Class Bivalvia (Pelecypoda)
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2nd largest mollusc class
~ 20,000 living; 20,000 fossil spp.
Appear in Cambrian
Height of diversity ~ 350 mya
Class Bivalvia (Pelecypoda)
• Sizes: 2 mm to > 1.35 m (Tridachna
gigas, giant clam can weigh 440 lbs.)
• 1. Bivalve shell
• 2. Bilateral symmetry, body compressed
laterally
• 3. Rudimentary head, no eyes, no
tentacles, no radula
Characters, cont.
• 4. Large blade-like foot
• 5. Pair of large, complex, ciliated gills
– Filter feed on plankton, sediments
• 6. Some sessile species have byssal
threads for attachment to substrate
– Mussels and oysters
– Secreted by gland at based of foot
Bivalves
• 7. Most are dioecious
• External fertilization, or in mantle
chamber (some marine and most
freshwater spp.)
• 8. Marine species: trochophore + veliger
larvae present
– Freshwater Unionacea veliger = glochidia
• Parasitic on fish gills
• Fishes are host to glochidia
Subclass Protobranchia
• Primitive, small, marine bivalves
• Nut clams Nucula yoldia
• Cryptodont Solemya
0.45 cm
1.5 cm
Subclass Lamellibranchia
98% of bivalves
• A. arks, Arca
– Hinge straight w/ numerous fine, regular
teeth
– Bitter sweet clams, Glycemeris
Subclass Lamellibranchia
• B. Mussels, winged or tree oysters, pen
shells
– Byssal threads for attachment
– Mytilus, Modiolus are edible mussels
– Pearl oysters Pinctada are winged oysters
Subclass Lamellibranchia
• C. Scallops Pectin, jingle shells, file
shells
– Scallops swim by clapping their valves
– Numerous blue eyes on mantle margin
Subclass Lamellibranchia
• D. Oysters Osstrea, Crassostrea (Asian)
– Form large attached colonies
– Oyster farming began over 100 years ago in
U.S.
Subclass Lamellibranchia
• E. Unionacea, freshwater clams
• Small commercial fishery in Indiana and
elsewhere in U.S. produced “mother of
pearl”
• Currently for Japanese pearl industry
You are required to know:
Megalonaias nervosa = washboard
Amblema plicata = three ridge
Quadrula metanevra = monkeyface
Elliptio crassidens = elephant ear
Obovaria subrotunda = hickorynut
Corbicula fluminea = Asian clam
Megalonaias nervosa = washboard
Amblema = three-ridge
Quadrula metanevra = monkeyface
Elliptio crassidens = elephant ear
Obovaria subrotunda = hickorynut
Corbicula fluminea = Asian clam
Subclass Lamellibranchia
• F. Lucines, jewel boxes, and cockles
• Cockles are edible and popular in
Europe, Cardium
Subclass Lamellibranchia
• G. Giant clams, Tridachna are tropical
sessile species
• Rely on commensal algae for much of
nutrition
• Most species are endangered in many
areas by shell collectors
Subclass Lamellibranchia
• H. Sphaeridae: freshwater fingernail
clams
• Corbicula, Asian clams
• Common, most < 1/4 inch
Subclass Lamellibranchia
• I. Razor clams - elongate valves
• J. venus clams
– Mercenaria is the tasty cherrystone or
hard-shelled clam
Subclass Lamellibranchia
• K. Soft-shell clams (in comparison with
Mercenaria)
– Mya is commercial clam in U.S.
– Geoducks
– Panopea
Corbicula fluminea, Asian clam
• Corbiculidae
• Hermaphroditic
• Introduced to North America early
1900’s
• Widespread E., S., and far W. United
States
Corbicula
• Life history adapted for unstable,
unpredictable habitats
• Highly invasive, replaces native
Sphaeriid populations
– Highest filtration and assimilation rates of
any freshwater bivalve
– Highest growth and production rate
– 3-6 mos. to maturity
– Single adult can produce 68,678 juv/year
Dreissena spp., zebra mussel
and quagga mussel
• Widespread colonial, sessile bivalve
from Europe
– Caspian Sea
• Spread through European drainages in
1700’s
Zebra and quagga mussels
• 1985 - Lake St. Claire by ship water
ballast
• 1990 - entire Great Lakes and St.
