Fig wasps and fig trees: the wasp's entire life happens inside figs, the tree can't reproduce without the wasp — 80-million-year mutualism. Cuscuta (amarbel) strangles host plants — a parasitic plant you can see on hedges. Barnacles on whales: free ride, whale unaffected — commensalism. Penicillium killing bacteria on a petri dish: amensalism that gave us penicillin.