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Biology 320Biology 320Invertebrate ZoologyInvertebrate ZoologyFall 2005Fall 2005
Chapter 12 – Phylum MolluscaChapter 12 – Phylum Mollusca
Part TwoPart Two
Class GastropodaClass Gastropoda
Largest and most diverse Largest and most diverse class of molluscsclass of molluscs
Slugs, snails, and sea slugs Slugs, snails, and sea slugs
Only molluscs that live Only molluscs that live terrestriallyterrestrially– Mantle cavity is Mantle cavity is
modified to form lung in modified to form lung in somesome
Approximately 60,000 Approximately 60,000 described spp.described spp.
All have some degree of torsionAll have some degree of torsion
Most have coiled shell Most have coiled shell
Much more active than polyplacophoransMuch more active than polyplacophorans
Higher degree of cephalization than Higher degree of cephalization than polyplacophoranspolyplacophorans– TentaclesTentacles– EyesEyes
The Evolution of The Evolution of TorsionTorsion What is torsion?What is torsion?
– 180180° counterclockwise rotation of visceral mass, relative to foot° counterclockwise rotation of visceral mass, relative to foot
Possible benefits of torsionPossible benefits of torsion– Allows entire animal to fit inside shellAllows entire animal to fit inside shell
Visceral mass always protectedVisceral mass always protected Head and foot can be extended for feeding, locomotion, Head and foot can be extended for feeding, locomotion,
reproduction, etc.reproduction, etc. May be retracted for protection and to minimize water May be retracted for protection and to minimize water
lossloss
– Mantle cavity is located anteriorlyMantle cavity is located anteriorly May improve ventilation of gills / lungMay improve ventilation of gills / lung Osphradia can test water before animal moves forwardOsphradia can test water before animal moves forward
– Improved center of gravity for someImproved center of gravity for some
Possible costs associated with Possible costs associated with torsiontorsion– Sanitation problemsSanitation problems
Anus, nephridiopore, and Anus, nephridiopore, and gonopore located in close gonopore located in close proximity to mouthproximity to mouth
May foul mouth and gillsMay foul mouth and gills
Many have anatomical Many have anatomical adaptations to combat this adaptations to combat this problemproblem– Shell perforations, as in Shell perforations, as in
abaloneabalone– Anal pore at apex of shell, as Anal pore at apex of shell, as
in keyhole limpetsin keyhole limpets– Unidirectional water flowUnidirectional water flow
One gillOne gill Water enters on left and Water enters on left and
leaves on right side of mantle leaves on right side of mantle cavitycavity
Gastropod ShellsGastropod Shells Hollow cone coiled Hollow cone coiled
around an axis known around an axis known as a columellaas a columella
BaseBase
ApexApex
ApertureAperture– Opening though which Opening though which
head and foot can be head and foot can be extended / retractedextended / retracted
– Visceral mass stays Visceral mass stays inside shellinside shell
Oftentimes, aperture Oftentimes, aperture has a siphonal canal has a siphonal canal or notchor notch
SiphonSiphon– Anatomical Anatomical
adaptation for adaptation for respirationrespiration
– Portion of the mantle Portion of the mantle skirt that is rolled and skirt that is rolled and elongateelongate
– Allows snail to take in Allows snail to take in water and test before water and test before moving into itmoving into it
– Acts like a snorkel for Acts like a snorkel for burrowersburrowers
Complete revolution Complete revolution around columella is around columella is called a whorlcalled a whorl
Columellar muscleColumellar muscle– Retracts head and footRetracts head and foot– Originates at columellaOriginates at columella– Inserts at footInserts at foot
OperculumOperculum– Proteinaceous or Proteinaceous or
calcified disccalcified disc– Located on dorsal / Located on dorsal /
posterior footposterior foot– Seals aperture to Seals aperture to
protect against protect against predation and predation and desiccationdesiccation
Shell growth occurs when Shell growth occurs when edge of mantle secretes edge of mantle secretes organic materials and organic materials and minerals on lips of apertureminerals on lips of aperture
Growth lines can often be Growth lines can often be seenseen
Amazing diversity of shell Amazing diversity of shell shapes, patterns, and shapes, patterns, and colorscolors
