Breaking down biodegradability

A few days ago, while eating in a restaurant, the drink I ordered came with a straw. As he handed it to me, the waiter volunteered that it was made from biodegradable plastic. “No more soggy paper straws,” he joked. But I had to wonder: would the straw really biodegrade in the landfill it was destined for or did the manufacturers evaluate its end-of-life behaviour exclusively under controlled composting conditions? Worse still, what would happen if it did make its way into the ocean? Would its impacts be as significant as those of demonized conventional plastic straws, or would it readily decompose in a marine environment?