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The senses of taste and smell in different fish classes are described as two distinct senses; smell is mediated by the nasal openings, and taste by epithelial taste buds.

They are both forms of chemoreception and therefore it is not clear to me what intrinsically defines them as two distinct senses. The more so because reception of chemical substances is mediated by water.

What defines them as different senses? Is it only the difference of receptors that defines them as two distinct senses, or are there other reasons to differentiate them?

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    $\begingroup$ One difference is that they are controlled by different parts of the brain. But both are GPCRs and perhaps have common evolutionary origins. I am in a hurry now so cant look up the answer but I can point you to this article that talks about functional difference between olfactory and gustatory response towards the same chemical. $\endgroup$ – WYSIWYG May 30 '15 at 19:51
  • $\begingroup$ @WYSIWYG - I hve waited a month before answering, but I am assuming you have other priorities now as a mod. I hope you don't mind me taking this opportunity to answer this interesting question :) $\endgroup$ – AliceD Jun 30 '15 at 5:10
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    $\begingroup$ @AliceD No problem at all. You can add my comment in your answer if you want. That article was not accessible to me then. $\endgroup$ – WYSIWYG Jun 30 '15 at 5:27
  • $\begingroup$ one thing is not clear to me... are you asking about taste-and-smell of fishes to us when we eat them? or you are asking about fishes's perceptions about their underwater environment? $\endgroup$ – Always Confused Nov 4 '16 at 14:44
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Short answer
One way to distinguish between the olfactory and gustatory system in fish is by their anatomical differences. For example, olfactory receptors are clustered in the nasal region, while gustatory receptors are scattered in the head region and beyond. In turn, olfactory information is sent via a single cranial nerve to the brain, while the gustatory system includes three cranial nerves.


Background
The olfactory and gustatory system both receive their inputs through chemoreceptor cells. The chemoreceptor proteins in both systems are G-protein coupled receptor systems. However, the olfactory and gustatory systems are anatomically separated.

The olfactory end organ is in fish comparable to that in humans, namely sensory epithelia situated in nostrils. The olfactory receptor cells form axons that are grouped into the olfactory nerve (cranial nerve I). The nerve innervates the olfactory bulb, where its axons synapse onto mitral cells. Mitral cell axons group into the olfactory tract that projects to the telencephalon in the brain.

fish
Schematic of olfactory system of a fish (not all structures shown). Source: Global Warming Australia.

The gustatory end organs are less well anatomically localized. The taste buds can be found in various structures including gills, barbels, fins, oral cavity and pharynx, dependent on the species. Unlike the olfactory system, gustatory information is transmitted to the brain via three separate cranial nerves: facial (cranial nerve VII), glossopharyngeal (cranial nerve IX) and vagal nerve (cranial nerve x). Normally, the facial nerve innervates taste buds on the extraoral surface, the glossopharyngeal on the anterior part of the oral cavity, and the vagal nerve on the pharynx. The facial and vagal nerves respectively terminate in the facial and vagal lobes in the medulla. The glossopharyngeal nerve terminates in a dorsal medullary region between the facial and vagal lobes.


Reference
- Hara, Rev Fish Biol Fisheries (1994); 4: 1-35

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