Coloration comes from Coloration comes from pigments that are pigments that are synthesized, or synthesized, or sequestered from food sequestered from food items items
Shell shape is related to Shell shape is related to habitathabitat– Small spires = better Small spires = better
adapted for attaching adapted for attaching upside-down or vertically upside-down or vertically to rocks or aquatic to rocks or aquatic vegetationvegetation
– Long spires = moving Long spires = moving horizontally over soft horizontally over soft substratessubstrates
– Low / broad shells = Low / broad shells = clinging to rocks in strong clinging to rocks in strong currentscurrents
– Some, like moon snails, Some, like moon snails, have a muscular foot that have a muscular foot that facilitates burrowingfacilitates burrowing
Gastropod DiversityGastropod Diversity Three main groups:Three main groups:
– ProsobranchsProsobranchs– OpisthobranchsOpisthobranchs– PulmonatesPulmonates
No longer called Subclass Prosobranchia, Opisthobranchia, No longer called Subclass Prosobranchia, Opisthobranchia, and Pulmonataand Pulmonata
Recent uncertainty of classification Recent uncertainty of classification
Such diversity that it is easiest to point out specifics of each Such diversity that it is easiest to point out specifics of each groupgroup– Shell size, shape, colors and patternsShell size, shape, colors and patterns– Radula structure Radula structure – Feeding ecology Feeding ecology – ReproductionReproduction– Etc.Etc.
ProsobranchsProsobranchs
20,000 spp.20,000 spp.
Most marine and Most marine and benthicbenthic
Few freshwater and Few freshwater and terrestrialterrestrial
Externally and internally Externally and internally tortedtorted
Marine limpets, Marine limpets, abalones, and snailsabalones, and snails
Shell may be coiled, or Shell may be coiled, or may posses one large may posses one large whorl like an abalone shellwhorl like an abalone shell
Snails posses an Snails posses an operculumoperculum
Respiratory systemRespiratory system– SiphonSiphon– Solitary left ctenidiumSolitary left ctenidium– MonopectinateMonopectinate
One kidneyOne kidney– Located on left sideLocated on left side– One nephridiopore empties into One nephridiopore empties into
mantle cavitymantle cavity
One gonadOne gonad– Right sideRight side– One gonopore empties into One gonopore empties into
mantle cavitymantle cavity
CephalizedCephalized– Two cephalic tentaclesTwo cephalic tentacles– Typically posses one Typically posses one
lateral eye at the base of lateral eye at the base of each tentacleeach tentacle
ReproductionReproduction– GonochoricGonochoric– Some direct developmentSome direct development
Unique egg cases Unique egg cases molded by pedal gland molded by pedal gland of footof foot
– Some produce a veliger Some produce a veliger larvalarva
Possesses a swimming Possesses a swimming organ known as a velum, organ known as a velum, which consists of two which consists of two circular, ciliated lobescircular, ciliated lobes
Has a shellHas a shell
Size: a few mm to 70 cmSize: a few mm to 70 cm
LimpetsLimpets– Well adapted to clinging to rocksWell adapted to clinging to rocks– Travel up to five feet from Travel up to five feet from
“homes” and use homing “homes” and use homing behavior to returnbehavior to return
– Follow chemical cues from Follow chemical cues from mucus they secretemucus they secrete
– Mucus also stimulates algal Mucus also stimulates algal growth; they feed on algaegrowth; they feed on algae
AbalonesAbalones– Also well adapted for clinging to Also well adapted for clinging to
rocks, due to low / broad shellrocks, due to low / broad shell
Turban snailsTurban snails
ConchsConchs
WhelksWhelks– CarnivoresCarnivores– Scavengers can detect carrion that Scavengers can detect carrion that
is 30 m awayis 30 m away– May wedge bivalves open using May wedge bivalves open using
the foot, and edge of aperture or the foot, and edge of aperture or siphonal canal siphonal canal
Moon snailsMoon snails– Some drill holes in the shells of Some drill holes in the shells of
bivalves, barnacles, and limpetsbivalves, barnacles, and limpets– Foot has a drill organ that drills for Foot has a drill organ that drills for
one minone min– Secretes acid into hole and lets it Secretes acid into hole and lets it
sit for 30 minsit for 30 min– Drills again to removed softened Drills again to removed softened
