Mollusca* 🐌

Body Plan:

  • Lophotrochozoans — Trochophore larvae

  • true coleom, bilateral cephalization, no segmentation (as a group), ALL 3 KINDS OF SUPPORT STRUCTURES

  • ~100,000 species, largest marine phyla, 3rd largest overall

  • Malacology: study of molluscs, more endangered bivalves

  • Synapomorphies: mantle + mantle cavity

Anatomy:

  • Head-foot region: feeding/sensory/cephalic/locomotor

  • Visceral mass: contains digestive, circulatory, respiratory, & reproductive organs

*while head-foot/visceral mass are present throughout mollusca, the defining trait is actually the mantle + mantle cavity

  • Head

    • well-developed w/mouth and sensory organs

      • radulae only in molluscs, lost in bivalves

    • Radula = rasping, tongue-like organ w/up to 250,000 teeth

  • Foot

    • Functions/Modifications:

      • Attachment - disc of limpets

      • Digging - foot of bivalves

      • Gliding - gastropods

      • Siphon - jet/arms of cephalopods

    • mucus aids in adhesion or gliding w/cilia

  • Mantle + Mantle Cavity

    • Mantle = dorsal body wall, secretes shell & covers visceral mass

    • Mantle Cavity = houses lungs or gills (gas exchange), digestive/excretory waste, gametes empty into mantle cavity

    • “The external environment under the shell”

  • Shell

    • Periostracum: outer protein layer, thick in freshwater molluscs, protects against acid from leaf decay

    • Prismatic Layer: closely packed CaCO3, main part of shell

    • Nacre: innermost layer, next to mantle, secreted continuously throughout life,

      • “mother of pearl” - secreted around irritants that enter mantle cavity

Digestion: radula; anus empties out into mantle cavity

Circulatory: open system; pumping heart, few vessels

  • Cephalopods have a closed system

Respiratory: gill or lung (vascularized chamber)

Excretory: kidneys or nephridia

  • Kidney ducts also discharge sperm and eggs

Nervous/Sensory:

  • cerebral ganglia near pharynx

    • Cephalopod — most advanced invertebrate brain

  • 2 nerve cord pairs — in foot and lower viscera

  • sense organs… vary…

Reproduction & Development:

  • Most have separate sexes; some gastropods are hermaphroditic

  • some external and some internal fertilization

  • trochophore larva

    • veliger = more developed larvae

    • glochidia = specialized parasitic larvae of many bivalves

Defense: shell, venom/poison, ink, mucus, camo, mutualism, etc.

Diversity:

  • C. Gastropoda — snail

  • C. Cephalopoda — octopus

  • C. Bivalvia — clam

  • C. Polyplacophora — chiton

  • C. Scaphopoda — tusk shell

Class Gastropoda

  • most diverse class; 70,000 species

  • all environments — marine, freshwater, terrestrial

  • Shells: main defense, one-piece

    • operculum = covers aperture to prevent water loss

    • ancestral shells: flat spiral → helix

  • Torsion = mantle cavity and anus rotate

    • Results: viscera shift left to right, nerve cords form figure 8

    • fouling = anus empties over mouth, sense organs (→ gill moves)

  • Feeding: radula adaptations = feeding variations (herbivore, carnivore, scavenger)

  • Respiration: gills (mantle cavity) or “lung” (vascular area in mantle)

    • Pneumostome = lung opening to outside

  • Reproduction: [Snails, Love Darts, & Slug Sex]

    • Dioecious and hermaphroditic species

    • NO self-fertilization

    • land snails lay eggs in gelatinous masses

Class Cephalopoda

  • Nautiluses, Cuttlefish, Squids, Octopuses

    • foot modified into tentacles + funnel

    • ALL predators

    • octopuses and cuttlefish produce venom in salivary glands

    • strong beak-like jaws grasp prey

  • Shells:

    • Nautilus = gas chambers in shell allow swimming

      • animal only inhabits last chamber

      • cord (siphuncle) extends through chambers and adjusts gases

    • Cuttlefish = “cuttlebone” in mantle

    • Squid = “pen” in mantle

    • Octopus = completely lost

  • Circulation/Respiration:

    • Increase in oxygen demands, increase in size

    • 1) ventricle contracts → oxygenated blood to body

    • 2) deoxygenated blood returns to branchial hearts

    • 3) branchial hearts contract to gills (exchange gases)

    • 4) blood returns to ventricle [& repeat]

  • Nervous/Sensory:

    • large diameter nerves (speed w/o myelination)

    • brain w/cartilaginous cranium

    • Sense Organs: well-developed; complex eyes + photosensitive chromatophores

    • tool use!

  • Movement:

    • expel water through siphon

      • controls direction and force/speed

    • squid/cuttlefish → lateral fins = stabilizers

    • octopus → crawls on sea floor or swim around with webbing

  • Reproduction:

    • sexes separate, internal fertilization, eggs are laid

  • Defense:

    • color change/camouflage: skin texture can change, can imitate other animals

      • chromatophores = cells in skin expand and contrast

    • ink: ink sac empties into rectum, secretes melanin, pseudomorph

Class Bivalvia

  • clams, scallops, oysters, mussels, shipworms

    • 2 shells, sedentary, suspension feeders → reduced head and no radula

  • Shells:

    • closed by adductor muscles

    • Umbo = oldest part of shell, growth occurs outward in rings

  • Feeding:

    • 1) H2O moves over gills

    • 2) gills secrete mucus and traps particles

    • 3) food in mucus → grooves at gills’ edge

    • 4) cilia move mucus to mouth

Class Polyplacophora

  • chitons

  • flattened with 8 dorsal plates

  • prefer rocky intertidal zones; scrape algae from the rocks

Class Scaphopoda

  • tusk shells

  • sedentary, marine

  • tubular shell, open both ends

    • mantle fused → tube around viscera

    • foot/tentacles from larger end burrow

  • eats detritus caught on tentacles