Transcript Chapter 16 Mollusks

Chapter 16
Molluscs
Kingdom: Animalia
Phylum: Mollusca
Class Gastropoda
Class Bivalvia
Class Cephalopoda
I. Intro to P. Mollusca
• + 90,000 living sp.
– 70,000 fossil sp.
• Chitons, snails, slugs, clams, mussels, oysters, squids,
octopuses, nautiluses
• Overview
– soft body
– microscopic → massive
• 80% under 10 cm
• giant squid - 900 kg, 20 m long
• giant clams - 250 kg, 1.5 m dia.
– herbivores/predators/filter-feeders/
parasites
– marine/freshwater/terrestrial/tropics/polar
– burrowers/borers
Intro to P. Mollusca
A. Evolutionary Contributions
– respiratory system
• gas exchange though body surface + gills or “lungs”
– open circulatory system
• ♥ that pumps + vessels + blood sinuses
• closed circulatory in cephalopods
– highly developed direct eye in cephalopods
• photosensitive cells in retina face light
• derived from skin
Intro to P. Mollusca
B. Mollusca Characteristics
– bilateral symmetry
• bilateral asymmetry
– often defined head
– ventral body wall forms foot
• locomotion
– folds of dorsal body wall form mantle w/ mantle cavity
• mantle forms gills/lungs and secretes shell
– ciliated surface epithelium w/ mucous glands + sensory nerve
endings
– coelom around ♥, sometimes gonads/kidneys/intestine
– complete digestive tract w/ anus
– rasping organ (radula)
B. Characteristics Continued…
– open circulatory system w/ ♥ (3-chambered) + blood vessels +
sinuse
– 1-2 kidneys (metanephridia)
– nervous system w/ paired cerebral/visceral ganglia w/ nerve cords
– sensory organs for touch/smell/taste/equilibrium/vision
– internal/external ciliary tracts
Phylum Mollusca
• Coelom
limited to
space
around the
heart,
gonads, and
kidneys
P. Mollusca: Form/Func.
C. Reproduction and life history
– most dioecious, some hermaphroditic
– egg hatches and produces a free-swimming larva (trochophore
larva)
– larva undergoes direct metamorphosis into small juvenile in
chitons
– many gastropods and bivalves, an intermediate larval stage
(veliger) is derived state
P. Mollusca: Form/Func.
a. Radula
• protruding, rasping, tongue-like organ
• unique to molluscs
– lacking in bivalves
• membrane w/ rows of tiny teeth pointed
backward
• scrapes/tears/cuts food particles from
surface
• carries particles to digestive tract
• some sp. bore through hard material or
harpoon prey
Radula
P. Mollusca: Form/Func.
b. Foot
– func. attachment or locomotion
– usually ventral
– modifications
• hatchet foot in clams
• siphon jet in squids
– mucus secretions fucn. adhesion or help glide on cilia
– uses
• snails/bivalves extend foot hydraulically by engorgement w/ blood
• burrowers extend foot into substrate, enlarge tip (anchor), draw
forward
• free-swimming sp. modified foot into wing/fin-like swimming agents
II. 1st Class of Molluscs: Gastropoda
A. Class Gastropoda (snails/slugs/conches/sea slugs)
– most diverse/abundant
• +70,000 living sp., +15,000 fossil sp.
– marine → air-breathing terrestrial snails/slugs
– sluggish/sedentary animals
– terrestrial gastropods restricted by soil mineral content/acidity/
temperature/dryness
– intermediate hosts to many parasites
• often harmed by larval stages
Gastropoda
B. Respiration
– performed by ctenidia in mantle cavity
– have a highly vascular area in mantle that serves as lung
– lung opens to outside by small opening (pneumostome)
Gastropoda
C. Nervous/sensory systems
– most have well-developed nervous systems
– sense organs include eyes/tactile organs/ chemoreceptors
– eyes
• simple cups w/ photoreceptors → complex eye w/ lens/cornea
Gastropoda
D. Reproduction
– monoecious/dioecious
– some discharge ova/sperm into water
• external fertilization
– young emerge as veliger larvae
or pass this stage inside egg
– some sp. ovoviviparous
III. 2nd Class of Molluscs: Bivalvia
A. Class Bivalvia Characteristics
(mussels/clams/scallops/oysters/shipworms)
– 1–2 mm → giant South Pacific clams
– most sedentary filter feeders
• ciliary currents bring in food
– lack head/radula/cephalization
– marine/freshwater streams/ponds/lakes
– native freshwater clams in U.S. most
jeopardized animal group
• +300 sp. once present
– 12 extinct
– 42 threatened/endangered
– 88 of concern
Bivalvia
B. Bivalvia Form/func.
– 2 shells or valves held together by hinge ligament
– valves drawn together by adductor muscles
– umbo = bulge
• oldest part of shell
• growth occurs
outward in rings
– pearls produced
when irritant lodged
btwn shell/mantle
• layers of nacre
secreted around
foreign material
B. continued…
– freshwater clams sensitive to water
quality changes (pollution/sedimentation)
– Zebra mussels
•
•
•
•
•
•
serious exotic pest
clog water intake pipes
outcompete native sp.