shellshell– May take 8 hr to drill through a 2 May take 8 hr to drill through a 2
mm thick shellmm thick shell
DrillsDrills– UrosalpinxUrosalpinx – American – American
oyster drilloyster drill– Can decimate oyster bedsCan decimate oyster beds
Cone snailsCone snails– Carnivores of polychaetes, Carnivores of polychaetes,
gastropods, and fishgastropods, and fish– Tropical – live in Indo-Pacific Tropical – live in Indo-Pacific
and Western Atlanticand Western Atlantic– Highly modified radulaHighly modified radula
One radular tooth bathed in One radular tooth bathed in and filled with neurotoxic and filled with neurotoxic venomvenom
Everted like a harpoonEverted like a harpoon Tooth replacedTooth replaced
– Occasional human deaths Occasional human deaths from cone snail venomfrom cone snail venom
OpisthobranchsOpisthobranchs
3000 marine spp.3000 marine spp.
Interstitial to 60 cm in lengthInterstitial to 60 cm in length
DetortedDetorted– Also seen in the nervous Also seen in the nervous
systemsystem– Therefore, they are Therefore, they are
essentially bilaterally essentially bilaterally symmetricalsymmetrical
Reduction / loss of mantle Reduction / loss of mantle cavitycavity
Reduction / loss of shellReduction / loss of shell
Modern species lack operculaModern species lack opercula
Because they lack a shell, Because they lack a shell, opisthobranchs have opisthobranchs have developed other defensesdeveloped other defenses– Sequester nematocysts for Sequester nematocysts for
defensedefense
– Some have lateral Some have lateral expansions of the foot expansions of the foot (called parapodia) that they (called parapodia) that they use for escape swimminguse for escape swimming
– Often have skin glands that Often have skin glands that produce sulfuric acid or produce sulfuric acid or other noxious substances other noxious substances that repel fish and other that repel fish and other predatorspredators
– Aposematic colorationAposematic coloration
– Cryptic colorationCryptic coloration
GillsGills– Absent in someAbsent in some– When present are unlike When present are unlike
prosobranch gillsprosobranch gills– Gill SA is provided by mantle Gill SA is provided by mantle
folds folds – May be located on body May be located on body
surface in some (anal gills or surface in some (anal gills or cerata)cerata)
Nervous systemNervous system– Trend towards cephalization Trend towards cephalization
and detorsionand detorsion– Cephalic tentaclesCephalic tentacles– RhinophoresRhinophores
Second pair of tentacles, just Second pair of tentacles, just posterior to the firstposterior to the first
ChemosensoryChemosensory Can be retracted into a sheathCan be retracted into a sheath
ReproductionReproduction– Simultaneous Simultaneous
hermaphroditeshermaphrodites– Reciprocal internal Reciprocal internal
fertilizationfertilization– Eggs usually oviposited in Eggs usually oviposited in
gelatinous stringsgelatinous strings– Veliger larvae or direct Veliger larvae or direct
developmentdevelopment
Bubble snailsBubble snails– Also called bubble shellsAlso called bubble shells– Still have a shell, Still have a shell,
although somewhat although somewhat reducedreduced
– Superficially resemble Superficially resemble sea slugssea slugs
– Most primitive of the Most primitive of the opisthobranchsopisthobranchs
Sea HaresSea Hares– Largest opisthobranchs Largest opisthobranchs
(up to 60 cm)(up to 60 cm)– Resemble sea slugsResemble sea slugs– Shell is reduced (in Shell is reduced (in
mantle) or absentmantle) or absent– Some swim with Some swim with
parapodia, or by jet parapodia, or by jet propulsionpropulsion
– Some release purple ink Some release purple ink when disturbedwhen disturbed
PteropodsPteropods– Sea butterfliesSea butterflies– Swim using large Swim using large
parapodiaparapodia– Small; some posses a light Small; some posses a light
shellshell– Some conduct gas Some conduct gas
exchange across body exchange across body surfacesurface
NudibranchsNudibranchs– Sea slugsSea slugs– Graze on cnidarians Graze on cnidarians
and sequester and sequester nematocysts or nematocysts or photosynthetic photosynthetic endosymbiontsendosymbionts
– Lack shell and Lack shell and mantle cavitymantle cavity
– Sometimes lack gillsSometimes lack gills– Cerata Cerata
Function as gillsFunction as gills Sites for nematocystsSites for nematocysts Each contain a branch Each contain a branch
of digestive cecumof digestive cecum
PulmonatesPulmonates 16,000 – 30,000 spp.16,000 – 30,000 spp.
