other Groups with Exoskeletons/Support Structures:
Corals: Possess a hard, calcitic (calcium carbonate) exoskeleton, forming the reef structures.
Bryozoans & Mollusks: Utilize various forms of exoskeletons, often calcified, for support and protection, demonstrating convergent evolution for such structures.
Chitin:
The primary polysaccharide (a complex carbohydrate) that forms the structural component of the cuticle layer in both nematodes and arthropods. Chitin is highly resistant to degradation and contributes to the strength and durability of the exoskeleton.
Nematodes (Roundworms)
Common Name: Roundworms, descriptive of their cylindrical body shape.
Diversity:
Approximately 30,000 recognized species have been formally described, but this is considered a very small fraction of their actual diversity, with estimates ranging into the millions of undescribed species.
Habitat: Ubiquitous, found in virtually every ecosystem – abundant in soil, diverse in aquatic systems (freshwater and marine), and often found inside plant and animal tissues as parasites.
Lifestyle:
Parasitic: Many species are obligate parasites, extracting nutrients and resources from a vast array of hosts, including plants, insects, vertebrates, and other invertebrates, often causing significant disease.
Free-living: Thousands of species thrive independently in soil, sediments, and other habitats, playing crucial roles in decomposition and nutrient cycling.
Size variation: Ranging from microscopic, barely visible to the naked eye (less than 1\;mm in length), to highly elongated forms over 25 feet in length (e.g., Placentonema gigantissima, a parasite found in the placenta of sperm whales).
Model Organism in Research:
C. elegans (Caenorhabditis elegans): A widely studied free-living nematode, serving as a foundational model organism in developmental biology, genetics, and neuroscience due to its simplicity, rapid life cycle, and transparent body.
Simplicity: It has an invariant cell lineage with approximately 1000 somatic cells, making its development precisely traceable.
Examples of studies: Pioneering research on dye tracing for tissue development, apoptosis (programmed cell death), and neuronal connectivity has been conducted using C. elegans.
Nematode Life Cycles:
Exhibit varied and complex life cycles; major ecological groups include plant parasites (causing significant agricultural losses), vertebrate parasites (responsible for numerous human and animal diseases), which often involve intermediate hosts, and numerous free-living soil and aquatic species that contribute to ecosystem health.
Parasitic Examples:
Trichinella (Trichinosis):
Disease: Trichinosis, a food-borne illness.
Adult residence: Adult worms typically reside in the mucosal lining of the small intestine (digestive tract) of carnivorous mammals.
Life cycle involving larvae: After mating, females produce live larvae that migrate from the intestine through the bloodstream and lymphatic system into muscle tissues, where they encyst (form a protective capsule).
Sylvatic cycle: Primarily occurs in wildlife populations (e.g., bears, wild boars, rodents, predatory birds), where transmission happens through predation.
Transmission to humans: Through consumption of undercooked or raw flesh containing infective cysts (most commonly pork, but also bear, rat, or other game meats).
Heartworm (Dirofilaria immitis):
Host & Location: A serious parasite primarily affecting canids (dogs, wolves, coyotes), with adult worms residing in the pulmonary arteries and right ventricle of the heart, sometimes reaching several inches in length.
Larval migration & transmission: Microscopic larval stages (microfilariae) circulate in the bloodstream of infected hosts. When a mosquito takes a blood meal, it ingests these microfilariae, which then develop into an infective stage within the mosquito. The mosquito subsequently transmits the infective larvae to a new host during another blood meal.
Arthropods Overview
Characteristics:
Jointed appendages: A defining feature, providing flexibility and specialized functions for locomotion, feeding, and sensory perception.
Segmented bodies: Their bodies are divided into distinct segments, often grouped into tagmata (e.g., head, thorax, abdomen or cephalothorax and abdomen) for specialized functions.
Exoskeleton: A rigid external skeleton, primarily composed of chitin, which offers protection, muscular attachment points, and prevents desiccation (drying out).
Shed exoskeleton: Like nematodes, arthropods must shed their exoskeleton through ecdysis to grow, making them temporarily vulnerable during this process.
Coelomate animals: Possess a true coelom (a fluid-filled body cavity completely lined with mesoderm), which allows for the development of complex internal organs and provides hydrostatic support.
Diversity:
Arthropods are the most successful animal phylum, comprising an astonishing 80% of all described animal species, reflecting their incredible adaptability and evolutionary success across nearly all habitats.
