You are fish. The guy above is your enemy, a Gnathiid isopod, a vicious parasitic relative of a roly-poly. Your defense? You cough up enough loogies to coat yourself in a protective layer of joyous mucus.
Of course you are not a fish and fish don’t need to cough 1,000′s of thick loogies. If you were a parrotfish all you would need is to secrete a thin layer of mucus to cover yourself every evening. You sleep and your mucus blanket protects you. What do you care? Sure you may not be attracting the opposite sex all covered in your own slime but that is little price to pay for not getting eaten alive. Moreover, that mucus blanket costs so little. A mere 2.5% of your daily caloric intake. Of course you could not produce that mucus and risk a 95% chance of having flesh picked at by your friend above. I personally pick the nice juicy mucus coat and cut my chances down to 10% of being picked off in my sleep by parasites.
Grutter, A., Rumney, J., Sinclair-Taylor, T., Waldie, P., & Franklin, C. (2010). Fish mucous cocoons: the ‘mosquito nets’ of the sea Biology Letters, 7 (2), 292-294 DOI: 10.1098/rsbl.2010.0916