20,000 animals /m2
30,000-1,000,000 eggs/yr
altering biomass distribution
Bivalvia
C. Locomotion
– slender foot extended out btwn valves
– blood pumped into foot
• it swells/anchors bivalve in mud
– shortening foot pulls clam forward
– scallops clap valves to create jet propulsion
• mantle edges direct the stream
Bivalvia
D. Gills
– both mantle/gills perform gaseous exchange
– gills derived from primitive ctenidia by lengthening filaments to
each side
– water mvmt through clam
• water enters incurrent siphon
• enters water tubes via pores
• out excurrent siphon
Classes of Molluscs: Bivalvia
E. Circulatory system
– 3-chambered ♥ w/ 2 auricles + 1 ventricle
– some blood oxygenated in mantle
• returns to ventricle via auricles
– rest circulates via sinuses/kidneys/gills before back to auricles
Bivalvia
F. Feeding
–
–
–
–
suspended organic matter enters incurrent siphon
gland cells on gills/labial palps secrete mucus to trap particles
food in mucous masses slide via grooves at lower edge of gills
suck in crustaceans by
creating sudden inflow
of water
Bivalvia
G. Digestive system
– stomach folded into ciliary tracts for sorting particles
• rotating style helps free digestive enzymes, roll mucous food mass
– dislodged particles directed to digestive gland or engulfed by
amoebocytes
Bivalvia
H. Reproduction and Development
– dioecious
– gametes discharged out in excurrent flow
– fertilization usually external
– internal fertilization
• freshwater clams
• sperm enter incurrent siphon to
fertilize eggs in water tubes of gills
– embryos develop as trochophore,
veliger, spat larval stages
– eventually sink to begin independent
life on streambed
IV. 3rd Class Molluscs: Cephalopoda
A. Class Cephalopoda (squid/octopus/nautilus/devilfish/cuttlefish)
– 2 cm → giant squid (largest invertebrate known)
– mostly marine
• octopuses mostly intertidal
• squids are deep-sea animals
– predators
– foot is in head region
• modified for expelling water from
mantle cavity
A. Class Cephalopoda continued…
– cuttlefish shell enclosed in mantle
– squid shell is thin strip enclosed in mantle (pen)
– octopus has completely lost shell
Cephalopoda
B. Locomotion
– swim by forcefully expelling water via ventral funnel/siphon
– octopuses crawl on bottom/swim backward by spurting water
jets
• webbing btwn arms allows swimming w/ medusa-like action
– control direction/force of water
• determines speed
– lateral fins of squids/cuttlefishes stabilize
• Active life of cephalopods
reflected in respiratory/
circulatory/nervous systems
Cephalopoda
C. Respiration/circulation
– 1 pair of gills
• 2 pair in Nautilus
– muscular pumping keeps water flowing through mantle cavity
when ↑ O2 demand
– network of vessels conducting blood through gill filaments
– blood goes to
systemic circulation
before gills
– accessory/branchial
♥ at base of each
gill ↑ pressure
Cephalopoda
D. Nervous/sensory systems
– largest brain of any invertebrate
– giant nerve fibers
– well-developed sense organs
• complex eyes (cornea/lens/retina)
– learn by reward/punishment/
observation of others
– lack hearing
– tactile/chemoreceptor cells in arms
Cephalopoda
E. Communication
– chemical/visual signals
– cells in skin contain pigment
granules (chromatophores)
• contractions of muscle fibers
attached to cell boundary cause
cell to expand and change color
pattern
– deep-water cephalopods have
luminescent organs
– ink sac empties into rectum
• ink gland secretes sepia
when animal is alarmed
Cephalopoda
F. Reproduction
– dioecious
– ♂ seminal vesicle
– 1 arm of ♂ modified as intermittent organ
• plucks spermatophore from mantle cavity and inserts it into ♀
– fertilized eggs leave oviduct, attach to substrate
– hatch into juveniles w/ no free-swimming
larval stage
Human eye
Anatomy of the human eye
PBS evolution of the eye
Read Ken Miller Evolutionary Eye
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