鱼嘴上的两个孔是鼻孔吗?

Y: I noticed that fish have holes that look like nostrils, does that mean they can smell?

我发现鱼的头上有两个孔，看起来像是鼻孔，这说明鱼能闻味道吗？

D: Smell, or olfaction, as scientists call it, is an important sense for many fish. Those little holes that look like nostrils are called nares. Nares don’t lead to the throat the way nostrils do in mammals, but open up into a chamber lined with sensory pads. Not all fish move water in and out through these nares in quite the same ways, but key to a strong sense of smell for fish is the ability to move water rapidly over these sensory pads. Some fish can pick up chemical signals when immobile by pumping water through their olfactory system via tiny hairs called cilia. Other fish can pump water by a muscular movement. Some fish, such as smaller species of mackerel, have an olfactory system that requires them to swim in order to get water moving through their nares. When the sensory pads pick up chemical signals, they transmit them to the fish’s forebrain, which interprets the signal and incites the fish to respond appropriately.

对很多鱼类来说，嗅觉是一种很重要的感官。与哺乳动物不同，鱼类的鼻孔不与喉咙相通，而是形成了单独的一个腔，里面与嗅觉感应组织相连。鱼并不都是以相同的方式让水在自己的鼻孔中进出的，但嗅觉敏锐与否的关键在于鱼是不是有能力让水快速通过自己的嗅觉感受器官。有些鱼，在静止不动时可以通过一种叫作纤毛的小绒毛让水通过自身的嗅觉器官，来接收（水中的）化学信号。而有些鱼则是通过肌肉运动来实现这一过程的。还有的鱼，如小型鲭鱼，则需要通过游动来让水通过它们的呼吸系统。当嗅觉感受组织接收到了化学信号，就会把这些信号传递给前脑，前脑则负责对信号进行分析，让鱼对此做出恰当的反应。

Y: You mean like if the chemical signal's food, the fish will pursue the food. Or, if the chemical signal's danger, they will flee?

你是指，比如说，如果这个化学信号是食物，鱼就会去捕食。或者，化学信号是危险，它们就会逃跑吗？

D: You got it. But fish use chemical cues in all sorts of ways. For instance, a large group of fishes release a chemical when they’re wounded that incites other fish to flee. And then there are salmon, which are known for a superb sense of smell that enables them to sense the stream where they were born, so that they may return to it to spawn.

完全正确！但化学物质所提供的线索对鱼来说有多种用途。例如，当受伤时，有很多种鱼会释放一种化学物质，让其他鱼逃跑。因嗅觉出众而闻名于世的大马哈鱼，能够通过水流来嗅出它们出生的地方，以便能重归故里进行繁衍。