What are the most remarkable symbiotic relationships in nature?

The answer

Cleaner wrasses remove parasites from larger fish that willingly open their mouths and gills - over 2,000 interactions daily per cleaning station on coral reefs. The relationship between clownfish and sea anemones is iconic: clownfish are coated in mucus that prevents anemone stings, gaining shelter while their movements increase anemone oxygen flow by 13%. Oxpeckers ride on African megafauna, eating ticks but also keeping wounds open to drink blood - a mutualism-parasitism continuum. Perhaps most extraordinary, the Hawaiian bobtail squid houses Vibrio fischeri bacteria in a specialized light organ; the bacteria's bioluminescence eliminates the squid's shadow under moonlight, providing camouflage in exchange for nutrients.