The range of roles which fish mucus is said to play is very large. Proposed roles in respiration, ionic and osmotic regulation, reproduction, excretion, disease resistance, communication, feeding, nest building and protection potentially make mucus a highly multifunctional material. While it is likely that mucus is involved in many of these, interpreting an 'action or involvement' as a 'function' is a fundamental step which, in particular, defines a purpose for mucus. In this sense, functional interpretation is, inevitably, teleological. In biology, the process of natural selection provides some justification for a teleological approach as natural selection accounts for the appearance and development of purposeful design. Mucus may have a specific action which enables the fish to survive where, otherwise, it might not. Its presence is, therefore, adaptive and is likely to be selected for. To interpret an action as a function, therefore, it is important to stress the adaptive consequences of the action and, for this purpose, to investigate the mechanisms involved as far as possible. Indeed the most appropriate question is probably 'how does it work?' rather than 'what does it do?' An answer to this former question requires information about the processes involved. An answer to the latter question might not, but will inevitably be a more subjective interpretation of what is happening.
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