Download Biology 320 Invertebrate Zoology Fall 2005

Biology 320
Invertebrate Zoology
Fall 2005
Chapter 12 – Phylum Mollusca
Part Four
Class Bivalvia
• Clams, oysters, mussels,
scallops, and shipworms
• 8000 described extant spp.
• Mostly marine, with 1300
freshwater spp.
• Range in size from two mm –
one m
– Giant clam (Tridacna) lives in
South Pacific and weighs
close to 300 kg
• Common features
– Lack radulas
– Lateral compression
• Facilitates burrowing
– Siphons
• For ventilation and feeding
while being buried
• Most are sedentary, but a
few can swim
• Some participate in
symbiotic relationships
– Commensals with
polychaetes, crustaceans,
and echinoderms
– Parasites of fish
Body Form
• Bilaterally symmetrical
• Laterally compressed
• Essentially lack cephalization
• Sensory structures typically
located on mantle margin
• Ventral blade-like foot
• Ventral mass is dorsal to foot
• Two lateral mantle
cavities are formed by
mantle skirts
• One gill is housed in
each mantle cavity
• One anterior and one
posterior adductor
muscle
– Connect valves
transversely
Shell
• Two valves: left and right
• Dorsal hinge
– Two ligaments
• Elastic, so when adductor
muscles relax, shell opens
– Teeth that interlock to
prevent slippage
• Oldest part of shell is the
umbo
• Shell grows outwards
from umbo
– Mantle lays down
concentric rings
• Some bivalves (such
as cockles and
scallops) have ridges
on the valve surface
– Adds strength to shell
Pearls
•
Formed when a sand grain or parasite
gets trapped in extrapallial space
•
Layer after layer of nacre is added to
the particle
•
Cultured pearls are “seeded”
– On average, three years is required to
obtain a pearl of marketable size
•
Freshwater pearl culturing in the
Southeastern US is having negative
environmental impacts
– Siltation
– Pollution
– Introduction of exotics
Gills
• Can be used solely for gas
exchange, solely for collecting
food, or both
• Bivalves are classified
according to gill structure and
function
• Three main morphological
groups
– Protobranchs – primitive
– Lamellibranchs – majority; of
economic importance
– Septibranchs – specialized
carnivores
Protobranchs
• Name means “first gills”
• Deposit feeders
• Gills used solely for
respiration
– Small
– Located in posterior of
mantle cavity
– Bipectinate
– Cilia create respiratory
current and prevent gill
fouling
• Two palpal tentacles
–
–
–
–
–
Located at posterior end of mantle cavity
Elongate
Covered with mucus
Gather particles from sediment
Transfer to labial palp
• Labial palp
– Covered with ciliated grooves and ridges for sorting
– Leads to mouth
– Sorting isn’t perfect
• Waste (pseudofeces) falls off of palp into mantle cavity
– Valves close rapidly to expel wastes
Lamellibranchs
• Name means “sheet gills”
• As organic matter ends up on
gills anyway, it was inevitable
that some bivalves would adapt
to exploit this
• Suspension feeders that trap
particulate matter on gill surface
– Gills are folded and therefore have
large surface area
– Secrete mucus
• Gills have ciliary sorting fields
• Labial palps receive strings of
mucus from gills
• Scallops (Pecten)
– Free-living, not attached
– Clap valves rapidly to swim
away from predators
– Large central posterior
adductor muscle
– Peripheral sensory structures
– Ridges on valves
• Mussels (Mytilus)
– Attached to substrate via a
byssus (more later)
– Often found in large clusters
(beds)
– Zebra mussels (Dreissena) are
a freshwater invasive in North
America
• Oysters (Crassostrea,
Ostrea)
– Cemented to substrate
– Produce pearls
• Clams
– Giant clams (Tridacna)
• Live on substrate
– Geoducks (Panopea)
• Bury
• Siphon is so large that it
cannot retract into valves
Septibranchs
• Small group of specialized
carnivores
• Gills only used for food capture
– Form two perforated muscular
septa
– Contraction of septal muscles
elevates septa and water
rushes in
– Animals / particles are sucked
into mantle cavity as water
rushes in enlarged inhalant
siphon
• Gas exchange occurs across
mantle surface
• Stomach is also
adapted for feeding on
