Clownfish are immune to anemone stings, so they can live inside them and be protected from bigger critters. The anemone, in return, gets the leftovers from clownfish dinners. One of the Great Diving Revelations TM I had in Papua New Guinea was my discovery of marine flatworms. A flatworm sounds so ugly and uninteresting, yet here were these beautiful, colorful critters crawling on the reef. They're hard to spot 'cause they're so tiny, but the effort is well worth it.
Scorpion fish like to sit around on the reef pretending to be rocks.
I was practically on top of this one in Papua, New Guinea before I realized it was a critter. One of my main reasons I dreamed of diving the South Pacific was the lionfish. I've seen pictures of them, and think they're some of the most spectacular undersea critters in existence. Papua, New Guinea certainly didn't disappoint in that regard. I was in lionfish heaven.