A heavy metal is a member of an ill-defined subset of elements that exhibit metallic properties, which would mainly include the transition metals, some metalloids, lanthanides, and actinides. One definition is metals with a density greater than 5 g/cm3. 
Biological essential and non-essential heavy metals
Heavy metals consist of both biological essential, as non-biological essential metals. Biological essential heavy metals include copper (Cu), nickel (Ni), iron (Fe) and zinc (Zn). Iron for instance forms an essential part of hemoglobin, a protein in our blood which transports oxygen from the longs to other tissues. Although they are necessary, they become toxic at high concentrations. Non-biological essential heavy metals include lead (Pb), mercury (Hg), cadmium (Cd) and tin (Sn). They can be tolerated at low levels, but become toxic as well at higher concentrations. The order of toxicity (from low to high) has been suggested as follows: cobalt, aluminum, chrome, lead, nickel, zinc, copper, cadmium and mercury.  In marine environments however, three metals are of primary concern: lead, mercury and cadmium.
Heavy metals are a natural part of the earth's crust. Heavy metals enter the sea usually through riverine influx (after weathering and erosion of rocks), atmospheric deposition (dust particles e.g. from volcano's) and anthropogenic activities. Humans add both to the riverine disposion (waste water of factories) and atmospheric depostion (cars, factories,...). Heavy metals are stable and can't be broken down, which makes it easy for them to accumulate in the environment.
When present above threshold concentrations, all heavy metals can be toxic. This threshold concentration depends on the metal, the animal species, but also on the environment, which determines the availability. The exposure of marine organisms to toxic levels of metal contaminants can cause damage to tissue, inability to regenerate damaged tissue, growth inhibition, damage to DNA,... . Although most marine organisms tend to accumulate heavy metals from the environment, they are capable to store, remove (trough feces, eggs, or molting) or detoxify (with metallothioneins) many of them. However, these abilities tend to differ between species, making some species more tolerant than others. It should also be considered that although detoxifying mechanisms exist, when environmental concentrations are to high, these can be unsufficient and the organisms will start showing effects. However, areas which experienced very high heavy metal pollution for centuries (like Restronguet Creek in England has for copper) contain populations which are more tolerant to the pollution than populations of the same species in uncontaminated regions. So there might be selection towards heavy metal resistance in these regions. 
Behavior in the ecosystem
Heavy metals introduced in the marine ecosystem are mostly concentrated in coastal areas, near densely populated and industrialized regions. Heavy metals are usually associated to particles. These particles are often very small, and can therefore stay in solution for a very long time. Nevertheless they will end up in the sediments, therefore concentrations in the sediments are often 10 to 100 times higher than those in solution. In the sediments, these particles may form an important secondary source of contamination, even after the primary source has disappeared. 
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