They mostly eat kelp which is "blown" into them by the current/wave action. It is kinda cool to see the abalone rear up and "attack" a piece of kelp you hold out to them.

As far as the way they look, these are really good eating too.
http://www.wdfw.wa.gov/fish/shelfish/beachreg/2clam.htm
but who in their right mind was the first person to say, "HEY!! Gimme somma dat!!"?

oysters and other filter feeders aren't dangerous either, unless there is bad pollution, or too much of a dangerous type of plankton in the water. Then you could get sick. you just have to check the danger warnings in your state. I have eaten a lot of local oysters, as well as self-gathered mussels, clams, etc in the right season...and no, I don't eat them raw either.