2nd largest mollusc class ~ 20,000 living; 20,000 fossil spp. Appear in Cambrian Height of diversity ~ 350 mya

1. Class Bivalvia (Pelecypoda)
• 2nd largest mollusc class
• ~ 20,000 living; 20,000 fossil spp.
• Appear in Cambrian
• Height of diversity ~ 350 mya

2. 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

3. 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

4. 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

5. • Fishes are host to glochidia

6. Subclass Protobranchia
• Primitive, small, marine bivalves
• Nut clams Nucula yoldia
• Cryptodont Solemya
0.45 cm
1.5 cm

7. Subclass Lamellibranchia
98% of bivalves
• A. arks, Arca
– Hinge straight w/ numerous fine, regular
teeth
– Bitter sweet clams, Glycemeris

8. 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

9. Subclass Lamellibranchia
• C. Scallops Pectin, jingle shells, file
shells
– Scallops swim by clapping their valves
– Numerous blue eyes on mantle margin

10. Subclass Lamellibranchia
• D. Oysters Osstrea, Crassostrea (Asian)
– Form large attached colonies
– Oyster farming began over 100 years ago in
U.S.

11. 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

12. 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

13. Megalonaias nervosa = washboard

14. Amblema = three-ridge

15. Quadrula metanevra = monkeyface

16. Elliptio crassidens = elephant ear

17. Obovaria subrotunda = hickorynut

18. Corbicula fluminea = Asian clam

19. Subclass Lamellibranchia
• F. Lucines, jewel boxes, and cockles
• Cockles are edible and popular in
Europe, Cardium

20. 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

21. Subclass Lamellibranchia
• H. Sphaeridae: freshwater fingernail
clams
• Corbicula, Asian clams
• Common, most < 1/4 inch

22. Subclass Lamellibranchia
• I. Razor clams - elongate valves
• J. venus clams
– Mercenaria is the tasty cherrystone or
hard-shelled clam

23. Subclass Lamellibranchia
• K. Soft-shell clams (in comparison with
Mercenaria)
– Mya is commercial clam in U.S.
– Geoducks
– Panopea

24. Corbicula fluminea, Asian clam
• Corbiculidae
• Hermaphroditic
• Introduced to North America early
1900’s
• Widespread E., S., and far W. United
States

25. 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

26. Dreissena spp., zebra mussel
and quagga mussel
• Widespread colonial, sessile bivalve
from Europe
– Caspian Sea
• Spread through European drainages in
1700’s

27. Zebra and quagga mussels
• 1985 - Lake St. Claire by ship water
ballast
• 1990 - entire Great Lakes and St.
Lawrence
• Continues to spread through rivers

28. Zebra and quagga mussels
• 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

29. Boring clams (Pholas) and
shipworms (Teredo)
• Shipworms cause economic damage by
boring in pilings and submerged
wooden structures

30. Subclass Septibranchia
• Watering pot shells
• Elongate tube-shaped with tiny shells
• Live in mud or bore in rock, coral, or
clamshells

31. 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

32. Cambrian = 500 mya
65 mya
present
250 mya
65 mya
550 mya
250 mya

33. • Largest invertebrate animal is giant
squid of North Atlantic (Architeuthes)
– to 16 m with long arms
• All marine + carnivores
Class Cephalopoda - octopus
and squids

34. 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

35. Characters
• 2. Bilateral symmetry
• 3. Well-developed head w/prominent, well-
developed eyes
• Eyes resemble vertebrate eyes
• 4. Prehensile tentacles - derived from
anterior of foot - suckers (except Nautilus)

36. 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 “jet-
propulsion”

37. 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.

38. 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

39. Classification
• 1. Subclass Nautiloidea
– Nautiloids, shelled
– Living Nautilus with 3? spp.
• Indo-Pacific
– Many fossil species
– Don’t confuse with “paper nautilus” - an
octopus

40. 2. Subclass Ammonidea
Ammonoids
• Shell w/ complex sutures
• Silurian - Cretaceous

41. 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

42. Subclass Coleoidea
• C. Cuttlefish
– straight, chambered, internal shell
– Sepia
– Rossia (bob-tailed squid)

43. D. Squids
• Shell chitinous, no chambers, serves as
body skeleton
• Many pelagic and deep-sea spp.; few in
shallow waters
• Loligo
• Lolliguncula
• Architeuthes

44. E. Vampire squid
• Octopus-like forms with webbed arms
• Vampyroteuthis infernalis
• Black skin

45. F. Octopods
• Eight arms, no fins
• Octopus
• Argonauta (produces shell-like egg-
case, has dwarf males)

46. 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