class gastropoda
DESCRIPTION
Class Gastropoda. Snails - largest class of molluscs ~ 85,000 living spp. ~ 15,000 fossil spp. Habitats: marine benthos, ocean plankton, freshwater, land. After torsion. Gastropod characteristics. 1. Torsion - primitive bilateral symmetry lost during development - PowerPoint PPT PresentationTRANSCRIPT
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Class Gastropoda
• Snails - largest class of molluscs
• ~ 85,000 living spp.
• ~ 15,000 fossil spp.
• Habitats: marine benthos, ocean plankton, freshwater, land
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Gastropod characteristics
• 1. Torsion - primitive bilateral symmetry lost during development
• Twisting of visceral mass, mantle, and mantle cavity
After torsion
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Gastropod characteristics
• 2. Definite head: eyes, tentacles
• 3. Flat ventral foot
• 4. Mantle cavity w/gills or lungs
• 5. Buccal cavity w/ radula
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Characters cont.
• 6. Coiled shell - one continuous piece– operculum on posterior foot of
some
helico-spiralplano-spiral
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Characters, cont.
• 7. Centralized complex nervous system
• 8. Hermaphroditic or dioecious
• 9. Oviparous or ovoviparous
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10. Larval form
• Archaeogastropoda have trochophore larva
• Many marine snails have veliger larva
• Freshwater + terrestrial species usually hatch as young snails
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Gastropod classification
• 3 groups:
• Caenogastropoda: includes gilled snails
• Opisthobranchs - reduced shell
• Pulmonata - lungs
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Caenogastropoda
• Mantle cavity anterior
• 1 or 2 gills
• shell and operculum usually present
• most are dioecious
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Caenogastropoda
• A. slit shells - deep water species– Abalones (Haliotis)– 9 spp on our Pacific coast– Commercially harvested
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Caenogastropoda
• B. keyhole limpets - Fissurella– conical shells
• C. Limpets - Acmaea
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Caenogastropoda
• D. topshells, turban shells, star shells– Astraea
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Neritimorpha
• E. Nerites (Nerita) intertidal in Caribbean– some freshwater + terrestrial
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Prosobranchs
• Male with penis
• Fertilization internal
• 1 monopectinate gill
• 1 auricle
• 1 coeloduct
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Prosobranchs
• A. Freshwater apple snails -– Viviparus, Pomacea
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Prosobranchs
• B. Turret shells - worm shells; caecums, ceriths, some freshwater genera, all have high shells
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Prosobranchs
• C. Violet snails - pelagic grazers on man o’war– Vellela, Porpita– Janthina: secretes bubble mass for
floatation
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Prosobranchs
• D. Cap shells, slipper shells– Crepidula change sex w/age
• see slides of veliger larva
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Prosobranchs
• E. Conchs - Strombus– Feed on algae, turtlegrass– Commercial harvest in Caribbean
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• F. Cowries; Cypraea - graze on algae, sponges, gorgonians, and tunicates
• Mantle covers most of shell when extended• Favorites of shell-collectors• No periostracum• Italians call them “porcellanos” = little pigs
– Porcelain got its name
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Cowries
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• G. Heteropods - pelagic, swimming snails w/reduced shell
• Carinaria (see Fig. 10-30, p. 399)
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• H. Moon shells - predators on molluscs:– bore into shells– Lunatia– Polinices
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Moon shells:
• Muscular suction disk holds snail on clam shell• Drill through shell with radula:• Wiggle proboscis into flesh• Remove most of flesh from clam
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• I. Helmet shells, tritons, tuns
• Predators on molluscs and echinoderms
• Long proboscis
• Large shells
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• J. Periwinkles - intertidal– Littorina
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• Mostly same characteristics as Mesogastropoda
• Radula has only 3 teeth in a transverse row
• Most are marine
• Most are carnivores
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• A. Drills - Murex, Urosalpinx
• Drill holes in shells of bivalves and barnacles
• Oyster drill and others cause economic losses
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• B. Whelks - Busycon
• Tulip shells - Fasciolaria
• Predators and carrion feeders
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• C. Olives, miters, and volutes
• Colorful, mostly tropical snails
• prey on invertebrates
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• D. Cones - Conus
• Most prey on inverts
• Those that prey on fish can be dangerous to humans
• Fig 10-42, p. 409
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Conus
• Long proboscis with harpoon-like radular tooth
• venom gland
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“Glory of the sea” cone shell
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Opisthobranchia
• Detorsion - brings mantle cavity to right side
• 1 gill, 1 auricle, 1 coelomoduct
• Shell often reduced or absent
• Mantle cavity often reduced or absent
• Many are secondarily bilateral symmetric
• Hermaphroditic
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Opisthobranchia
• A. Bubble shells - Bulla
• Predators with thin, reduced shell
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Opisthobranchia
• B. Sea hares - Aplysia
• Thin, caplike shell overgrown by mantle
• Vegetarians (herbivores)
• Can eject milky fluid from mantle cavity
• A. californica has largest body of all gastropods
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Opisthobranchia
• C. Sea slugs
• With or w/o shell
• Berthelinia is a “bivalve” gastropod
• Fig. 10-23, p. 392
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Opisthobranchia
• D. Pteropods or sea butterflies
• 2 orders with and w/o (naked) shells
• Planktonic with foot modified into wing-like flaps for swimming
• Often occur in enormous numbers
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More Pteropods
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Opisthobranchia
• E. Nudibranchs
• Often with secondary gills and cerata (hornlike dorsal projections)
• Many endemics
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Opisthobranchia
• F. Parasitic Opisthobranchs
• 2 orders
• One ectoparisitic on bivalves and annelids
• One endoparasitic in sea cucumbers
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Pulmonata
• Charactistics like Opisthobranchia, but w/o gill
• Mantle cavity converted to lung
• Shell usually present
• No operculum
• Hermaphroditic
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Pulmonata
• A. freshwater snails: – Lymnaea, Physa, Planorbis– Freshwater limpets
• Most come to surface for air
• Some have developed secondary gills
• Descended from terrestrial ancestors
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Pulmonata
• B. Land snails and slugs– Helix– Limax
• C. Intertidal slugs w/posterior anus
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Class Scaphopodatusk shells
• ~ 300 spp.• Sedentary, marine burrowing, in 6 - 1800 m
depth• Shells resemble elephant tusks• Most burrow in sand, few in mud• Feed on microscopic organisms;
foraminiferans• Considered offshoot of early bivalve ancestors
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Scaphopod characteristics
• 1. Tusk-like shell open at both ends
• 2. Bilateral symmetry (like bivalves)
• 3. Rudimentary head– no eyes – Head has threadlike, food gathering
tentacles (captacula)
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Foot
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• 4. Radula present
• 5. Circulatory system reduced - sinuses– Gills absent
• 6. Dioecious, trochophore and veliger larval stages
Scaphopod characteristics
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Scaphopod classification
• Two families (representative species):
• Dentalium - conical shell
• Cadulus + Siphonodentalium - globular shell, enlarged foot