Like many other invertebrates, sea urchins may be vulnerable to increasing acid levels in the ocean. The hard parts of urchins (such as the sea urchin in our photo) are made up of calcite, magnesium-rich calcium carbonate.
Ocean acidification could interfere with the urchins’ ability to transform calcium, magnesium and other minerals from seawater into their “tests” (the white shell left when the animal dies), spines and teeth. Other animals that can be affected include oysters, clams and other mollusks, and crabs and other crustaceans.
In the top photo, Strongylocentrotus franciscanus is using the teeth on its five part jaw (see photo at left) to gnaw on the exoskeleton of a hermit crab (at arrow). These herbivores will also scavenge on other animal material such as dead fish and eggs.
At left is the oral or underside of the red urchin, showing its central mouth. The light spot (at the upper end of the arrow) is the tip of the five teeth partially emerging from the mouth. The teeth are part of the whole powerful mouth structure (shown at lower left) which is known as Aristotle’s lantern.
These teeth, although composed of calcite which is relatively soft, are able to gnaw algae off of rocks and other hard surfaces (such as plexiglass) and even, because they’re self-sharpening, create holes in limestone and other hard materials. Their unique structure allows the worn surfaces of each tooth to break off at specific fault lines, continuously revealing new sharp cutting edges.