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This article was originally published as a chapter in the book “Design and Catastrophe: 51 Scientists Explore Evidence in Nature"
In the study of animal physiology, we often seek to answer two central questions about how animals function: (1) what is the mechanism by which modern-day animals carry out their functions, and (2) why do modern-day animals possess the mechanisms they do? In this essay I describe how most biologists view the mechanism and origin of water-salt physiology of fishes and offer an alternative hypothesis to explain the same processes.
In terms of their water-salt physiology, aquatic animals confront challenges in maintaining a constant water and salt content of their bodies when they inhabit a range of environments on Earth—salt water, fresh water, salt lakes, glacial ponds, estuaries, etc. The body fluids of freshwater animals are more concentrated than the water in which they live. For example, in freshwater teleost (bony) fish, blood osmotic pressure averages about 300 milliosmolar (mOsm) higher than the osmotic pressure of freshwater, which is about 0.5–10 mOsm.[1] These fishes tend to gain water by osmosis and lose ions by diffusion, especially across permeable gill membranes, and in their urine. These passive fluxes of water and ions tend to dilute their body fluids. To void excess water, the kidneys of freshwater animals produce copious amounts of urine. For example, a goldfish might excrete urine equivalent to one-third of its body weight per day. The daily osmotic water influx of a goldfish will also equal one-third of its body weight. To replace lost ions, freshwater animals take up ions such as Na+, Cl-, and Ca2+ through active transport from their ambient water and food uptake.
On the other hand, the body fluids of marine fishes are far more diluted than the seawater in which they live. Such fish have blood osmotic pressure of approximately 300–500 mOsm, whereas the osmotic pressure of seawater is approximately 1,000 mOsm. Therefore, these fishes tend to lose water by osmosis and gain ions by diffusion. To replace water, they drink; however, to absorb H2O from the seawater in their gut, they must actively take up NaCl, increasing their problem of salt loading. Their kidneys make urine that is approximately isosmotic to their blood plasma but is rich in divalent ions, and monovalent ions are excreted across the gills.[2]
These differences in environmental salinity require different designs in gill and kidney function.[3] For example, freshwater teleosts pump divalent cations out of the filtrate, while saltwater teleosts pump them in. The distal segment and collecting tubule are permeable in saltwater fish but impermeable in freshwater fish.
Most biologists describe these water-salt relations in fishes as an evolutionary vestige: their body fluids evolved as they adapted to their origins in the oceans, their later invasion to fresh waters, and re-invasion of the oceans millions of years ago.
Using the water-salt physiology of modern vertebrates, I offer an alternative interventionist hypothesis to suggest that marine and freshwater fishes were created with different designs for differing water salinities. Some species are designed to be able to adjust their gill and kidney function to enable them to move between environments with radically different salinities.
Anadromous fishes hatch in fresh water, migrate to marine water where they mature, and then return to fresh water to spawn. Depending on species, anadromous fishes spend one to several years at sea feeding and growing and then return to their natal stream where they breed. Salmon are an excellent example. These fishes are said to be euryhaline; they tolerate wide swings in salinity as they migrate between fresh and salt water. When these fishes swim into fresh water, the major physiological challenge is coping with salt loss across the gills. As euryhaline fishes pass part of their lives in fresh water and part in salt water, a period of adjustment usually involving several weeks (not millions of years) in brackish water is often required to allow acclimation. In such fishes, less gill activity occurs at isotonic environments than at low and high salinities.
Catadromous fishes, on the other hand, migrate in the opposite direction, from seawater to fresh water. They mature in streams and migrate to the ocean to breed. European and American eels are examples. Again, these fishes often require acclimation over a few weeks (not millions of years) to adjust.
Anadromous and catadromous fishes illustrate how the interventionist hypothesis of creation of fishes, living in streams and seas, and a subsequent global flood explains the data better than the hypothesis of gradual evolution. The interventionist model proposes that a design was already in place to permit them to survive in a variety of environments, because they could adapt to rapid changes in salinity, including those that may have occurred during the Flood.
If we believe that fish were created, they could have been designed with the genetic information to adapt to changing water conditions, possibly a requirement necessary to fulfill the mandate of filling the waters of the seas (Gen. 1:21–22). On the contrary, random processes seem utterly incapable of producing the molecular structures responsible for gill and kidney function in waters of differing salinity. I conclude that the manner in which fish regulate their water balance and internal ion concentrations points to an intelligent design.
NOTES
[1] DH Evans. Teleost fish osmoregulation: what we have learned since August Krogh, Homer Smith, and Ancet Keys. American Journal of Physiology–Regulatory, Integrative and Comparative Physiology 2008; 295:R704–R713.
[2] RW Hill, GA Wyse, M Anderson. Animal physiology. 4th ed. Sunderland (MA): Sinauer; 2016, pp. 741–753.
[3] AP Salati, A Baghbazadeh, M Soltani, R Peyghan, G Riazi. Effect of different levels of salinity on gill and kidney function in common carp Cyprinus carpio (Pisces: Cyprinidae). Italian Journal of Zoology 2011; 78(3):298–303.
Noble Donkor is the vice president for academic administration and a professor of biology at Burman University. He has a PhD in Wildlife Ecology and Management from the University of Alberta. He has maintained an active research program in the ecology and physiology of ungulates. His teaching of ecology, vertebrate biology, biogeography, and animal physiology has heightened his interest in the mechanisms and design of animal function.