Filter-feeding mollusks

Marine biotoxins are produced by naturally occurring marine phytoplankton. Marine algal toxins are responsible for more than 60,000 intoxication/year with an overall mortality of about 1.5%. These substances can accumulate in aquatic animals intended for human consumption like filter-feeding mollusks. The toxins are thermoresistant compounds therefore, normal cooking, freezing, or smoking cannot destroy them. 

Phytoplankton in areas of oceanic upwell-ing contain raised Cd levels (Martin and Broenkow 1975), and filter-feeding mollusks can accumulate significant concentrations of Cd, even in coastal localities that are only moderately contaminated (Bryan et al. 1980). Oysters, in particular, are well-known Cd accumulators, with levels of up to 8 mg kg wet weight having been recorded in New Zealand (Nielsen 1975). Certain edible crustaceans such as crab and lobster also contain relatively high Cd concentrations, the metal being localized in the hepatopancreas or brown meaf ... 

Clams, oysters, and mussels accumulate chromium from the medium or from contaminated sediments at comparatively low concentrations. For example, oysters subjected to 5.0 (xg Cr+ /L for 12 weeks contained 3.1 mg Cr/kg DW in soft parts and retained 52% of the accumulated chromium after they were transferred to chromium-free seawater for 28 weeks. Mussels (Mytilus edulis) subjected to the same dose-time regimen contained 4.8 mg/kg, but retained only 39% after 28 weeks of depuration. Both oysters and mussels contained higher residues after exposure to 10.0 jig Cr /L for 12 weeks 5.6 and 9.4 mg Cr/kg DW in soft parts, respectively, and both contained substantial (30-58%) residues after 28 weeks in a chromium-free environment. In studies with mussels and softshell clams (Mya arenaria), it was demonstrated that chromium in New Hampshire sediments (contaminated with Cr+ from tannery wastes) was bioavailable to clams by diffusion from seawater, and that both diffusion and particulate uptake were important pathways for mussels. Accumulation was observed at sediment chromium concentrations as low as 150.0 mg/kg. Kaolinite sediments containing up to 1200.0 mg Cr+ /kg produced the most pronounced adverse effects on filtration rates and ciliary activity of bivalve mollusks, leading the authors to conclude that chromium that has accumulated in areas affected by industrial wastes might have serious consequences to filter feeding bivalves. 

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