• Christopher Stephens, MSc

Pacific Sand Dollars: Just What Are They?

Updated: May 14

3 Surprising Facts About These Flat Sea Urchins

Pacific sand dollar taking cover in the substrate, Parksville Bay, BC, Canada

The Pacific Coast of North America is a place of extreme geography. It is where landscapes and seascapes vary from vast sandy beaches to cliffs emerging straight from the ocean in just a short distance.

In the sandy zones of this coast, like British Columbia, Canada's Parksville Bay and Pacific Rim National Park, Eccentric sand dollars, also known as Pacific sand dollars, thrive. Often found sunbleached as pale white tests, which are the skeletons of deceased sand dollars, these creatures are highly underestimated. They are not coins from Atlantis, let's get that out of the way first. The Pacific sand dollar has a huge natural range, extending from Alaska to Baja California, Mexico.

Now for fascinating fact number one: Pacific sand dollars are in fact flat sea urchins! Yes indeed. If you look carefully at a live specimen at low tide, you will discover that the purplish black animal feels slightly felt-like or bristly. That is because they are covered with spines seen on a typical sea urchin. There are just a lot more of the spines, which are far finer, as well as countless tube feet.

Fascinating fact number two is the ability of seemingly sedentary Pacific sand dollars to actively chew up their prey. Yes, these creatures have five-part jaws, which process food caught by their spines and tube feet. Prey taken by hungry sand dollars may include small crustaceans such as copepods as well as the larvae of larger crustacean species.

The third and most remarkable fact about the Pacific sand dollar in particular is its hydrodynamic function and resultant social cooperation in feeding. Engineered by physics and biology, the Pacific sand dollar is in fact a hydrofoil, as described in Steven Vogel's fascinating book Comparative Biomechanics: Life's Physical World. These sand dollars group up in lines, angled up against the water flow. As a result, food is brought right up to the mouths of each sand dollar in the feeding group for easier capture. The cumulative effect of multiple sand dollars feeding as hydrofoils was compared to the function of triplane or biplane wings in Vogel's text.


 © 2020 by Christopher M. Stephens Consulting & Writing

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