ABSTRACT
A common nominator for the chemosensory systems throughout the vertebrate phylum is the early appearance and functionality. The variation in form and function of chemosensory organs throughout the many thousands of fish species is enormous and fascinating. There are three chemosensory systems in fish, which can be distinguished due to form, distribution and function. The most salient features of the olfactory system are that it can detect substances in minute quantities in nano- or picomolar concentrations; that activation of the sense of smell elicits distinct behaviours, and that these behaviours are related to essential life processes. The other requirement for an efficient olfactory organ is that ambient water is made to pass over, or be transported through the organ. The olfactory placodes in zebrafish develop in parallel with the olfactory bulbs.
