Table 2. Life history stages and habitats of shellfish that have been tested for biological responses to suspended sediment dosages (see Table 1)

ERDC TN-DOER-E9

May 2000

Table 2

Life history stages and habitats of shellfish that have been tested for biological

responses to suspended sediment dosages (see Table 1)

Habitat

Eggs

Larvae

Juveniles

Adults

Pelagic

Eastern oyster

Mysid shrimp

Hard clam

Pacific oyster

Hard clam

Shrimp (P. japonicus)

Benthic and

Spot-tailed sand shrimp

Mobile

Dungeness crab

Black-tailed sand shrimp

Grass shrimp

Dungeness crab

Demersal Eggs

Coast mussel

and Sessile

Blue mussel

Organisms

Hard clams

Eastern oyster

Soft-shell clam

Hard clam

Surf clam

Bay scallop

Juvenile and Adult Stages

Bivalve Molluscs. Primary mechanisms used by bivalves to deal with high suspended-sediment

concentrations include the reduction of net pumping rates (Foster-Smith 1976) and rejection of

excess filtered material as pseudofeces (Robinson, Wehling, and Morse 1984; Turner and Miller

1991; Hawkins et al. 1996). When suspended-sediment concentrations rise above a threshold at

which bivalves can effectively filter material, a dilution of the available food occurs (Widdows,

Fieth, and Worrall 1979). The responses of suspension-feeding bivalves to relatively low sus-

pended-sediment concentrations are varied. Mussels (Mytilus edulis) (Kiorboe, Mohlenberg, and

Nohr 1981), surf clams (Spisula subtruncata) (Mohlenberg and Kiorboe 1981), and eastern oysters

(Urban and Langdon 1984) exhibit increased growth rates in high algal concentrations after the

addition of silt. Hard clams, however, decrease their algal ingestion with increasing sediment loads

(Bricelj and Malouf 1984), resulting in no difference in their growth rates compared with those of

clams exposed to an algal diet alone (Bricelj, Malouf, and De Quillfeldt 1984). The presence of

suspended clay (20 mg/L) interferes with the ability of juvenile eastern oysters to preferentially

ingest algae, but does not reduce the overall amount of algae ingested (Urban and Kirchman 1992).

The summer growth of European oysters (Ostrea edulis) in the field was enhanced at low levels of

sediment resuspension and inhibited as sediment deposition increased (Grant, Enright, and Griswold

1990). Sediment chlorophyll in suspension at low levels may act as a food supplement, thus

enhancing growth, but at higher concentrations may dilute planktonic food resources and suppress

food ingestion.

The progression of sublethal effects associated with relatively low concentrations is demonstrated

by soft-shelled clams (Mya arenaria) exposed to suspended sediments in the 100- to 200-mg/L

range. The clams first exhibit reduced gape width and partial retraction of their siphons and mantles

over the first 7 days of exposure, a decreased response to mechanical stimuli by 15 days, and

prolonged protrusion of their siphons by day 30 (Grant and Thorpe 1991). In laboratory studies,