Sunday, November 22, 2009

Arachnid Lungs Evolved From Horseshoe Crabs

I have been watching videos of the BBC television series Life in the Undergrowth with David Attenborough. It starts off by explaining how life got in the undergrowth to begin with, in other words, how it crawled out of the sea. One example he discusses is the horseshoe crab.

The horseshoe crab is actually not even a crab, it is an arthropod of the subphylum chelicerata, which means it is more closely related to the arachnids like spiders, scorpions and ticks than it is to crabs. Sir Attenborough showed thousands of horseshoe crabs crawling out of the water for their annual spawning. Living in prime horseshoe crab territory in New Jersey, I have seen many of these ancient creatures gracing our beaches.

Attenborough explained that since horseshoe crabs were among the first animals to develop the ability to venture onto land, they had a distinct advantage in that they could keep their eggs away from their enemies who were still in the water. They no longer have this advantage, since many birds look forward to the annual horseshoe crab spawn every year. This is especially true of the red knot, which feasts on the horseshoe crab eggs during its migration stop-over in the Delaware Bay. This species of sandpiper makes an amazing annual migration from one end of the Americas to the other, from the Canadian Arctic to Tierra del Fuego at the southern tip of South America!

The adaptation that allowed the horseshoe crab to venture onto land was the "book gill", which you can see in the top photo just above his tail. Each of the folds that is visible in the photo has many more folds within it, with the overall structure looking like the pages of a book. This gill is on the outside of the horseshoe crab's body, so as long as he keeps it moist with the small amount of water in the wet sand, he can live out of the water up to a week. The many folds increase the surface area for gas exchange to his blood.

Fast forward to modern-day spiders, close relatives of the horseshoe crab. Below is a cross-section diagram, with #16 being the "book lung". This is essentially the book gill of the horseshoe crab, evolved to be located within the spider's body, and supplied with air through a small opening. Scorpions have a similar setup. Pretty amazing that these small land animals have lungs evolved from an ancient sea creature. Thanks to Wikimedia Commons for the photo and diagram.


  1. Very interesting, I'll pay more attention to my local spiders (since we don't have horseshoe crabs at our beaches in Panama).

  2. Wow, very interesting indeed... i've seen those poor little horseshoe crabs on the beach, a fine dinner for the seagulls and crows... thanks for sharing

  3. We are David Attenborough fans too. Thanks for pointing out the relationships of horseshoe crabs to spiders.

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