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Biology 320 Invertebrate Zoology Fall 2005

Biology 320 Invertebrate Zoology Fall 2005. Chapter 12 – Phylum Mollusca Part Two. Class Gastropoda. Largest and most diverse class of molluscs Slugs, snails, and sea slugs Only molluscs that live terrestrially Mantle cavity is modified to form lung in some

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Biology 320 Invertebrate Zoology Fall 2005

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  1. Biology 320Invertebrate ZoologyFall 2005 Chapter 12 – Phylum Mollusca Part Two

  2. Class Gastropoda • Largest and most diverse class of molluscs • Slugs, snails, and sea slugs • Only molluscs that live terrestrially • Mantle cavity is modified to form lung in some • Approximately 60,000 described spp.

  3. All have some degree of torsion • Most have coiled shell • Much more active than polyplacophorans • Higher degree of cephalization than polyplacophorans • Tentacles • Eyes

  4. The Evolution of Torsion • What is torsion? • 180° counterclockwise rotation of visceral mass, relative to foot • Possible benefits of torsion • Allows entire animal to fit inside shell • Visceral mass always protected • Head and foot can be extended for feeding, locomotion, reproduction, etc. • May be retracted for protection and to minimize water loss • Mantle cavity is located anteriorly • May improve ventilation of gills / lung • Osphradia can test water before animal moves forward • Improved center of gravity for some

  5. Possible costs associated with torsion • Sanitation problems • Anus, nephridiopore, and gonopore located in close proximity to mouth • May foul mouth and gills • Many have anatomical adaptations to combat this problem • Shell perforations, as in abalone • Anal pore at apex of shell, as in keyhole limpets • Unidirectional water flow • One gill • Water enters on left and leaves on right side of mantle cavity

  6. Gastropod Shells • Hollow cone coiled around an axis known as a columella • Base • Apex • Aperture • Opening though which head and foot can be extended / retracted • Visceral mass stays inside shell

  7. Oftentimes, aperture has a siphonal canal or notch • Siphon • Anatomical adaptation for respiration • Portion of the mantle skirt that is rolled and elongate • Allows snail to take in water and test before moving into it • Acts like a snorkel for burrowers

  8. Complete revolution around columella is called a whorl • Columellar muscle • Retracts head and foot • Originates at columella • Inserts at foot • Operculum • Proteinaceous or calcified disc • Located on dorsal / posterior foot • Seals aperture to protect against predation and desiccation

  9. Shell growth occurs when edge of mantle secretes organic materials and minerals on lips of aperture • Growth lines can often be seen • Amazing diversity of shell shapes, patterns, and colors • Coloration comes from pigments that are synthesized, or sequestered from food items

  10. Shell shape is related to habitat • Small spires = better adapted for attaching upside-down or vertically to rocks or aquatic vegetation • Long spires = moving horizontally over soft substrates • Low / broad shells = clinging to rocks in strong currents • Some, like moon snails, have a muscular foot that facilitates burrowing

  11. Gastropod Diversity • Three main groups: • Prosobranchs • Opisthobranchs • Pulmonates • No longer called Subclass Prosobranchia, Opisthobranchia, and Pulmonata • Recent uncertainty of classification • Such diversity that it is easiest to point out specifics of each group • Shell size, shape, colors and patterns • Radula structure • Feeding ecology • Reproduction • Etc.

  12. Prosobranchs • 20,000 spp. • Most marine and benthic • Few freshwater and terrestrial • Externally and internally torted • Marine limpets, abalones, and snails

  13. Shell may be coiled, or may posses one large whorl like an abalone shell • Snails posses an operculum • Respiratory system • Siphon • Solitary left ctenidium • Monopectinate • One kidney • Located on left side • One nephridiopore empties into mantle cavity • One gonad • Right side • One gonopore empties into mantle cavity

  14. Cephalized • Two cephalic tentacles • Typically posses one lateral eye at the base of each tentacle • Reproduction • Gonochoric • Some direct development • Unique egg cases molded by pedal gland of foot • Some produce a veliger larva • Possesses a swimming organ known as a velum, which consists of two circular, ciliated lobes • Has a shell

  15. Size: a few mm to 70 cm • Limpets • Well adapted to clinging to rocks • Travel up to five feet from “homes” and use homing behavior to return • Follow chemical cues from mucus they secrete • Mucus also stimulates algal growth; they feed on algae • Abalones • Also well adapted for clinging to rocks, due to low / broad shell • Turban snails