larger particles / whole
organisms
– Muscular
– Lined w/ cuticle
– Functions as a gizzard
• Grinds / crushes prey
– Enzymes are dumped in
Generalized Digestive System
• Mouth connected to labial
palps
• Stomach has:
– Sorting fields
– Gastric shields
– Crystalline style
• Huge rod made of solid
digestive enzymes
• Rubs against gastric shield to
release enzymes
• In some cases, rotation of rod
pulls mucus strings into
stomach
• Intestine loops through visceral
mass several times
Burrowing
• Pedal hemocoel functions as a hydrostat; controlled by coordinated
muscular contractions
• Foot elongates and is forced into sediment
– Shell acts as penetration anchor
• Blood forms terminal anchor in distal foot
• Pedal retractor muscles contract
– Shell rocks back and forth, and moves forward
• Process repeats
• Mantle is sealed in several spots
– Prevents fouling of mantle cavity
Siphons
• Extensions of mantle
– Ventral inhalant siphon
– Dorsal exhalant siphon
• Tube length varies
– Longest in deep burrowers
• Siphons are often grazed upon by fishes and other predators
– Regenerated
– Can be retracted
• Sensory organs are often
located on the tips of
siphons
– Retract when shadows
pass overhead
• Geoduck clams have
huge siphons and can
burrow up to one meter
• Some live in permanent
burrows lined with mucus
Attachment
• Many attach to rocks, shells,
wood, coral, jetties, wharf
pilings, etc.
• Anterior adductor muscle,
siphons, and foot are usually
reduced / absent
• Attached via:
– Byssus
• Bundle of strong protein
threads secreted by glands in
the foot
– Cement
• Permanently attaches one
valve to substrate
Boring
• Some bore into hard
substrates such as rocks,
wood, shells, or coral
• Animal settles and begins to
burrow
– Uses valves as a drill
– Some use chemicals to soften
substrate
• Especially true of those that
bore into corals
• Also secrete chemicals that
inhibit nematocyst firing
• Permanently enclosed in
burrow
• Drilling rates (substrate controlled for)
vary across species
– 2.6 mm / month – 11.4 mm / month
• Shipworms
– Lamellibranchs that are ecologically
important degraders of wood
– Elongate, cylindrical, and almost
worm-like
– Burrows can be as long as two feet
– Feed on sawdust, so they can do
great damage to piers, pilings, and
wooden boats
• Symbiotic bacteria to break down
cellulose and fix nitrogen (to
compensate for low protein diet)
Circulation
• Open system
• Blood travels through
mantle for:
– Auxiliary respiration
– Diffusion for sensory
structures
• Blood is important to
hydrostat
– Comprises approximately
50% of soft volume
Gas Exchange
• Inefficient system when compared with other molluscs
– Many extract less than 10% of the available O2 in a given volume
of water
– Most lack respiratory pigments
– Not detrimental as metabolic rates are usually low
• Compensated by filtering huge volumes of water
– 40 L / hr in oysters
• Mantle skirts aid in gas exchange
– Especially at low tides when gills are collapsed
Nervous System
• Not cephalized, but have a bilaterally symmetrical nervous system
consisting of ganglia and nerves
• Sensory organs (statocysts, ocelli, osphradia, etc.) situated on
periphery structures
–
–
–
–
Foot
Siphons
Pallial tentacles
Mantle margins
• Important structures under muscular control:
–
–
–
–
Pedal retractor, adductor muscles, etc.
Pallial tentacles
Labial palps
Siphons
Reproduction
• Mostly dioecious
• Two gonads surround intestinal loops in visceral mass
• External fertilization, sometimes in mantle cavity
– Some brood eggs in exhalant chamber
• Two gonads surround intestinal loops in visceral mass
• Most have a free-swimming larval phase (trochophore,
veliger, or both) that later settles
• Most freshwater species lack
free-swimming larvae
– Get swept away with currents
– Harder to remain buoyant in
FW
– Have direct development
• Many attach to fish, so that
they may be taken upstream to
an optimal habitat
– Zebra mussels (Dreissena)
release parasitic glochidia
• Life spans are variable
– 20 – 30 yrs is common
– Over 150 yrs in some