They change colour. 颜色发生变化

168. The top of this waterfall.
     这座瀑布顶端
169. Surely an impossible journey for a fish.
     对鱼来说 无疑是一处不可能的目的地
170. But over the coming days, the gobies transform themselves.
     但在接下来的几天里 虾虎鱼会改变自己
171. They change colour.
     颜色发生变化
172. Their mouth rotates and acts like a kind of suction cup...
     嘴部转动 如同一个吸盘
173. ..which they will certainly need.
     这正是它们需要的
174. Because the only way...is up.
     华山一条路 它们必须登上顶端

来自《行星地球 第三季 第四集 Planet Earth S03E04 淡水》第 171 行