The reading gives third characteristics of symbiotic relationships.
The first characteristic is mutualism that both organisms gain some benefit from relationship.
The second characteristic is parasitism that one of the organism benefits, other one is harmed.
The third characteristic is commensalism that one of the organism benefits, while other one neither benefits nor is harmed.
The professor gives two examples: bacteria on our skin and shrimp and crab that living in coral.
Bacteria on our skin are commensalism relationship because its colonies don’t harm us. Nevertheless, when we get burned, the bacteria will turn into opportunistic pathogen that caused infections. So this relationship is called parasitism.
Shrimp and crab that living in coral is commensalism relationship because they neither harm or benefit the coral colonies. Nonetheless, when coral-eating sea stars attacks the coral, shrimp and crab will protect it. Therefore this relationship is called mutualism.