Class Bivalvia (Pelecypoda)

• 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

• 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

65 mya

present

250 mya

65 mya

550 mya

250 mya

• Largest invertebrate animal is giant squid of North Atlantic (Architeuthes)– to 16 m with long arms

• All marine + carnivores

Class Cephalopoda - octopus and squids

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, well-developed eyes

• 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 “jet-

propulsion”

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 AmmonideaAmmonoids

• 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 egg-case, 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