PrimitivePrimitive– Intertidal and freshwater Intertidal and freshwater
snails snails – Intertidal and freshwater Intertidal and freshwater
limpetslimpets
ModernModern– Terrestrial snails, and slugsTerrestrial snails, and slugs
DetorsionDetorsion
Slugs lack shellsSlugs lack shells– Original distribution center of Original distribution center of
slugs has low soil calciumslugs has low soil calcium
Pulmonate snails lack an Pulmonate snails lack an operculumoperculum
Mantle cavity on right side, but no gills; mantle Mantle cavity on right side, but no gills; mantle cavity converted to a lungcavity converted to a lung
Opening to lung is termed pneumostome Opening to lung is termed pneumostome
Roof of mantle cavity is highly vascularizedRoof of mantle cavity is highly vascularized– Capillary bedCapillary bed
Floor of cavity is elevated and depressed to Floor of cavity is elevated and depressed to ventilateventilate
Physiological adaptations Physiological adaptations to terrestrial existenceto terrestrial existence
– Secrete a mucus plug that Secrete a mucus plug that acts like an operculumacts like an operculum
– Secrete mucus Secrete mucus Prevents desiccationPrevents desiccation Defends against bacteriaDefends against bacteria Discourages predatorsDiscourages predators Facilitates locomotionFacilitates locomotion
– Uricotelic instead of Uricotelic instead of ammonotelicammonotelic
– Tolerant to water lossTolerant to water loss HelixHelix can lose 50% of body can lose 50% of body
waterwater 80% in 80% in LimaxLimax
Behaviors associated with Behaviors associated with terrestrial existenceterrestrial existence– Periods of torpor during unfavorable Periods of torpor during unfavorable
weatherweather Estivation during hot / dry weatherEstivation during hot / dry weather Hibernation during cold weatherHibernation during cold weather
– Most inhabit humid environmentsMost inhabit humid environments
– Those living in xeric environments Those living in xeric environments are only active at night or after rainsare only active at night or after rains
Most are herbivorous, Most are herbivorous, and some are serious and some are serious crop pestscrop pests
Many have been Many have been introducedintroduced– Giant African snail has Giant African snail has
been introduced to Hawaii been introduced to Hawaii and continental U.S.and continental U.S.
Up to 23 cm in heightUp to 23 cm in height
Second largest group of Second largest group of snails can reach 15 cm snails can reach 15 cm and are found in South and are found in South AmericaAmerica
HermaphroditesHermaphrodites
Reciprocal internal fertilizationReciprocal internal fertilization– Could be considered an adaptation to terrestrial lifeCould be considered an adaptation to terrestrial life– Many deposit spermatophoresMany deposit spermatophores
Direct developmentDirect development
Interesting courtship ritualsInteresting courtship rituals– Twist around a mucus strand in LimaxTwist around a mucus strand in Limax– Love darts in HelixLove darts in Helix