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Molluscs: What are they? Definition: Soft-bodied animals that have an internal or external shell at some point in their development Molluscus means 'soft' in Latin evolved 600 MYA 100,000 + species 7 classes size: grain of sand to 20m long Molluscs – Characteristics that Unify Have true coelom Are protostomes Have trochophore larvae Trochophore larvae Swim in open water Feed on tiny floating plants Also seen in annelids Common ancestor Tuft of Cilia Ring of Cilia Protostomes, Coelomates Molluscs – Characteristics that Unify Have foot, mantle, visceral mass, shell Some lack shells, but evolved from shelled ancestors Variations on same basic body plan used to group into classes Mollusk form: Visceral Mass Contains internal organs Just below mantle Mollusk form: Mantle Thin tissue layer covering most of a mollusc's body Usually covers visceral mass Usually secretes shell Mollusk form: Shell Found in most molluscs Secreted by mantle Made of Calcium carbonate (CaCO3) Snail shell Whelk Shell Abalone shell Necklace made from Abalone Shell Mollusk form: Foot Soft and muscular Usually contains mouth and other structures associated with feeding Adapted - Flat to crawl Spade-shaped to burrow Tentacled to capture prey Molluscs – classified by the modifications of the Foot Class Gastropoda Gastropods – “stomach foot” Foot is broad and muscular found along the ventral surface used for moving, eating Examples: Snails, slugs, nudibranchs Class Gastropoda - Snails Gastropods - Sea slugs/nudibranchs Class Cephalopoda the tentacled molluscs “Head –foot” Foot is modified into head with tentacles coming out of it Have very little shell if any Cephalopods include: Octopi Squid Cuttlefish Nautiluses May have internal shell Swim with jet propulsion Squid – 8 arms + 2 long tentacles Class Bivalvia Have 2 shells – foot can extend out of shell Clams Oysters Mussels Geoduck, or king clam. A delicacy! Yumm Yumm Mollusk: Feeding Many forms of feeding: herbivores, carnivores, filter feeders, detritus feeders and parasites Feeding Many molluscs have a structure that is like a very rough tongue – has hundreds of tiny teeth for grinding through shells or scraping algae off of rocks Feeding Radula: tongue shaped structure made of flexible skin with hundreds of tiny teeth, like sandpaper Stiff supporting rod of cartilage Scrapes over surface Predators use to drill holes in animals Cartilage Rod Radula in grey Magnified 50x Mollusk: Feeding cont'd Some carnivorous mollusks use sharp jaws instead Filter feeding mollusks (bivalves) use gills to sift food from water Phytoplankton get stuck in sticky mucus Cilia pass into mouth Mollusk: Feeding cont'd A Squid's beak-used to kill and dismember prey. Molluscs – Filter feeders They also use the gills to BREATHE Clam – How it eats Incurrent siphon pulls water across gills Food particles stick to gills Coordinated cilia move food to mouth Food is stored/digested in stomach and intestines Solid wastes exit via the anus Mollusk: Respiration Usually by gills Aquatic: inside mantle Exception: nudibranchs Exception - Nudibranchs Means “naked gill” b/c gills are exposed Nudibranchs 3000+ species Carnivorous 6 mm – 31 cm Hermaphrodites Avg life: up to 1 year Some solarpowered! Store algae and live off sugars Nudibranchs Respiration Land snails and slugs use a moist sac inside the mantle Sac’s lining is convoluted (to increase surface area) and lined with blood vessels Mollusc: Internal Transport Open circulatory system: blood pumped by simple heart through body tissue in open spaces called sinuses (not contained in blood vessels) Sinuses lead to vessels that pass through gills where gas exchange occurs Works well for slow-moving (sessile) organisms like slugs, snails and clams, but inefficient for faster or larger molluscs Internal Transport Closed circulatory system: blood always moves inside vessels Good for fast moving molluscs such as squids and octopi (hunters) Closed circulatory system Open circulatory system Circulatory System Comparisons Molluscs: Excretion Solid waste excreted via anus as feces Nitrogen-containing waste, such as ammonia, removed by tube-shaped organs called nephridia Nephridia remove ammonia from blood and release it to outside environment Mollusc: Response Nervous system complexity highly varied Two-shelled molluscs usually simple nervous systems (they just dig in sand) Small ganglia near mouth, a few nerve cords, simple sense organs, statocysts (organ for balance) and ocelli (eye spots) Clam burrowing in sand Statocyst Swimming scallop Scallops Eye spots allow for detection of predators Sustainably harvested in BC as food source Only migratory bivalve Mollusc: Response Tentacled, fast moving molluscs highly developed nervous systems (to hunt) Well developed brain; capable of memory; complex sense organs Octopi: Well-developed brain (with memory) Eyes that are similar to ours – form images Octopi Highly intelligent Short and long-term memory Die soon after mating 3 hearts Can crawl, swim, jet propulsion, walk Some camouflaged Mollusc: Reproduction Usually separate sexes with external fertilization (snails, most two-shelled mollusks) Release egg + sperm into open water in large amount They meet by chance, developing into freeswimming trochophore larvae In tentacled mollusks and some snails, fertilization occurs inside female Fertilization also internal in hermaphrodites Reproduction Oysters – hermaphrodites that change sexes each season – sometimes they are the males, sometimes the females. Defence Mechanisms - Gastropods Slugs hide under rocks or only come out at night Some gastropods are poisonous – brightly colored to warn off predators Some nudibranchs save the nematocysts from the cnidarians they ate, and sting their predators with them Defence Mechanisms- Cephaloods Octopi move rapidly backwards with their jet propulsion of water Some release foul-tasting black ink Some can change colours Read text p. 593 (How Mollusks Fit Into Our World) Mollusks in our world: Positives Roles: eat plants, prey on animals, clean up surrounding, eat detritus, host symbionts or parasites, are parasites Important food sources, sometimes to humans Mollusks in our world: Positives Concentrate dangerous pollutants Monitor levels of toxins Environmental “Watchdogs” Filter feeders remove the pollutants from the water and concentrate them in their bodies Environmentalists can use these animals to learn information about levels of pollution in the water. Don’t eat bivalves during a RED TIDE What is red tide? Red Tide Mollusks in our world: Positives Snails never develop cancer how? Teeth of Mollusc: Dental use (toughened with iron biominerals- harder than steel) Stronger, more flexible, resistant to fracture Waterproof strong adhesive Mollusks in our world: Negatives Damage crops Shipworms eat hulls Termites of the sea Problem as food Eating concentrated pollutants Red tide Poisonous !
