Invertebrate Phyla: Sponges, Cnidarians, Lophotrochozoans, Flatworms, Ribbon Worms, Annelids, and Mollusks

Sponges (Porifera)

• Reproduction:
  • Most sponges are hermaphrodites, producing both sperm and eggs.
  • Unlike most plants, they cannot self-fertilize; sperm must find a partner organism.
  • They can also reproduce asexually through fragmentation (e.g., the "blender test" where a broken piece can grow into a new sponge).
  • Larvae are protected during development.

Cnidarians

• Etymology: "Cnidarian" literally translates to "nettle," referring to their stinging cells.
• Key Characteristic: Possess stinging cells (nematocysts).
• Examples of Cnidarians:
  • Box Jelly (Cubozoans): Considered one of the most dangerous animals on the planet.
    • Can cause death in approximately 45 seconds, primarily by freezing the respiratory system, leading to drowning as the victim is often swimming.
    • Toxin is excruciating but potentially survivable if the victim is quickly removed from the water.
  • Hydrozoans: Include colonial organisms like the Portuguese Man-o-War, which are multiple multicellular organisms forming larger colonies.
  • Staurozoans: Rare in shallow waters, common in the deep ocean; these are jellies that have affixed themselves.
  • Anthozoans: Typically characterized by having tentacles facing upwards (e.g., anemones).
  • Scyphozoans: "True jellies" with tentacles facing downwards (medusiform).
• Body Plan:
  • Approximately 10,000 known species, relatively diverse.
  • Possess a sac-like body, with a middle gel layer called mesoglea.
    • Mesoglea is not a true tissue layer forming a coelom or organs (like in vertebrates), but rather a jelly-like substance providing volume and distance between layers.
  • Gastrovascular cavity (gastrodermis): This is where gas exchange, vascular functions, and feeding occur.
  • Respiration: High surface area-to-volume ratio and thin body structure allow for sufficient oxygen diffusion across their body surface, eliminating the need for specialized respiratory organs (lungs, gills).
  • Locomotion: Mostly planktonic; they lack heavy musculature and cannot swim against currents, drifting instead.
• Polyps:
  • Are sessile, attaching to surfaces using an adhesive.
  • Can release and reattach (e.g., anemones can be moved and reattach).
  • Deeper-sea stalked jellies will release and roll to find better locations/food.
  • They primarily rely on chemical cues for finding suitable attachment sites, indicating a rudimentary nervous system.
• Nematocysts:
  • Stinging cells that resemble harpoons under a scanning electron microscope (SEM).
  • Covered by an operculum (a covering).
  • Contain a long-chain helical protein that functions like a spring, storing potential energy and attached to the harpoon.
  • When stimulated (e.g., tickled), the operculum pops open, releasing the coiled pressure and firing the nematocyst.
  • The harpoon-like thread is coated with a toxin, penetrating and stinging the target.
  • All cnidarians possess nematocysts, although the length and strength vary (e.g., true jellies have longer, stronger threads to puncture fish scales).
  • Fired nematocysts must be regrown; a single cell is spent, and new cells/proteins need to be generated, taking time (e.g., 12-15 hours, though they possess millions of these cells).
• Bioluminescence: Some jellies are bioluminescent, but often microscopic glowing observed in water is due to dinoflagellates (which can produce toxins and cause red tides).
• Taxonomy: Two major clades:
  • Medusozoa: Includes Scyphozoans (true jellies), Cubozoans (box jellies), and Hydrozoans.
  • Anthozoa: Includes anemones and corals.

Ctenophores (Comb Jellies)

• Distinguishing Features:
  • Not cnidarians, despite superficial similarities; they belong to a completely different phylum.
  • Radially symmetrical.
  • Lack nematocysts; they do not have stinging cells.
  • Predators that use sticky mucus (mildly toxic, mostly paralyzing) to capture small prey like zooplankton, amphipods, and krill.
  • Relatively few species: only 100-150 known species, compared to 10,000 for cnidarians.
  • All are marine, mostly planktonic, though some can attach.
• Locomotion and Appearance:
  • Characterized by eight ciliated rows.
  • The beautiful, rainbow-like colors are not bioluminescence but rather light refraction caused by the cilia being only a few hundred nanometers apart, breaking light wavelengths as they beat.

