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How Can Fish Live at Different Ocean Depths?

Recent work by scientists from around the world has uncovered the biochemical mechanism that allows fish to thrive at extreme ocean depths. This mechanism exemplifies the splendor of biological designs found throughout nature—designs worthy of a Creator.

“Let the fish in the sea inform you. 
Which of all these does not know that the hand of the Lord has done this?”

Job 12:9b–10

As the old adage goes, “You only get one chance to make a first impression.” And first impressions often last.

One of my first reactions to studying biology and biochemistry was wonder at the elegance and sophistication of living systems. Thirty-three years later, the initial impression that biological and biochemical designs had on me remains unchanged. In fact, it was the beauty and sophistication of biochemical systems that first convinced me that a Creator must be responsible for bringing life into existence.

This was the same reaction I had upon reading about recent research into the biochemical mechanisms that fish employ to live at extreme ocean depths. This work, done by an international group of scientists led by Paul Yancey, exemplifies the splendor of biological designs found throughout nature.1

Where Are All the Fish?

Marine organisms dwell in regions in Earth’s oceans most distinctly characterized by hydrostatic pressure. The deepest areas of the ocean (between 6,000 and 11,000 meters) experience hydrostatic pressures around 1,000 atm (atmospheres). Pressure changes can be quite dramatic, increasing by 10 atm per 100 meters from the ocean’s surface to its depths. Oceanographers have noted that some fish are not found below certain levels. For example, elasmobranchs (sharks, rays, etc.) are not found below 4,000 meters and teleosts (bony fish) can’t live below 8,400 meters. In fact, only two types of bony fish—snailfish and cusk-eels—live below 6,000 meters.

Why Aren’t Bony Fish Found Below 8,400 Meters?

One reason fish can’t occupy extreme depths stems from the effects of pressure on protein structure. High pressures cause the delicate three-dimensional architectures of proteins to unravel, compromising the function of these biomolecules.

Researchers think that small organic molecules found inside cells may counteract the negative effects of pressure. Normally, these small molecules balance the osmotic pressure of salts (such as sodium chloride) in the extracellular fluids bathing the tissue—but they also appear to double as protein stabilizers. One of these organic compounds, trimethylamine N-oxide (TMAO), is known to stabilize protein structure under conditions that would usually cause its three-dimensional configuration to unravel. The research team led by Yancey speculated that if this compound allows fish to tolerate high hydrostatic pressures, then the amount of TMAO found in cells would correlate with the capture depth of the fish.

They discovered that fish residing at relatively shallow regions of Earth’s oceans have a relatively low level of TMAO associated with muscle cells. (Interestingly, in the absence of any perturbant, TMAO actually compromises protein structure and function. Because of this property, it is advantageous for TMAO to exist at low levels when it’s not required to stabilize protein structure.) And when the team analyzed the muscle cell contents for fish that lived at depths below 1,000 meters, they found a striking correlation between TMAO content and capture depth. Interestingly, once bony fish reach a depth of about 8,400 meters, the levels of TMAO in the cells necessary to counteract the harmful effects of pressure on proteins becomes so high that it would compromise the ability of the fish to balance the osmotic pressure of the cell’s interior with the extracellular tissue fluid. The research team concluded that the depth limit for bony fish results from this osmoregulatory constraint.

Elegant Biological Designs

As a biochemist, I find this mechanism to be incredibly sophisticated. In principle, a number of compounds, such as glycine or taurine (both amino acids), could be used within the cell’s interior to balance the osmotic pressure of the extracellular fluids. But none of these alternatives stabilize protein structure at high pressures; this property seems to be unique to TMAO. To say it another way, TMAO displays optimal properties as a protein stabilizer and a regulator of osmotic pressure.

It’s also remarkable that the levels of TMAO required to stabilize proteins at differing ocean depths exactly corresponds to the levels required to maintain osmotic balance with the extracellular fluid. (Fish regulate the osmotic pressure of extracellular fluid with respect to the oceanic osmotic pressure by using their kidneys, which are equipped with special filtering structures called glomeruli.)

The biochemical mechanism that allows fish to thrive at extreme ocean depths is just one of many remarkable biological designs found in nature. The biological realm is replete with elegant, sophisticated systems—just like one would expect if life, indeed, stems from the work of a Creator. And as a believer, who is also a biochemist, I view these types of awe-inspiring systems as a powerful reflection of God’s wisdom.

How many are your works, Lord! 
In wisdom you made them all;
the earth is full of your creatures.

Psalm 104:24

  1. Paul H. Yancey et al., “Marine Fish May Be Biochemically Constrained from Inhabiting the Deepest Ocean Depths,” Proceedings of the National Academy of Sciences, USA 111 (March 25, 2014): 4461–65.