Major groups include trilobites (extinct), cheliceriforms (spiders, scorpions), myriapods (centipedes, millipedes), crustaceans (crabs, lobsters), and hexapods (insects), each displaying unique morphological and ecological characteristics.
Subdivisions of Arthropods:
Trilobites:
An extinct group of marine arthropods, with 20,000 species known from the fossil record.
Morphology: Characterized by distinctive three-lobed bodies and segmented structures.
Extinction: Went extinct dramatically during the Permian-Triassic extinction event (around 250\;Ma), which wiped out about 90\%\ of marine species.
Cheliceriformes:
Examples: Include familiar animals such as spiders, scorpions, ticks, mites, and horseshoe crabs.
Unique feature: Possess chelicerae, which are specialized fang-like or pincer-like mouthparts used for grasping and injecting venom (in spiders) or tearing food.
Body structure: Their body is typically divided into two main tagmata: the cephalothorax (prosoma, a fused head and thorax region) and the abdomen (opisthosoma, containing digestive and reproductive organs).
Myriapods:
Terrestrial arthropods that include millipedes (Class Diplopoda) and centipedes (Class Chilopoda).
Millipedes (herbivores): Characterized by having two pairs of legs per body segment and a generally more rounded body; they are detritivores, feeding on decaying plant matter.
Centipedes (carnivores): Characterized by having one pair of legs per body segment and a flattened body; they are predatory, possessing venomous fangs (forcipules) for subduing prey.
Crustaceans:
A diverse group primarily found in aquatic environments (marine and freshwater), but some are terrestrial.
Include lobsters, crabs, shrimp, barnacles, copepods, and isopods (e.g., rolly pollies/pill bugs, which are terrestrial crustaceans).
Anatomy includes a cephalothorax and abdomen (like cheliceriformes), often covered by a carapace; they typically have branched (biramous) appendages adapted for various functions like swimming, feeding, and sensing.
Hexapods (Insects):
The largest and most diverse class of arthropods, with about 1 million described species, though estimates of actual diversity are much higher (possibly 5-30 million).
Habitat: Primarily terrestrial, adapted to nearly every land habitat and many freshwater environments, with only a few marine species.
Distinct body plan: Characterized by a distinct head, thorax, and abdomen; the thorax bears three pairs of appendages (legs) and typically one or two pairs of wings in adults.
Significance: Play significant roles as pollinators, herbivores, predators, and decomposers, making them crucial to ecosystem functioning.
Insect Development and Physiology
Insect Anatomy: The insect body is clearly segmented into three functional regions: the head (sensory and feeding), the thorax (locomotion with three pairs of legs and usually wings), and the abdomen (digestion and reproduction).
Instars: The developmental stages between each molt (ecdysis) are called instars. Insects undergo a series of instars, growing larger with each shedding of the cuticle, progressing towards maturity.
Holometabolous Development: Also known as complete metamorphosis, this developmental pathway includes distinct larval stages that are morphologically and ecologically very different from the adult (e.g., the life cycle of a butterfly: egg {\to} larva (caterpillar) {\to} pupa (chrysalis) {\to} adult). This allows larvae and adults to exploit different resources and avoid competition.
Larvae Roles: Many insect larvae (e.g., aquatic larvae of dragonflies, caddisflies) are essential in freshwater food webs as primary consumers (grazers, detritivores) and also as predators, playing a critical role in nutrient cycling and maintaining ecological balance in aquatic ecosystems.
Ecological Roles of Arthropods
Ecosystem Services:
Vital for food webs: Arthropods serve as primary consumers (herbivores feeding on plants), decomposers (breaking down organic matter), and as significant food sources for a multitude of other animals (e.g., birds, fish, mammals).
Pollination services: Bees, butterflies, moths, and other insects are crucial for the reproduction of over 80\%\ of flowering plants and a significant portion of human food crops, enabling fruit and seed production.
Economic value from pollination: The global economic value of pollination services provided by insects is estimated at up to 300\;billion annually, highlighting their immense contribution to agriculture and food security.
Other services: Soil aeration, pest control (predatory insects), and decomposition of organic waste.
Scientific Models: Organisms like Drosophila melanogaster (the common fruit fly) serve as foundational models in genetic research, developmental biology, neuroscience, and toxicology due to their rapid life cycle, ease of breeding, and well-understood genome, yielding fundamental insights into biological processes.
Forensics: The study of insect colonization of remains (forensic entomology) can provide crucial information in criminal investigations, particularly in estimating the Post Mortem Interval (PMI) or time of death, based on the species composition, developmental stages of insects found, and ecological succession patterns.
