The horseshoe crab is one of the most ancient animals on the planet today. They have survived massive upheavals throughout the Earth's history and have remained intact and unchanged. Recently their numbers have been in decline, and this is thought to be due to coastal habitat destruction and over-harvesting; they are often used as bait, in fertilizer, or by pharmaceutical companies. However, new research from the US Geological Survey (USGS) indicates that their population size also parallels changes in the climate. With predicted climate change in the future, their numbers may continue to decline.