Why can’t mussels get rid of the heavy metal in their bodies?
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Because evolution takes the easiest path to the best outcome, it doesn’t optimize per what humans might think is “Best”, it optimizes for *what works*.
In this case, it was easier for sea creatures to learn to tolerate toxic metals that it was to isolate them, concentrate them, and expel them.
There may be additional secondary effects in which by removing toxic metals from the surrounding area mussels ended up creating a “healthier than normal” region where other sea life could thrive. This in turn benefits the mussels by creating a more ideal ecosystem in their other life functions, such as increasing the general availability of nutrients or attracting predatory fish that will eat the organisms that prey on young/larval mussels.
Heavy metals tend to be fat soluble, and they’re toxic because they act *similar* to other atoms in your body that you need without being similar *enough* to fulfill the function of those other elements.
When a heavy metal is ingested – not just by mussels, but also by you – it dissolves into the lipids in your body, which are difficult for all of the water-soluble enzymes to get to. The metals just kind of…sit there, slowly leaking out. That also means the heavy metal can easily slip through the cell membrane. Once inside a cell, it’s much harder to get rid of because that individual cell may not have the tools for that. Usually, getting rid of toxins involves immune cells with special tools to contain and remove them. If the heavy metal is *inside* a cell, the immune cells can’t find it to remove it.
So, that means the heavy metals are either accumulating in fatty tissue, where they leak out, or slipping inside cells to wreak havoc. Inside a healthy cell, the heavy metal gets incorporated into protein structures. That happens because the heavy metal reacts similarly to an enzyme as some other element that your body is trying to use, or the protein has an open binding site and the heavy metal wanders closely enough to get caught.
Your cells can’t really do anything about that, because it’s difficult for your body to recognize something being wrong, since the protein *looks* like a normal protein, it’s just slightly broken by the heavy metal. Even if the cell does try to recycle the protein, that just releases the heavy metal back into the tissues. If your body builds enzymes designed to react to the heavy metal and remove it, it might accidentally react to the elements that you *want* that are similar and remove them instead.
The best, safest course of action is to just shove them into the fatty tissues where they can’t react to anything and have enough redundancy in the rest of the cells so that when it leaks out and causes problems it’s not bad enough to cause serious harm to the animal. And that strategy worked for hundreds of millions of years because heavy metals don’t usually end up in the water. It’s only recently that heavy metals have become such a huge problem, because of human industry that creates a lot of heavy metal waste.
Mussels are particularly vulnerable because they are filter feeders and can’t really control what flows through them. (Almost) all water leads to the ocean, and rain washes industrial waste into rivers and streams (if the waste isn’t being dumped directly into a waterway). All that industrial waste will pass through the waters where the mussels live. The metals get sucked up by plankton, which get sucked up by the mussels.
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