Negative Aspects of Arthropods
Invasive Species:
Examples include the Japanese beetle (Popillia japonica), which causes extensive damage to lawns, gardens, and agricultural crops, and the Argentine fire ant (Linepithema humile), which displaces native species, disrupts ecosystems, and inflicts painful stings.
Economic impact: Crop damage, forestry losses, and harm to livestock and infrastructure caused by invasive arthropods result in billions of dollars in economic losses worldwide.
Vectors for Diseases:
Arthropods act as vectors, transmitting a wide array of pathogens including nematode diseases (e.g., filariasis transmitted by mosquitoes), viruses (e.g., West Nile virus, dengue, Zika transmitted by mosquitoes), bacteria (e.g., Lyme disease by ticks, plague by fleas), and protozoa (e.g., malaria by mosquitoes, Chagas disease by kissing bugs) to humans, livestock, and wildlife, causing significant global health burdens.
KEY TERMS
Ecdysis: The process of shedding the old cuticle (a tough, rigid outer layer) by nematodes and arthropods to allow for growth; this is a metabolically demanding and vulnerable stage for the animal.
Metamorphosis (Holometabolous Development): Also known as complete metamorphosis, this developmental pathway in insects includes distinct larval stages that are morphologically and ecologically very different from the adult (e.g., egg \to larva (caterpillar) \to pupa (chrysalis) \to adult). This allows larvae and adults to exploit different resources and avoid competition.
Instar: The developmental stages between each molt (ecdysis) in insects, during which they grow larger with each shedding of the cuticle, progressing towards maturity.
Nematode Life Cycle (e.g., Trichinella and Dirofilaria):
Trichinella (Trichinosis): Adult worms typically reside in the mucosal lining of the small intestine of carnivorous mammals. After mating, females produce live larvae that migrate through the bloodstream and lymphatic system into muscle tissues, where they encyst (form a protective capsule). Transmission to humans occurs through consumption of undercooked or raw flesh containing infective cysts.
Dirofilaria immitis (Heartworm): This serious parasite primarily affects canids, with adult worms residing in the pulmonary arteries and right ventricle of the heart. Microscopic larval stages (microfilariae) circulate in the bloodstream of infected hosts. When a mosquito takes a blood meal, it ingests these microfilariae, which then develop into an infective stage within the mosquito. The mosquito subsequently transmits the infective larvae to a new host during another blood meal.
Chelicera: Specialized fang-like or pincer-like mouthparts found in Cheliceriformes (e.g., spiders, scorpions) used for grasping and injecting venom (in spiders) or tearing food.
Head, Thorax vs. Cephalothorax, Abdomen:
Head and Thorax: In Hexapods (insects), the body is clearly segmented into three functional regions: the head (sensory and feeding) and the thorax (locomotion, bearing three pairs of legs and usually wings).
Cephalothorax: A fused head and thorax region (prosoma) found in Cheliceriformes (e.g., spiders, scorpions) and Crustaceans (e.g., crabs, lobsters), which combines sensory, feeding, and locomotory functions.
Abdomen: The posterior body region (opisthosoma) in both insects and other arthropod groups (like Cheliceriformes and Crustaceans), housing organs for digestion and reproduction.
Spinnerets: This term is not explicitly detailed in the provided lecture notes.
Silk Gland: This term is not explicitly detailed in the provided lecture notes.
Food Web: An ecological concept where arthropods are vital, serving as primary consumers (herbivores), decomposers (breaking down organic matter), and significant food sources for a multitude of other animals (e.g., birds, fish, mammals).
Pollination: A crucial ecosystem service performed by arthropods (e.g., bees, butterflies, moths) that enables the reproduction of over 80\% of flowering plants and a significant portion of human food crops, thereby facilitating fruit and seed production.
Forensic Entomology: The study of insect colonization of remains, which provides crucial information in criminal investigations for estimating the Post Mortem Interval (PMI) or time of death, based on the species composition, developmental stages of insects found, and ecological succession patterns.
Disease vectors: Arthropods, such as mosquitoes, ticks, and fleas, act as vectors for a wide array of pathogens. These include nematode diseases (e.g., filariasis transmitted by mosquitoes), viruses (e.g., West Nile virus, dengue, Zika transmitted by mosquitoes), bacteria (e.g., Lyme disease by ticks, plague by fleas), and protozoa (e.g., malaria by mosquitoes, Chagas disease by