How do horseshoe crabs breathe?
Horseshoe crabs require a steady supply of oxygen to stay alive. Their blood contains a molecule call hemocyanin which carries oxygen throughout the crabs body. Hemocyanin is very similar to the hemoglobin that human blood uses to carry oxygen and that gives our blood it's red color. The main difference between these two molecules is that hemoglobin is an iron-based molecule whereas hemocyanin is based on copper.
Oxygen is transferred from the outside environment to the inside of the body through gills. Very small animals can obtain oxygen directly through the body wall because they are small enough that they don't require much oxygen and the amount of material in their bodies that require it is relatively small. As an animal gets larger it's volume increases exponentially and there isn't enough outside surface area for oxygen to get in. The way large animals deal with this is by increasing the surface area where oxygen comes into the body. It folds. We have lungs which have branches and aveoli which give us an effective surface area of some 2000 square feet. Horseshoe crabs use a system that is also employed by their closest relatives, spiders and scorpions. They use book gills.
Book gills are a fairly straightforward design for getting a lot of surface area exposed to the outside environment in as little space as possible. Imagine a paperback book of several hundred pages in your hand. It might only be an inch or so thick and perhaps 4 by 6 inches tall and wide. Imagine cutting the pages out of the book and laying them side by side. Very quickly you would see just how much print area even a small book represents. Book gills are of a very similar design and each "page" or lamella is a thin double-walled chamber full of blood which can exchange oxygen and carbon dioxide with the outside water. By keeping the gills in motion the crab moves water and oxygen over them and also moves blood through the lamellae. In addition, young horseshoe crabs can propel themselves by using their gills as paddles.
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