Phylum Mollusca

 

Class Bivalvia

 

•~300 native & a few introduced species in NA FW systems

–most species in Southeast

•Mantle a single folded tissue layer

–Extend anteriorly, posteriorly, ventrally

–Secrete halves of shell (valves)

•Hinged dorsally

 

Shell morhpology

•3 layers

–Periostracum – outer layer, mostly protein, variable thickness

•Protects CaCO3 from chemical weathering

–Prismatic layer – 90% of shell, CaCO3 laid down vertically

•Very thin in slow water inhabitants

–Nacre – innermost layer, CaCO3 laid down horizontally

•White, purple, peach, pink

•Thin in Sphaeridae, lacking in Corbiculidae

 

Shell structures

•Umbo – dorsal most portion of shell (oldest portion of shell)

•Hinge – ligament connecting valves

•Teeth – provide stability for valves

–Cardinal/Pseudocardinal

–Lateral

•Alae – posterio-dorsal extension of valve

•Flutings – grooves/channels on outer part of valves

•Pustules/tubercles – bumps on surface of valves

•Muscle scars – attachment points for adductor muscles

•Sulcus – depression on outer side in center of valve

•Rays – linear green pigment of valve

 

 

Internal morphology

•Head absent

•Mantle - single folded flap of tissue

•Adductor muscles – anterior and posterior muscles located dorsally & attached to valves

•Foot – large muscular extension

•Inhalent siphon – opening used to bring water into mantle cavity (posteriorly located)

•Exhalent siphon – opening used to expel water from mantle cavity (posteriorly located immediately dorsal to Inhalent Siphon)

•Gills (ctenidium) – large, conspicuous “striated” flaps b/w mantle & foot on both sides (each side w/ outer & inner “demibranchs”

 

Visceral Mass

•Coelom – restricted to dorsal area surrounding ventricle

•Ventricle – muscular heart located dorsally w/in coelem (open circulatory system)

•Auricle – opening into coelem

•Intestine – long coiled tube w/in viscera

•Gonad – surrounding intestine

•Mouth – located antertiorly just dorsal to foot

•Digestive gland – “green” mass dorsal to mouth

•Labial palps – flanking mouth

 

Feeding

•Draw water into mantle cavity through Inhalent siphon

•Algae, detritus, microorganisms filtered by gills (stick to mucous)

•Cilia & Labial palps sweep food to mouth

•Inedible matter expelled out of mantle cavity

•Edible particles ingested & waste expelled through anus

•Filtered water & waste expelled through Exhalent Siphon

 

Reproduction

•Mostly dioecious

–Some sexually dimorphic

•Long-lived (up to 100+ years)

–Late sexual maturity (6-12 years)

•Males release sperm into water

•Females “filter” sperm to fertilize eggs

– thousands to millions of eggs/year

•Fertilized eggs held in gills (marsupia) for development

 

Larval Development

•Embryos held for 3-9 months

•Glochidia = parasitic larval stage released directly onto fish, attach to fins and/or gills

–Detach after ~3 weeks and settle on substrate

•Adaptations to increase glochidial infestation

–Conglutinates – small mucous bags filled w/ glochidia released into water

–Superconglutinates – long strands of mucous waving in current filled w/ glochidia

–Lure – extension of mantle waving in current

 

 

 

 

 

 

More cool stuff

•http://unionid.missouristate.edu/

 

Family Unionidae

•Vast majority of NA FW bivalves

•Worldwide family, but greatest diversity in SE United States

–269 species in Southeast (175 in AL, 132 in TN, 98 in GA)

•Distinct regional faunas (similar pattern in fishes)

–Likely associated w/ drainage evolution

–Mobile, Apalachicolan, Cumberlandian, Mississippian, Ozarkian

 

 

Shells and Habitat

•Found mostly in riffles or flowing areas in medium to large rivers (a few small stream species)

•Valves mostly thick and heavy (Anadonta sp., Utterbackia sp. thin)

•Thicker/heavier shelled mussels less likely to get washed downstream by current

•Thinner shells take less energy to produce

•Shell morphology reflects microhabitat

 

 

 

 

 

Unionidae

Declining Unionidae

•Only about ~25% of NA unionid species considered stable

•Reasons for decline

–Highly sensitive to disturbance

•Specialized feeding & reproduction

•Long lived

–Habitat degradation (dams, siltation, alteration of host  & algal communities)

–Overharvest

•Pearly button & now cultured pearl industries

–Competition from exotics

 

 

 

 

 

 

 

Sphaeriidae

•Fingernail clams

•~ 30 species (most native)

•Headwaters – big rivers

•Very small (<10 mm)

•Round valves, lack nacre

•Monoecious

•No glochidial stage (release fully formed juveniles)

 

Corbiculidae

•Corbicula fluminea

•Exotic (native to Asia)

•Small – medium size (<40 mm)

•High density populations

–Vast majority of bivalve biomass in most streams

–Very effective filterers

•Monoecious (simultaneous hermaphrodites)

•No glochidia stage (fully formed juveniles released)

•Tolerant of degraded conditions

 

Dreissenidae

•         Exotic, native to Europe

•         Dreissena polymorpha

•         Small size (<30 mm)

•         Incredibly high density populations

–        100,000+ individuals/m2

–        Vast majority of bivalve biomass in large rivers

–        Very effective filterers (reversed eutrophication of Lake Erie)

•         Dioecious, reproduce after 1st year

•         No glochidia stage (veliger larvae)

•         Attach via byssal threads to hard substrate

 

 

Conservation

•297 species native to U.S.

–269 in SE

•7% extinct

•42%  threatened or endangered

–Realistically, much higher %

•25% stable

Conservation efforts/strategies

•Propagation

–Collecting mature adults

–Rearing larvae in labs

–Releasing subadults

•Problems w/ propagation

–Maintaining genetic diversity?

–Expensive & labor intensive

–Sustainable?

–Where do you put subadults?

 

Habitat Protection

•Dam construction largely stopped

•Siltation from logging less of a problem than in past (BMP implementation)

•Development/urban sprawl currently biggest problem

–Inadequate silt containment

–Increased impervious surface

–Inadequate stream buffers

–Reduced profitability for developers/increased home cost associated w/ implementing BMPs

 

Habitat Restoration

•Bank stability (increased riparian vegetation)

•Increased O2 levels below dams

•Decreased daily water level fluctuations below dams

•Increased water temperature below dams

 

 

Duck River, TN

•Increases in # of species

–Some thought to be extinct rediscovered

• Dramatic increase in # of individuals

–Especially T/E species