Specific contaminants

Mixed effluents

Comparisons of larval and adult sensitivity

Generalising about sensitivity to chemical pollutants

The bioaccumulation of environmental contaminants and their effects on the physiology of mussels was reviewed by Widdows & Donkin (1992). At the molecular and cellular level the effects were reviewed by Livingstone & Pipe (1992). An example of the use of mussels in monitoring the bioavailability of contaminants released by dredging sediments from a harbour was given by Bergen et al. (1993) #247]. They also give bioconcentration factors for six PCB congeners from the seawater.

The survival time of mussels in air is a simple to measure yet sensitive response to pollution induced environmental stress (Smaal et al., 1991). Survival was significantly lower after 6 weeks experimental exposure in contaminated parts of the Western Scheldt (Smaal et al., 1990), particularly when the mussels accumulated higher tissue concentrations of the lower PCB congeners. Clearance rates were also reduced at the highest tissue concentrations.

As part of the US Mussel Watch, histological examination for neoplasias was included as well as chemical analyses. Significant increases were found in the incidence of neoplasias where there were higher concentrations of combustion related PAHs, cis-chlordane pesticides and cadmium (Hillman, 1993).

Mesocosm experiments over two years with the water accommodated fraction of diesel showed that Mytilus was more sensitive than many of the other common intertidal invertebrates such as Carcinus maenas and Littorina littorea, being lost from mesocosms dosed with 30.1 µg/L and above (Bokn et al., 1993).

When mussels from areas where they had been exposed to PCBs were moved to a clean location, little depuration occurred over 3 months until gametogenesis (Hummel et al., 1990). Accumulated PCBs decline when mussels spawn, but different congeners are not all lost at the same rate, suggesting differential partitioning in different tissues or lipid pools (McDowell Capuzzo et al., 1989).

A number of studies have demonstrated toxic effects of TBT, including mortalities, at concentrations in water of 0.4 m g L^-1 or less (see Widdows & Donkin, 1992 for a summary).

Even substances that generally seem relatively benign sometimes cause unexpected impacts. In 1991 a special products tanker carrying sunflower oil was wrecked off the Anglesey coast. Mussels ingested droplets of the vegetable oil, with consequences for their reproductive metabolism resulting in mortality some months later when they spawned (Mudge et al., 1993).

In general, it is thought that bioaccumulation and toxicity of mixtures of organic compounds is additive, although antagonistic interactions (where the combined effects of toxicants is less than would be expected from simple addition of the effects of the separate toxicants) have sometimes been reported (see Widdows & Donkin, 1992, for a summary). Studies of interactions between structurally unrelated toxicants petroleum hydrocarbons and copper have revealed a simple additive effect (Widdows & Donkin, 1991), although this contradicted earlier work by Stromgren (1986), who found antagonism. Antagonism between the effects of TBT and hydrocarbons has been reported in laboratory studies (Widdows & Donkin, 1991), and this was supported by field observations which found mussels surviving with high tissue levels of TBT at sites thought to have hydrocarbon contamination (Page & Widdows, 1991). Reports of synergistic effects of combinations of toxicants were not found during preparation of this report.

Although there is a widespread supposition that larval stages of invertebrates are more sensitive to pollutants than adults, this may not be the case, at least in Mytilus; scope for growth studies have suggested that adults are around 10 times more sensitive than larvae in respect of copper (Beaumont et al., 1987), hydrocarbons (Widdows et al., 1987) and sewage sludge (Butler et al., 1990) and around 4 times for TBT (Page & Widdows, 1991). Results of toxicity studies of these and other contaminants from a variety of studies, in adult and larval stages, are summarised in Widdows & Donkin (1992), as also are the measured levels of a variety of contaminants within adult mussels.

Mussels were missing from a wider area of a Cumbrian shore than were other organisms around a large, phosphate rich outfall, the effluent from which was contaminated by a number of heavy metals (Pope et al., 1997). On the other hand, distribution of shore organisms around other industrial or mixed outfalls has shown mussels to be among the least sensitive shore organisms (eg McKenzie & Perkins, 1979). Laboratory based toxicity tests tend to give the same mixed results, with few organisms, including Mytilus, being consistently tolerant or sensitive. Attempts to generalise about sensitivity to chemical pollutants are probably unwise (Holt et al., 1997a; Holt et al., 1997b).

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