Coastal Watch, Part 4 - Drinking Water at Sea

An intriguing variety of marine vertebrate animals maintain a salt level in their blood that is similar to ours but only a third that of the surrounding sea. All of them need to replace water lost daily from their bodies, as in the exhalations of this Minke whale. How are they able to do something that we cannot?

Minke whale off Halibut Point

Nearly all marine mammals subsist on fish and other organisms with similar salt levels to their own. They gain most if not all the 'fresh' water they need from their prey without taking in excess salt. However they are generally equipped with kidneys much larger and more efficient than our own to filter out harmful salt that reaches their blood through swallowing or digestion.

Mammals constitute the only class of vertebrates in which the kidney is always the major organ of osmoregulation. * While we lack full agreement among scientists it appears that whales can at least occasionally drink from the sea to maintain their necessary physiologic balance. As in humans, but much more efficiently, excess salt is eliminated from their bloodstream in highly concentrated urine. ¹

Red-breasted merganser and horned grebe

Birds that spend much of their lives on the ocean have a similar challenge in meeting their fresh water needs. Fish eaters such as mergansers and grebes do obtain from their prey water of tolerable salt content.

Iceland gull

Some oceanic birds like gulls visit land at least occasionally to drink fresh water from terrestrial sources.

Northern gannet

Others like the gannet never intentionally come to land except in their breeding season.

Dovekie

Pelagic birds like the dovekie spend most of their time far from any shore and the possibility of drinking fresh water.

Common loon

Birds have a reptilian type of kidney considerably less efficient than a mammalian kidney at concentrating and secreting salt. If they drink sea water they must have some other means for eliminating excess salt from their bloodstream.

King eider

This problem is especially critical for birds like eiders that subsist on mollusks or invertebrates which, unlike fish, are in osmotic equilibrium with sea water. Their food sources are salty.

The salt gland in an albatross ²

All birds (except passerines, according to one source) have a salt-secreting nasal gland, but in terrestrial species it is very small. The size of the gland in marine birds is 10 to 100 times as large. Its size varies at least in part with the degree of exposure to salt in the species' ecology.
 

A large order of seabirds like the albatross illustrated above has developed tubed nostrils to increase their salt discharging capability. Within this group called the Procellariiformes  are some occasionally seen from shore at Halibut Point, like shearwaters, storm-petrels, and fulmars.
 

Great black-backed gull

Comparative physiologist Knut Schmidt-Nielsen studied the functions of salt glands in great black-backed gulls. In one experiment, a gull ingested about 1/10 of its body mass in seawater (the equivalent of a 150-pound human drinking about 2 gallons of seawater, more than a lethal amount). After three hours, the bird had totally eliminated the salt load, mostly via excretions from its salt glands (which were 10 times higher than salt elimination from its kidneys). ²

 

Razorbill

With this remarkable adaptation oceanic birds have been able to establish themselves far out to sea for most of their lifetime.

* Osmoregulation is the maintenance by an organism of an internal balance between water and dissolved materials regardless of environmental conditions. In many marine organisms osmosis (the passage of solvent through a semipermeable membrane) occurs without any need for regulatory mechanisms because the cells have the same osmotic pressure as the sea. Other organisms [including vertebrates such as mammals and birds], however, must actively take on, conserve, or excrete water or salts in order to maintain their internal water-mineral content.

Courtesy of Britannica.com
 

Sources

1. Knut Schmidt-Nielsen, "The Salt-Secreting Gland of Marine Birds,"  Journal of the American Heart Association, May 1960.

2. "Why Can Some Birds Drink Salty Seawater?" Living Bird magazine, Summer 2017.