🔬General Biology I Unit 28 – Invertebrates

What Are Invertebrates?

• Animals that lack a vertebral column or backbone
• Comprise over 95% of all animal species on Earth
• Includes a wide variety of organisms such as insects, spiders, crabs, snails, and jellyfish
• Can be found in nearly every habitat on the planet from the deepest oceans to the driest deserts
• Range in size from microscopic zooplankton to giant squids measuring over 40 feet long
• Many invertebrates have a hard exoskeleton made of chitin for protection and support
• Some invertebrates like jellyfish and worms have soft bodies without a hard external covering

Major Invertebrate Groups

• Porifera (sponges): Aquatic, sessile animals with porous bodies and no true tissues
• Cnidaria (jellyfish, corals, sea anemones): Radially symmetrical with stinging cells called nematocysts
• Platyhelminthes (flatworms): Soft, flattened worms with bilateral symmetry and no body cavity
• Nematoda (roundworms): Unsegmented worms with a pseudocoelom and complete digestive system
• Annelida (segmented worms): Includes earthworms and leeches with segmented bodies and a true coelom
• Mollusca (snails, clams, octopuses): Soft-bodied animals, often with a hard shell and a muscular foot
  • Classes include Gastropoda (snails and slugs), Bivalvia (clams and oysters), and Cephalopoda (octopuses and squids)
• Arthropoda (insects, spiders, crabs): Segmented bodies, jointed appendages, and a hard exoskeleton
  • Subphyla include Chelicerata (spiders and horseshoe crabs), Crustacea (crabs and shrimp), and Hexapoda (insects)
• Echinodermata (sea stars, sea urchins, sea cucumbers): Spiny-skinned marine animals with radial symmetry and a water vascular system

Invertebrate Body Plans

• Asymmetry: No symmetry, irregular shape (some sponges)
• Radial symmetry: Body parts arranged around a central axis (cnidarians and echinoderms)
  • Advantages include equal exposure to the environment and efficient movement in any direction
• Bilateral symmetry: Mirror image halves, anterior (head) and posterior (tail) ends, dorsal (back) and ventral (belly) surfaces (most invertebrates)
  • Allows for cephalization (concentration of sensory organs and nervous tissue in the head)
  • Facilitates directed movement and more complex behaviors
• Coelomate: Body cavity (coelom) completely lined with mesoderm (annelids, mollusks, arthropods)
  • Provides space for organ development and efficient circulation
• Pseudocoelomate: Body cavity not completely lined with mesoderm (nematodes)
• Acoelomate: No body cavity (flatworms)

Adaptations and Survival Strategies

• Exoskeletons provide protection, support, and prevent water loss (arthropods)
• Shells offer protection and can be closed to avoid predation (mollusks)
• Camouflage helps animals blend in with their surroundings to avoid detection (many invertebrates)
• Mimicry involves resembling another organism to deceive predators or prey (some insects and spiders)
• Bioluminescence produces light for communication, attraction, or deterrence (some jellyfish and squid)
• Regeneration allows for the regrowth of lost body parts (some flatworms and echinoderms)
• Venomous stings or bites can immobilize prey or deter predators (spiders, scorpions, some mollusks)
• Jet propulsion enables rapid escape from predators (squid and octopuses)

Ecological Roles

• Decomposers break down dead organic matter and recycle nutrients (many worms and insects)
• Filter feeders strain small particles from the water (sponges, clams, some crustaceans)
• Herbivores consume plants and algae (many snails, sea urchins, and insects)
• Carnivores prey on other animals (spiders, octopuses, some insects)
• Parasites live on or in a host organism and obtain nutrients at the host's expense (some flatworms and nematodes)
• Pollinators transfer pollen between flowers, enabling plant reproduction (many insects)
• Keystone species have a disproportionately large impact on their ecosystem (corals, sea stars)
• Ecosystem engineers modify their environment, creating habitats for other species (beavers, termites)

Invertebrate Evolution

• Invertebrates first appeared during the Ediacaran Period, about 600 million years ago
• The Cambrian Explosion (541 million years ago) saw a rapid diversification of invertebrate phyla
• Key innovations such as exoskeletons, shells, and segmentation allowed for the exploitation of new niches
• Invertebrates were the first animals to colonize land during the Silurian Period (444-419 million years ago)
• The evolution of flight in insects (Carboniferous Period, 359-299 million years ago) led to their widespread success
• Mass extinctions, such as the End-Permian (252 million years ago), significantly impacted invertebrate diversity
• Coevolution with flowering plants during the Cretaceous Period (145-66 million years ago) led to the diversification of pollinating insects

Key Invertebrate Systems

• Nervous system: Ranges from simple nerve nets (cnidarians) to complex brains (cephalopods)
  • Ganglia are concentrations of nerve cells that control specific functions
  • Some invertebrates have well-developed sensory organs (eyes, antennae, statocysts)
• Circulatory system: Open (hemolymph bathes organs directly) or closed (blood confined to vessels)
  • Some have a heart that pumps hemolymph or blood
• Respiratory system: Gas exchange through body surface, gills, book lungs, or tracheal systems
  • Tracheal systems (insects) are a network of tubes that deliver oxygen directly to tissues
• Digestive system: Ranges from incomplete (one opening) to complete (separate mouth and anus)
  • Specialized structures for feeding include the radula (mollusks) and mandibles (arthropods)
• Excretory system: Removes metabolic wastes through structures like nephridia (annelids) and Malpighian tubules (insects)
• Reproductive system: Asexual reproduction (budding, fragmentation) and sexual reproduction (gonads, external or internal fertilization)
  • Hermaphroditism (having both male and female reproductive organs) is common in some groups (snails, earthworms)

Cool Invertebrate Facts

• Tardigrades (water bears) can survive extreme conditions like the vacuum of space and temperatures close to absolute zero
• The box jellyfish has 24 eyes, including two complex eyes with lenses and retinas
• The giant squid has the largest eyes in the animal kingdom, measuring up to 10 inches in diameter
• The peacock mantis shrimp can strike with a force of 1,500 Newtons, accelerating faster than a .22 caliber bullet
• The immortal jellyfish (Turritopsis dohrnii) can revert to its juvenile stage after reaching maturity, essentially becoming biologically immortal
• The Antarctic krill (Euphausia superba) is one of the most abundant species on Earth, with an estimated biomass of 500 million tons
• The silk produced by the Darwin's bark spider is one of the strongest biological materials known, ten times stronger than Kevlar
• The Japanese spider crab has the longest leg span of any arthropod, reaching up to 12 feet from claw to claw