The growth of modern giant clams is supercharged compared to growth measured from fossil clams. A recent study from the Red Sea has shown this, finding that growth lines from modern species are larger than those of fossils from similar animals dated to the Holocene and Pleistocene.
These increased growth rates appear to be related to higher amounts of nitrate aerosols in the modern atmosphere. These come from many different sources. Some are natural, such as lightning, biomass burning, and soil processing, but most are from anthropogenic activity like burning fossil fuels and agricultural fertilization.
This fast growth may seem like a good thing, but growth doesn't mean anything about the overall health of the clams. Additionally, aerosols may actually reduce the productivity of marine phytoplankton, which represent almost half of the world's primary production.
The overall effects of nitrate and other aerosol pollution on global land and ocean cycles are not well understood. They may appear to reduce global warming by improving carbon dioxide uptake and reflecting the sun's heat, but they contribute to poor air quality. We can congratulate today's super clams on their impressive growth. But in the long run, fewer emissions on our part are probably better for them.