PLANET EARTH Freshwater

In their upper reaches, mountain streams are full of energy. Streams join to form rivers, building in power, creating rapids（湍流）. The water here is cold. Low in nutrients, but high in oxygen. The few（few表示几乎没有，a few表示有一点儿） creatures that live in the torrent have to hang on for dear life. Invertebrates（vertebra脊柱，in- 表示与XXX相反的，Invertebrate无脊椎动物） dominate these upper reaches（河流上游）.



The hellgrammite（翅虫的幼虫）, its body flattened to reduce drag（除了表示拖拽，还有阻力的意思，如空气阻力等，这里表示水流阻力）, has bushy gills to extract oxygen from the current（这里表示水流）. Black fly larvae（黑蝇幼虫） anchor themselves with the ring of hooks（钩环）, but if these become unstuck, they're still held by a silicon safety line. There are advantages to life in the fast stream - bamboo shrimps can just sit and sift out passing particles with their fan-like forearms.