Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

an 87% clearance from the exoskeleton due, in part, to molting. Other organs or tissues that take
up significant amounts of Pb include the gills, hepatopancreas, muscle, and hemolymph, in
decreasing order. These parts cleared >50% of accumulated Pb over the 3-week clearance
period, with the exception of the hepatopancreas. The hepatopancreas is the organ of metal
storage and detoxification, although the molecular mechanisms of metal balance in crayfish have
yet to be extensively investigated (Anderson et al., 1997).
Fish
Most fish use mucus as a first line of defense against heavy metals (Coello and Khan,
1996). In fish, some epithelia are covered with extracellular mucus secreted from specialized
cells. Mucus contains glycoproteins, and composition varies among species. Mucosal
glycoproteins chelate Pb, and settle, removing the metal from the water column. Fish may
secrete large amounts of mucus when they come into contact with potential chemical and
biochemical threats. Coello and Khan (1996) investigated the role of externally added fish
mucus and scales in accumulating Pb from water, and the relationship of these with the toxicity
of Pb in fingerlings of green sunfish, goldfish and largemouth bass. The authors compared trials
in which fish scales from black sea bass (Centropristis striata) and flounder (Pseudopleuronectes
americanus) and mucus from largemouth bass were added to green sunfish, goldfish, and
largemouth bass test systems and to reference test systems. On exposure to Pb, fish immediately
started secreting mucus from epidermal cells in various parts of the body. Metallic Pb stimulated
filamentous secretion, mostly from the ventrolateral areas of the gills, while Pb-nitrate stimulated
diffuse molecular mucus secretion from all over the body. The addition of largemouth bass
mucus significantly increased the LT50 (the time to kill 50%) for green sunfish and goldfish
exposed to 250 mg/L of Pb-nitrate. In contrast, Tao et al. (2000) found that mucus reduced the
overall bioavailability of Pb to fish but that the reduction was insignificant. Coello and Khan
(1996) found that scales were more significant in reducing LT50 than mucous. Fish scales can
accumulate high concentrations of metals, including Pb, through chelation with keratin. Scales
were shown to buffer the pH of Pb-nitrate in solution and remove Pb from water after which they
settled out of the water column. Addition of scales to test water made all species (green sunfish,
goldfish, and largemouth bass) more tolerant of Pb.
AX7-155