The Red Encrusting Sponge is an odd animal scientists long thought to be a plant. Sponges are the simplest animals with multiple cells, and they take many forms. This one's name comes from its bright color ranging from red to orange-red and the fact that it's shaped like a crust, usually on stone. Around Puget Sound, look under rock overhangs at low tide to see these red splotches and feel their velvety texture.
Some sponges have branches or grow large bodies that look and feel — like sponge. But Red Encrusting sponges are just a quarter inch thick and can spread a yard across the rock where they stay attached for life. As all sponges, this one has many identical cells but no specific mouth or stomach or any specialized organs at all. Tiny waving hairs in the sponge's cells send sea water into microscopic pores. Oxygen and bits of food get filtered out; then the sponge expels water and waste through bigger holes. Get close enough and you can see these star-shaped pores.
While you're having a close look you may also notice an identically-red ½ -inch long lump on the sponge's surface. This is the Red Nudibranch, a mollusk without a shell that eats the sponge and, while eating, gets the pigment it needs for perfect red camouflage. This little creature even lays red eggs, depositing a spiral of them atop the sponge.
Red Encrusting Sponges and the many other creatures living between the tides may look durable, but they're easily damaged. When exploring their world, be sure to replace rocks just as you found them and avoid walking on seaweeds because they shelter creatures.