Lophotrochozoans

• Etymology (Portmanteau): Derived from three words:
  • Zoa: Animal.
  • Lophophore: A feeding apparatus; a catcher's mitt-like spread of feathery extensions (not tentacles) that capture food and retract to the mouth. Common in sessile, sediment-dwelling animals (e.g., innkeeper worm).
  • Trochophore: A planktonic larval stage that aids in dispersal.
    • Resembles a dreidel, with an apical tuft of cilia at the top for vertical orientation and an equatorial ring of cilia for spinning.
    • Allows animals that settle (become sessile) as adults to spread through the water.
    • Contains a primitive gut and often a simple heart.
• Classification Basis: The name reflects a genetic association proven by gene sequencing, demonstrating an evolutionary link despite significant morphological differences.
• Included Groups: Contains animals with a lophophore, a trochophore, both, and even some that have neither but share genetic kinship.

Phylum Cycliophora

• Discovery: A single microscopic species (Symlophora) was discovered in the mid-1990s, leading to the classification of an entire new phylum.
• Habitat: Lives exclusively on the mouthparts and antennae of decapods (lobsters, crabs).
• Feeding: Captures food bits slopped off by its messy-eating decapod hosts.
• Significance: Represents the most recently discovered animal phylum, highlighting major genetic differences.

Platyhelminthes (Flatworms)

• Body Plan:
  • Triploblastic (three tissue layers) but acoelomic (lacking a body cavity), maximizing surface area.
  • Extremely flat, eliminating the need for respiratory organs like gills.
  • Possess neural ganglia that extend the length of the body.
• Regeneration: Known for high regenerative capabilities (e.g., two-headed planaria in lab).
• Classes: Four classes, but the main distinction is between parasitic and free-living forms.
  • Tubellarians: The only non-parasitic class.
    • Approximately 4,500 species, all marine.
    • Small carnivores that hunt or scavenge.
    • Simultaneous hermaphrodites (possessing both male and female reproductive organs) engaging in "penis fencing" to determine which individual will bear the offspring, as pregnancy is a high-cost endeavor.
    • Example: Persian carpet flatworm.
  • Other classes: Primarily parasitic (e.g., tapeworms, flukes), responsible for many diseases.

Nemertea (Ribbon Worms)

• Characteristics:
  • Often very long; one species, Lineus longissimus (the bootlace worm), can grow up to 50 meters (approx. 180 feet), potentially making it the longest animal on the planet.
  • Predatory, often feeding on annelids.
  • Have a closed circulatory system.
  • Hide primarily under sand, coiling their bodies.
  • Can strike prey with a proboscis that may deliver a neurotoxin to immobilize. Their large size often deters larger predators due to the potent neurotoxin.

Annelids (Segmented Worms)

• Diversity and Habitat:
  • Very diverse and important group (e.g., earthworms).
  • Found in marine, freshwater, and terrestrial environments.
• Segmentation: Their defining characteristic; each segment is a repetition of the previous one, though some are specialized (e.g., tube worm segments for holding symbionts, leech segments for blood).
  • Each segment may contain small, staggered "hearts" that collectively circulate blood, creating a pulsing motion.
• Classification (Modern vs. Historical):
  • Historically: Polychaetes (many bristles), Oligochaetes (few bristles), Hirudinea (leeches).
  • Modern (Cladistics based on genetics): Errantia (mobile worms) and Sedentaria (less mobile, sedentary worms).
• Cross-Sectional Anatomy:
  • Possess a full body cavity (coelom).
  • Endoderm forms the intestine, ectoderm is a tough outer layer (especially for burrowers).
  • Setae (or chaetae): Tough, bristle-like structures made of chitin (similar to fingernails), used like oars for burrowing through sand or for swimming.
  • Rudimentary eyes that can detect light/dark.
  • Good regenerative capabilities, especially in the posterior segments, as they are vulnerable to attack from behind.
• Errantia:
  • Mobile, marine, and typically hunters (heterotrophs).
  • Have paddle- or ridge-like structures called parapodia, which end in setae, aiding in movement through sand or water.
• Sedentaria:
  • Less mobile; some burrow and stay in place (e.g., Christmas tree worm).
  • Christmas Tree Worms: Dissolve coral to create burrows.
    • Often display external, multi-colored gills (these are the "tree" structures) as they are largely enclosed and need external oxygen exchange.

