Pencil Urchin Reefscape
Urchins have an interesting relationship with coral reefs. In short, a healthy reef system also supports a healthy urchin population. Coral competes with algae for space on the reef. Both coral and algae require substrate and light. The problem for coral is that it is an extremely slow growing organism, while algae grows very rapidly. Algae would easily out-compete coral if not for herbivorous grazers that keep the algae from getting out of control. These grazers can be fish (like parrotfish or surgeonfish) or invertebrates (like urchins). Urchins are some of the most efficient grazers around, and in their absence, a pristine coral reef begins to undergo a phase shift to a dead reef covered in algae.