But as King and his team discovered, the current genetic diversity among American horseshoe crabs is surprisingly low - a bad sign for an already threatened species.
Using the genetics of today's crab communities, the researchers pieced together historical population sizes. If a population was started by a few individuals, this causes a genetic bottleneck and results in a population with low genetic variation. This low diversity becomes apparent when a species' numbers start declining.
Scientists suspect that rising sea levels and temperatures since the last Ice Age likely washed away huge swaths of the crabs' habitat, separating them among newly isolated regions, and setting up these communities to bottleneck.
The low genetic variation observed in the crabs feeds a vicious cycle seen across the animal kingdom: low genetic diversity hurts a species' ability to adapt to environmental changes, which then often results in a population decline that further decreases genetic diversity.
King still believes human activities, such as the harvesting of horseshoe crabs as bait for American eel and whelk fisheries, play a major role in the recent crab declines, but he wants to raise awareness about climate change's potential influence.