Mollusks

• Diversity and Habitat:
  • One of the largest animal phyla, with over 100,000 species.
  • Most are marine, but some are freshwater (bivalves, gastropods) and terrestrial (gastropods).
  • Gastropods (snails and slugs): Constitute approximately 75,000 of the 100,000 species, making them the most diverse mollusk group.
    • Only gastropods have successfully colonized terrestrial environments.
    • Slugs are vulnerable to desiccation due to lack of shell; snails have a watertight shell and an operculum (a protective door) to seal themselves, allowing them to inhabit drier environments.
  • Arthropods: The most diverse animal group globally (estimated 2,000,000 species, 50-60\% of all species), dwarfing molluscan diversity.
• General Body Plan: Despite diverse forms, they share common structures:
  • Muscular Foot: Used for movement.
    • In snails/slugs: a single large structure.
    • In bivalves: internal, used for digging into sediment.
    • In cephalopods: subdivided into arms and tentacles.
  • Visceral Mass: A "plastic bag" containing all internal organs, including gonads (which can make up over 50\% of body weight during reproductive cycles due to short lifespans).
    • Many exhibit a semelparous lifestyle (one big spawning event, then death), e.g., octopuses.
    • Exception: Some snails can live up to 100 years.
  • Mantle: A protective layer.
    • In cephalopods: the muscular outer layer (forms calamari).
    • In bivalves: forms a seal to hold the shell closed.
  • Radula: A chitinous, chitinous, rasping, tongue-like structure acting as a "chainsaw" or "metal chainsaw for a tongue."
    • Used for grasping and scraping food.
    • In some cases, can incorporate metals to increase hardness.
    • In squids: part of the beak; in snails, it can cut grooves.
  • Nudibranchs ("Naked Lungs"): Sea slugs with external gills (branchial structures), allowing for better oxygen uptake and active swimming (e.g., Spanish dancer).
• Gastropods (Snails and Slugs):
  • Most have a single spiral shell for protection, hydration, and defense.
  • Most are herbivores, but some have evolved predatorial radulae (e.g., boring snails drill into bivalves and digest them with acid).
  • Land snails may lose the ability to fully retract into shells if they live in moist environments.
• Bivalves (Clams, Oysters, Mussels, Scallops):
  • Named for their two-halved shell, drawn together by adductor muscles.
  • Some (e.g., scallops) have numerous rudimentary eyes that can detect light/dark, useful for their sedentary lifestyle.
• Cephalopods (Squid, Octopus, Cuttlefish, Chambered Nautilus):
  • Etymology: "Head-foot," reflecting their body plan.
  • Muscular foot is divided into arms and tentacles.
  • Most possess a venomous saliva to quickly immobilize prey, compensating for their fragile bodies (they are stronger than jellies but still vulnerable to thrashing fish).
  • Squid: Fast swimmers.
  • Octopus: Mostly benthic hiders, capable of squeezing through very small openings due to lack of rigid body structure.
  • Cuttlefish and Octopus: Masters of optical illusion and camouflage.
    • Can change skin texture (only animal group known to have fine control over skin dimensionality via sight, not touch).
    • Possess chromatophores (pigment dots in yellow, red, brown) which can expand up to 15 times their diameter.
    • Reflectors under the pigments produce short wavelengths (blues, greens).
    • Remarkably, they are colorblind but can achieve perfect color matching.
    • Exhibit twitching chromatophores and camouflage continuously, relying on it for predator protection.
    • Utilize a few basic pattern templates: uniform (little contrast), mottle (small scale light/dark splotches), and disruptive (patterns that break up the body outline).
    • Camouflage is not exact background matching but rather an ability to fool a predator's visual perception, suggesting other animals process visual information differently. The "moving rock" trick is an example of deceptive camouflage. The internal structure for squid is usually a chitinous "pen."
• Ink: Used as a defense mechanism, often from the pen.