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Do fish sleep and drink water?

Asked by: Anne Marie Mantei
School: O-A Middle School
Grade: 5

Hobbies/Interests: Acting, Reading, Playing Volleyball, Horseback Riding
Career Interest: Professional Volleyball Player, Teacher, Book Editor, and Journalist

Answer from Stanley N. Salthe

Visiting scientist, Binghamton University

Department: Biological Sciences
Research Area: Natural philosophy
Interests/hobbies: ecology, evolutionary biology, semiotics, systems science, and thermodynamics. Woodland gardening, nature walks, all of the arts
Ph.D: Columbia University
Family: wife Barbara, two children Becky and Eric

To answer the first part of your question, yes fish do sleep.  Most rest at night, except for night feeders who rest during the day.  Without eyelids, fish cannot close their eyes, so they must switch their minds to a different mode while sleeping. Some fish, like open ocean sharks, are constantly moving so water can keep passing over their gills. Because there are so few obstacles, they may be able to sleep while moving slowly.  Bottom living sharks can park in places where a current of water keeps flowing in one direction for a period of time, such as the tides; they just need to open their mouth and take a nap.  Regular (bony) fish can find places to hide on the bottom of the waters where they live. They keep water circulating over their gills by moving their gill covers. This is automatic, much like our own breathing. Parrotfish secrete a thin mucous blanket around themselves as they sleep. This can act as an alarm if an intruder were to make contact.

In answering the second part of your question we must first look at the type(s) of fish in question.  Fresh water fish have no need to drink as water is continually taken in through diffusion.  They also cannot eat without taking in water. In fact, their problem is having too much water. To control the extra water intake, their kidneys produce large amounts of dilute urine, which helps to get rid of the extra water, while clearing out waste products.

Salt-water fish take in seawater when they eat because the also need water.  Just like us, salt-water fish absorb water in the gut.  However, along with this water comes an excess amount of salt, which can be unhealthy for these fish. They eliminate this extra salt through a special process in their gills.  Sharks also take in seawater with their food, and absorb it in their gut.  However, because their tissues are in balance with the salt, they do not absorb it and therefore do not need to clear it out.

Last Updated: 3/1/17