The Mutualist Life of Bivalves

Wendy StickelEVPP Seminar on Mutualism

March 4, 2010

Some Basic Facts

• Belong to bivalvia class of molluscs

• 30,000 species

• Include clams, oysters, mussels, scallops

• Very ancient and successful taxonomic group

Bivalve Anatomy• Two-part shell operated by

adductor muscles• Mantle covering soft body• Siphons pull in and release

water• Gills take oxygen out of

water, filter food and detritus, and circulate water

• Two palps extend from mouth collect and sort incoming particulates

Distribution

• Marine• All depths• All latitudes• Most substrates-rock, sand, compacted

mud • Often dominant on coasts and estuaries,

also offshore sediments• Tolerate extreme environments (deep sea

vents)

Ecological Roles

• Filter-feeder

• Bioturbator

• Bioengineer – reef and mat builder

• MutualistZooxanthallaeSeagrassesEpibiontsAnemones

Cockles and Zooxanthallae• Fragum erugatum • Hypersaline waters in Shark

Bay, Australia• Densities >4,000/m2• Photosynthetic zooxanthallae

provide nourishment in return for stable environment and access to CO2 and N wastes

• Light-harvesting and light-filtering services (in some species) Source: Hickman 2003

Mussels and Seagrasses

• Spartina alterniflora – perennial deciduous grass, saline-tolerant, builds up land at seaward edge of marsh

• Grass height, biomass, and flowering correlate with mussel (Conkensia demissa) density

• Experimental evidence:– Manipulated mussel density to observe effect on

Spartina biomass – Nutrient enrichment of sediment was important on

marsh flats but stabilization of substrate more important at marsh edge

Source: Bertness 1984

Source: Bertness 1984

More on Seagrasses

• Broadened findings to mussels (Mytilus edulis) and eelgrass (Zostera marina L)

• Western Baltic• Sediment porewater concentrations of

ammonium and phosphate doubled from mussels fertilization

• Fertilization affected eelgrass growth (largest fraction of nutrient demands met via roots)

Source: Reusch 1994

Some limitations on seagrass-bivalve mutualisms

• Interference competition of eelgrass by mussels squeezing roots – space limitations? (Ruth 1991; Kobarg 1993)

• Interaction variable with non-native mussel (Musculista senhousia)– Effects ranged from facilitation to interference– Consistently impaired eelgrass rhizome

elongation rates (Reusch and Williams, 1998)

Another benefit to seagrasses

• Bivalves increase structural complexity of habitat• Spaces between shells offer refuge for small

epiphytic grazers (gastropods, etc.)• Reduced predation on grazers increased grazing

from seagrass leaves=>increased light absorption

• Tested with Thalassia testudinum (turtle grass) and Modiolus americanus (tulip mussel)

Source: Peterson and Heck 2001

Bivalve-seagrass mutualism

Source: Peterson and Heck 2001

Clams and Epibionts

• Chama pellucida lives attached to shallow rock surfaces

• Covered by dense growth of sessile plants and animals

• Removal of epibionts increases predation on chama

• Chama likewise offers low-mortality habitat to diverse sessile biota

• Hypothesis that epibiont larvae preference for rough vs. smooth-surfaced substrates is selected for, increasing likelihood of interaction

Source: Vance 1978

Some common themes

• Habitat alteration is central

• Conditional and dynamic– Contingent on broad processes rather than

particular species-specific characteristics– Powerful force in shaping community structure

• Need to look at in community context

• Factor in restoring/preserving ecosystem function

How applicable to PRV?

• Species common to coastal conditions• Bring ecosystem services which are needed for

very disturbed environment– Filter-feeding– Increased rate of nutrient cycling– Reef and marsh building– Grazing of epiphytes– Light-harvesting and filtering

• Ability to manage environmental changes– Increased nutrient loading– Climate change impacts: sea level rise, erosion from

storm events

Partial List of References• PETERSON, BRADLEY J. AND KENNETH HECK, JR.

Positive interactions between suspension-feeding bivalves and seagrass—a facultative mutualism MARINE ECOLOGY PROGRESS SERIES, Vol. 213: 143–155, 2001.

• BERTNESS, MARK D. RIBBED MUSSELS AND SPARTINA ALTERNIFLORA PRODUCTION IN A NEW ENGLAND SALT MARSH' in Ecology, 65(6), 1984, pp. 1794-1807 6c) 1984

• VANCE, RlCHARD R A MUTUALISTIC INTERACTION BETWEEN A SESSILE MARINE CLAM AND ITS EPIBIONTS, in Ecology, 59(4), 1978, pp. 679-685 Cv) 1978.