Lawrence
• Continues to spread through rivers
Zebra and quagga mussels
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Tend to foul pipes - industry, boats, etc.
Dioecious, trochophore and veliger larvae
Adults attach by byssal threads
Very small eggs,
– 30,000 - 40,000 per female
– Kill natives by overcolonization
Boring clams (Pholas) and
shipworms (Teredo)
• Shipworms cause economic damage by
boring in pilings and submerged
wooden structures
Subclass Septibranchia
• Watering pot shells
• Elongate tube-shaped with tiny shells
• Live in mud or bore in rock, coral, or
clamshells
Class Cephalopoda - octopus
and squids
• Extremely complex, advanced molluscs
• Nervous and sensory system
development surpasses all other invert’s
• ~ 1000 extant spp.
• ~ 10,500 fossil spp.
• Appear in Cambrian
– Peaks in abundance Paleozoic + Mesozoic
Cambrian = 500 mya
present
250 mya
65 mya
65 mya
250 mya
550 mya
Class Cephalopoda - octopus
and squids
• Largest invertebrate animal is giant
squid of North Atlantic (Architeuthes)
– to 16 m with long arms
• All marine + carnivores
Characteristics:
• 1. Primitively - straight or coiled shell
divided into compartments by septa
– Shell used for buoyancy
– Many cephalopod shells are reduced or
absent
– Compartments connected by siphuncle
• Secretes or absorbs nitrogen-rich gas into
chambers
– New chambers secreted w/growth
Characters
• 2. Bilateral symmetry
• 3. Well-developed head w/prominent, welldeveloped eyes
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Eyes resemble vertebrate eyes
• 4. Prehensile tentacles - derived from
anterior of foot - suckers (except Nautilus)
Characters
• 5. Dorsal-ventral axis of body has
become functional posterior-anterior
axis, by elongation
• 6. Mantle encloses body
– Thick and muscular
– Opening to cavity is funnel-shaped siphon
– Pump water out for backwards “jetpropulsion”
Characters
• 7. Mouth equipped with pair of chitinous
jaws - resemble hawk’s beak
– Radula present
• 8. Respiration- pair of gills in mantle
cavity.
– No cilia
• 9. Ability to change skin color by
melanophores in most spp.
Characters
• 10. Brain large + complex.
– Behavior and learning highly developed
• 11. Dioecious: copulation by transfer of
spermatophore by one of tentacles
• 12. Egg development direct - no larva
Classification
• 1. Subclass Nautiloidea
– Nautiloids, shelled
– Living Nautilus with 3? spp.
• Indo-Pacific
– Many fossil species
– Don’t confuse with “paper nautilus” - an
octopus
2. Subclass Ammonidea
Ammonoids
• Shell w/ complex sutures
• Silurian - Cretaceous
3. Subclass Coleoidea - shell
internal or absent
• A. Belemnites - extinct, w/chambered
internal shell. Probably ancestral to
other coleoids
• B. Spirula - coiled chambered internal
shell.
– Deep-water denizen of tropics
Subclass Coleoidea
• C. Cuttlefish
– straight, chambered, internal shell
– Sepia
– Rossia (bob-tailed squid)
D. Squids
• Shell chitinous, no chambers, serves as
body skeleton
• Many pelagic and deep-sea spp.; few in
shallow waters
• Loligo
• Lolliguncula
• Architeuthes
E. Vampire squid
• Octopus-like forms with webbed arms
• Vampyroteuthis infernalis
• Black skin
F. Octopods
• Eight arms, no fins
• Octopus
• Argonauta (produces shell-like eggcase, has dwarf males)
It appears that:
• Molluscs have evolved from ancestors
of flatworms by acquisition of:
– 1. Complete gut
– 2. A shell
– 3. “body cavity” (elements of circulatory
system spaces)
– See online Mollusca Phylogeny paper