Nematode Study Guide

📖 Core Concepts Nematodes (roundworms): Tubular, unsegmented worms with a thick collagenous cuticle and a hydrostatic skeleton. Ecdysozoa: Superphylum that includes nematodes, arthropods, and tardigrades; they molt their cuticle. Free‑living vs. Parasitic: Most feed on microbes; many species parasitise plants, animals, or humans. Hydrostatic Skeleton: Longitudinal muscles contract against the cuticle; no circular muscles. Nerve Ring: Dense circular ganglion around the pharynx acting as a simple brain. Model Organism – Caenorhabditis elegans: Fully sequenced genome, invariant cell lineage, used for development, ageing, neurobiology. --- 📌 Must Remember Abundance: 4.4 × 10²⁰ nematodes in topsoil → 60 billion per human; 90 % of ocean‑floor animals. Human Pathogens: Ascaris, Trichuris trichiura, hookworms, Enterobius vermicularis, Trichinella spiralis, filarial nematodes, Anisakis, Toxocara. Plant‑Parasitic Nematodes: Root‑knot (Meloidogyne), cyst (Globodera, Heterodera), lesion (Pratylenchus), dagger (Xiphinema). Biological Control: Steinernema & Heterorhabditis (entomopathogenic), Phasmarhabditis (slug control). Reproductive Modes: Dioecious (separate sexes), androdioecious (C. elegans hermaphrodites + males), parthenogenesis (e.g., Meloidogyne). Phylogenetic Position: Sister to Nematomorpha → group Nematoida; may belong to Cycloneuralia. --- 🔄 Key Processes Feeding & Digestion Ingest via mouth → muscular, cuticle‑lined pharynx (enzyme secretion). No stomach; food passes directly to a thin‑walled intestine for absorption. Locomotion Longitudinal muscles contract → body lengthens, then cuticle recoils → sinusoidal wave. Reproduction (dioecious) Male fertilizes female → eggs laid (embryonated or unembryonated). Hermaphroditic self‑fertilization in C. elegans → rapid population growth. Biological Control Cycle Infective juvenile (IJ) enters insect host → releases symbiotic bacteria → kills host → IJ emerges to seek new hosts. --- 🔍 Key Comparisons Free‑living vs. Parasitic Nematodes Free‑living: feed on microbes, regulate bacterial populations, no host tissue invasion. Parasitic: possess stylet or specialized mouthparts, inject enzymes, complete life cycle inside host. C. elegans vs. Other Nematodes C. elegans: hermaphroditic, 1 mm adult, model organism, invariant cell lineage. Plant‑parasitic (Meloidogyne): large stylet, induces root galls, primarily agricultural pest. Entomopathogenic vs. Plant‑parasitic Nematodes Entomopathogenic (Steinernema): kill insects, used for biocontrol, carry symbiotic bacteria. Plant‑parasitic (Globodera): form cysts on roots, cause crop loss, managed by rotation/cover crops. --- ⚠️ Common Misunderstandings “Nematodes decompose organic matter” – They regulate microbes that decompose; nematodes themselves do not digest plant debris. All nematodes are parasites – Only ⅓ of genera parasitise vertebrates; the majority are free‑living. Cuticle = exoskeleton – Unlike arthropods, nematode cuticle is non‑mineralised, flexible, and not articulated. --- 🧠 Mental Models / Intuition Hydrostatic Skeleton = “Water‑filled balloon”: Muscles squeeze the tube, the fluid pressure pushes outward, creating motion. Life‑cycle “Switch”: Free‑living stage → infective juvenile (IJ) → host invasion → reproductive adult → egg → free‑living. Ecological “traffic controller”: Nematodes = traffic lights for bacterial populations – they keep microbial numbers in check, shaping nutrient cycles. --- 🚩 Exceptions & Edge Cases Ovoviviparity: Some nematodes retain eggs until they hatch inside the mother (e.g., certain Ascaris spp.). Androdioecy: Rare system where hermaphrodites self‑fertilize but males still occur (C. elegans). Stylet‑less parasites: Certain animal parasites lack a stylet but rely on cuticular secretions to penetrate tissue (e.g., hookworms). --- 📍 When to Use Which Identify species → Use DNA barcoding of ribosomal ITS regions (high resolution). Control plant‑parasitic nematodes → Start with crop rotation & resistant varieties; add cover crops (e.g., Tagetes) if infestation persists. Biocontrol choice → Use Steinernema for soil‑dwelling insect larvae; choose Phasmarhabditis when targeting slugs/snails. Model organism selection → For genetics & development → C. elegans; for extreme survival studies → Panagrolaimus davidi. --- 👀 Patterns to Recognize High nematode density → soil health indicator (≥10⁶/m² suggests fertile, microbe‑rich soils). Root galling + “nematode egg mass” on roots → likely root‑knot (Meloidogyne). Cuticular ridges & amphids in microscopy → characteristic of nematodes vs. other meiofauna. Larval migration to muscle → think Trichinella spiralis infection. --- 🗂️ Exam Traps Distractor: “Nematodes directly decompose organic matter.” – Wrong; they regulate microbes. Choice: “All nematodes have a stomach.” – Incorrect; they lack a true stomach. Option: “Only male nematodes are motile.” – False; both sexes move; males often have a copulatory spicule. Answer: “The ventral nerve cord is a sensory organ.” – Misleading; it integrates sensory and motor signals, not a sensory organ itself. ---