The algae mats floating in a stretch of California's Bear River look pretty disgusting. The "didymo" -- short for its scientific name, Didymosphenia geminata -- looks something like shredded toilet paper. It also resembles another of its nicknames: "rock snot." Its behavior has changed in recent years, and it is spreading. It appears in trout streams and threatens trout. New Zealand has banned felt-soled wading boots (often used by fly fishers), which can spread it.

Matt Weiser reports for the Sacramento Bee August 31, 2010. [2]