  16. Conchs • Whelks • Carnivores • Scavengers can detect carrion that is 30 m away • May wedge bivalves open using the foot, and edge of aperture or siphonal canal • Moon snails • Some drill holes in the shells of bivalves, barnacles, and limpets • Foot has a drill organ that drills for one min • Secretes acid into hole and lets it sit for 30 min • Drills again to removed softened shell • May take 8 hr to drill through a 2 mm thick shell

  17. Drills • Urosalpinx – American oyster drill • Can decimate oyster beds • Cone snails • Carnivores of polychaetes, gastropods, and fish • Tropical – live in Indo-Pacific and Western Atlantic • Highly modified radula • One radular tooth bathed in and filled with neurotoxic venom • Everted like a harpoon • Tooth replaced • Occasional human deaths from cone snail venom

  18. Opisthobranchs • 3000 marine spp. • Interstitial to 60 cm in length • Detorted • Also seen in the nervous system • Therefore, they are essentially bilaterally symmetrical • Reduction / loss of mantle cavity • Reduction / loss of shell • Modern species lack opercula

  19. Because they lack a shell, opisthobranchs have developed other defenses • Sequester nematocysts for defense • Some have lateral expansions of the foot (called parapodia) that they use for escape swimming • Often have skin glands that produce sulfuric acid or other noxious substances that repel fish and other predators • Aposematic coloration • Cryptic coloration

  20. Gills • Absent in some • When present are unlike prosobranch gills • Gill SA is provided by mantle folds • May be located on body surface in some (anal gills or cerata) • Nervous system • Trend towards cephalization and detorsion • Cephalic tentacles • Rhinophores • Second pair of tentacles, just posterior to the first • Chemosensory • Can be retracted into a sheath

  21. Reproduction • Simultaneous hermaphrodites • Reciprocal internal fertilization • Eggs usually oviposited in gelatinous strings • Veliger larvae or direct development • Bubble snails • Also called bubble shells • Still have a shell, although somewhat reduced • Superficially resemble sea slugs • Most primitive of the opisthobranchs

  22. Sea Hares • Largest opisthobranchs (up to 60 cm) • Resemble sea slugs • Shell is reduced (in mantle) or absent • Some swim with parapodia, or by jet propulsion • Some release purple ink when disturbed • Pteropods • Sea butterflies • Swim using large parapodia • Small; some posses a light shell • Some conduct gas exchange across body surface

  23. Nudibranchs • Sea slugs • Graze on cnidarians and sequester nematocysts or photosynthetic endosymbionts • Lack shell and mantle cavity • Sometimes lack gills • Cerata • Function as gills • Sites for nematocysts • Each contain a branch of digestive cecum

  24. Pulmonates • 16,000 – 30,000 spp. • Primitive • Intertidal and freshwater snails • Intertidal and freshwater limpets • Modern • Terrestrial snails, and slugs • Detorsion • Slugs lack shells • Original distribution center of slugs has low soil calcium • Pulmonate snails lack an operculum

  25. Mantle cavity on right side, but no gills; mantle cavity converted to a lung • Opening to lung is termed pneumostome • Roof of mantle cavity is highly vascularized • Capillary bed • Floor of cavity is elevated and depressed to ventilate

  26. Physiological adaptations to terrestrial existence • Secrete a mucus plug that acts like an operculum • Secrete mucus • Prevents desiccation • Defends against bacteria • Discourages predators • Facilitates locomotion • Uricotelic instead of ammonotelic • Tolerant to water loss • Helix can lose 50% of body water • 80% in Limax

  27. Behaviors associated with terrestrial existence • Periods of torpor during unfavorable weather • Estivation during hot / dry weather • Hibernation during cold weather • Most inhabit humid environments • Those living in xeric environments are only active at night or after rains

  28. Most are herbivorous, and some are serious crop pests • Many have been introduced • Giant African snail has been introduced to Hawaii and continental U.S. • Up to 23 cm in height • Second largest group of snails can reach 15 cm and are found in South America

  29. Hermaphrodites • Reciprocal internal fertilization • Could be considered an adaptation to terrestrial life • Many deposit spermatophores • Direct development • Interesting courtship rituals • Twist around a mucus strand in Limax • Love darts in Helix

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