Biology 320 Invertebrate Zoology Fall 2005

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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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.

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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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