Though less salty than the ocean, we too have a sea within us. Our blood offers a clue to our marine past because the fluids in our bodies mimic the primeval seas in which life began. Without salt, our nerves would misfire, our muscles would cramp, and our blood would thicken. Most of the ocean’s bony fish evolved from fresh water fish (fresh water still carries a bit of salt), which actively internally regulate how much salt they hold onto and how much they excrete. When some of these fresh water fish migrated to the sea, they brought with them the ability to regulate their internal salinity, a critical consideration because seawater is saltier than the fluids in most bony fish. The problem is that while human skin is relatively watertight, the skin of bony fish is leaky. Being porous and surrounded by much saltier water means that the higher salt concentration outside wants to diffuse into the lower salt environment inside the fish.
Staying hydrated when the only available water is salty is a chore because the only option is to, well, “drink like a fish,” which they do. To get rid of excess salt accompanying the water, as well as salt passively entering through the pores, demands an efficient desalination mechanism, which they have. It attacks the salt from two angles, one being the kidneys, which separate out the fresh water to be used for bodily functions, and two, when the excess salt is sent to specialized chloride cells in the gills, which secrete it directly to the outside. Though a labor-intensive strategy, these fish can adjust their fluids from becoming too diluted or too concentrated. Being able to accommodate shifts in surrounding salinity is a definite perk for individual survival and for overall perpetuation of the species. Human kidneys perform the same seawater desalination process but without chloride cells and gills, more fresh water is required to excrete the excess salt taken in with the seawater in the first place. As a result, for us, drinking seawater promotes rather than alleviates dehydration. And for the fish, any urination taking place is minimal.
Many of the most primitive fish (having skeletons of cartilage, not bone) including marine sharks, skates and rays don't lose water like most bony fish but not because their skin isn’t leaky and not because their inside fluids are as salty as seawater (no high-low salt gradient issues). These fish instead thwart the outside salt from entering by stocking their internal fluids with a surfeit of urea and trimethylamine oxide (TMAO). The urea concentrations retained would kill most other vertebrates but by harboring even higher concentrations of detoxifying TMAO, the urea is neutralized. Jointly, these two metabolic wastes operate like a “salt equivalent,” blocking seawater salt from entering through the pores but allowing in the “now-filtered” fresh water. In fact, these fish register a slightly higher salt-equivalent concentration than seawater. Consequently, they are spared both from constantly drinking seawater and constantly eliminating salt (no chloride cells needed). There is a downside to this simplicity. Since these fish are limited to tolerating only a narrow range of salt conditions and ocean salinity changes with temperature, depth, and proximity to shore, in kind species that travel far or deep are less apt to survive sudden environmental changes.
“One man’s poison is another man’s cure” is an adage that holds up over time. For a segment of organisms, the ocean may be “poisoned” but for an ocean of fish, it is the elixir of life.
— Judith Lea Garfield, naturalist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. Send comments to firstname.lastname@example.org.