Transcript Biology 320 Invertebrate Zoology Fall 2005

Biology 320
Invertebrate Zoology
Fall 2005
Chapter 12 – Phylum Mollusca
Part Two
Class Gastropoda
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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.
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All have some degree of torsion
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Most have coiled shell
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Much more active than polyplacophorans
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Higher degree of cephalization than polyplacophorans
– Tentacles
– Eyes
The Evolution of Torsion
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What is torsion?
– 180° counterclockwise rotation of visceral mass, relative to foot
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Possible benefits of torsion
– Allows entire animal to fit inside shell
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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
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May improve ventilation of gills / lung
Osphradia can test water before animal moves forward
– Improved center of gravity for some
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Possible costs associated with
torsion
– Sanitation problems
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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
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One gill
Water enters on left and leaves
on right side of mantle cavity
Gastropod Shells
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Hollow cone coiled around
an axis known as a
columella
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Base
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Apex
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Aperture
– Opening though which head
and foot can be extended /
retracted
– Visceral mass stays inside
shell
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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
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Complete revolution
around columella is
called a whorl
Columellar muscle
– Retracts head and foot
– Originates at columella
– Inserts at foot
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Operculum
– Proteinaceous or calcified
disc
– Located on dorsal /
posterior foot
– Seals aperture to protect
against predation and
desiccation
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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
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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
Gastropod Diversity
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Three main groups:
– Prosobranchs
– Opisthobranchs
– Pulmonates
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No longer called Subclass Prosobranchia, Opisthobranchia, and
Pulmonata
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Recent uncertainty of classification
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Such diversity that it is easiest to point out specifics of each group
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Shell size, shape, colors and patterns
Radula structure
Feeding ecology
Reproduction
Etc.
Prosobranchs
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20,000 spp.
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Most marine and benthic
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Few freshwater and
terrestrial
Externally and internally
torted
Marine limpets, abalones,
and snails
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Shell may be coiled, or may
posses one large whorl like
an abalone shell
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Snails posses an operculum
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Respiratory system
– Siphon
– Solitary left ctenidium
– Monopectinate
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One kidney
– Located on left side
– One nephridiopore empties into
mantle cavity
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One gonad
– Right side
– One gonopore empties into
mantle cavity
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Cephalized
– Two cephalic tentacles
– Typically posses one lateral
eye at the base of each
tentacle
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Reproduction
– Gonochoric
– Some direct development
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Unique egg cases molded by
pedal gland of foot
– Some produce a veliger larva
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Possesses a swimming
organ known as a velum,
which consists of two
circular, ciliated lobes
Has a shell
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Size: a few mm to 70 cm
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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
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Abalones
– Also well adapted for clinging to
rocks, due to low / broad shell
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Turban snails
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Conchs
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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
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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
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Drills
– Urosalpinx – American oyster
drill
– Can decimate oyster beds
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Cone snails
– Carnivores of polychaetes,
gastropods, and fish
– Tropical – live in Indo-Pacific and
Western Atlantic
– Highly modified radula
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One radular tooth bathed in and
filled with neurotoxic venom
Everted like a harpoon
Tooth replaced
– Occasional human deaths from
cone snail venom
Opisthobranchs
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3000 marine spp.
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Interstitial to 60 cm in length
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Detorted
– Also seen in the nervous system
– Therefore, they are essentially
bilaterally symmetrical
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Reduction / loss of mantle cavity
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Reduction / loss of shell
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Modern species lack opercula
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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
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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)
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Nervous system
– Trend towards cephalization and
detorsion
– Cephalic tentacles
– Rhinophores
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Second pair of tentacles, just
posterior to the first
Chemosensory
Can be retracted into a sheath
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Reproduction
– Simultaneous
hermaphrodites
– Reciprocal internal
fertilization
– Eggs usually oviposited in
gelatinous strings
– Veliger larvae or direct
development
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Bubble snails
– Also called bubble shells
– Still have a shell, although
somewhat reduced
– Superficially resemble sea
slugs
– Most primitive of the
opisthobranchs
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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
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Pteropods
– Sea butterflies
– Swim using large parapodia
– Small; some posses a light
shell
– Some conduct gas exchange
across body surface
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Nudibranchs
– Sea slugs
– Graze on cnidarians and
sequester nematocysts
or photosynthetic
endosymbionts
– Lack shell and mantle
cavity
– Sometimes lack gills
– Cerata
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Function as gills
Sites for nematocysts
Each contain a branch of
digestive cecum
Pulmonates
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16,000 – 30,000 spp.
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Primitive
– Intertidal and freshwater snails
– Intertidal and freshwater limpets
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Modern
– Terrestrial snails, and slugs
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Detorsion
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Slugs lack shells
– Original distribution center of
slugs has low soil calcium
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Pulmonate snails lack an
operculum
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Mantle cavity on right side, but no gills; mantle cavity
converted to a lung
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Opening to lung is termed pneumostome
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Roof of mantle cavity is highly vascularized
– Capillary bed
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Floor of cavity is elevated and depressed to ventilate
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Physiological adaptations to
terrestrial existence
– Secrete a mucus plug that acts
like an operculum
– Secrete mucus
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Prevents desiccation
Defends against bacteria
Discourages predators
Facilitates locomotion
– Uricotelic instead of
ammonotelic
– Tolerant to water loss
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Helix can lose 50% of body
water
80% in Limax
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Behaviors associated with terrestrial
existence
– Periods of torpor during unfavorable
weather
Estivation during hot / dry weather
 Hibernation during cold weather
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– Most inhabit humid environments
– Those living in xeric environments are
only active at night or after rains
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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.
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Up to 23 cm in height
Second largest group of
snails can reach 15 cm and
are found in South America
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Hermaphrodites
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Reciprocal internal fertilization
– Could be considered an adaptation to terrestrial life
– Many deposit spermatophores
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Direct development
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Interesting courtship rituals
– Twist around a mucus strand in Limax
– Love darts in Helix