Breathless

I AM MESMERISED as a banded sea snake slithers to the surface a few metres away from me. Here in this enveloping blueness, we are not such different creatures. We both have a need for air, and our survival depends on ascending to fill our lungs with it.
Our difference is that I make the choice to visit the oceans depths. The sea snake and others like it do not. Almost their entire lives must be spent living in a medium in which they cannot breathe.
With a landlubber’s basic physiology, the sea snake and other air-breathing marine creatures have developed highly specialised mechanisms to enable them to dive longer and deeper.
As divers, we are all too aware of the physiological problems that plague humans during a scuba dive.
The most pressing is our need for oxygen. Dangers such as nitrogen narcosis, hypoxia and the crushing effects of increased pressure, as well as the dreaded bends, are real threats that limit our ability to go deep for anything but short periods. But evolution has equipped a wide range of creatures with mechanisms that seem to solve such problems. Just how do they do it?
I take a dive, and my heart rate automatically slows down – it’s an evolutionary mechanism to conserve oxygen while under water.
But my reptilian dive buddy is able to reduce its metabolic rate in a way in which I never can. After 40 minutes or so, it’s time for me to make my ascent; the sea snake can stay submerged for up to two hours, and with a maximum depth of an astounding 100m.
Sea snakes have a remarkable tolerance for low oxygen levels, as well as enlarged lungs running to the tip of the tail, where they can store a large amount of oxygen relative to their size.
It’s not just a case of breath-holding; sea snakes have special valves covering their nostrils that prevent water entering their lungs. Combine this with the fact that more than 20% of the snake’s oxygen is acquired by absorption through the skin, and it becomes clear just how sea snakes can attain such world-class diving feats.

BUT SEA SNAKES AREN’T THE ONLY reptiles to have evolved such a specialised system of diving equipment. Marine turtles are also champions of deep diving. The leviathan leatherback turtle is able to dive up to an astonishing 1200m.
Like sea snakes, turtles have evolved additional means of harnessing oxygen in addition to their lungs. They are capable of bringing in water through their nostrils, their mouths and, peculiarly, their anal opening, where oxygen is extracted by the pharyngeal lining, acting much as gills do in fish.
Turtles’ blood is able to tolerate a much larger concentration of carbon dioxide than most air-breathers, so over long periods they use their blood’s oxygen supply very efficiently.
Muscle tissue and blood further stores large amounts of oxygen, enabling the animals to remain submerged for longer periods.
This adaptation of the blood, probably the most significant evolutionary characteristic in air-breathers, is not unique to turtles. Our aquatic cousins, the marine mammals, have also evolved mechanisms that greatly increase the amount of oxygen that their blood tissues can carry.
Seals, for example, have more red blood cells taking up oxygen in the tissues of their body than we do.
And they don’t use their lungs in the same way to store oxygen. Studies of seal respiration have shown that there is less oxygen in the lungs of a diving seal than in a human.
If they were to store air in the same way humans do, they too would be at a high risk of the bends, and for an animal that spends its life diving to depths, this would be evolutionary suicide.
Just to prove how efficient this system is, elephant seals have been recorded at depths of 1500m. But this is still nowhere near the diving record.
The most severe effects of depth for human divers are the risks of high- pressure nervous syndrome (HPNS), thoracic and middle ear squeeze, nitrogen narcosis, decompression illness and oxygen toxicity – illnesses that air-breathing marine animals rarely exhibit.
This is down to one simple fact. They all conduct breath-hold dives, so carry limited amounts of air in their lungs during a dive.
We now know that repeated breath-hold dives with short surface intervals, in both humans and marine animals, can result in decompression symptoms.
And recent studies suggest that marine mammals may live permanently with elevated nitrogen. It is, however, our reliance on scuba that creates many of our problems and limits our time below.
Seals, whales and dolphins have reduced the problems caused by nitrogen and oxygen accumulation by collapsing their lungs. Nitrogen and oxygen levels stay low, and the toxic effects of these gases are limited.
During deep and repeat dives they do retain nitrogen, but how they are able to tolerate formation of nitrogen bubbles in the body remains unclear to scientists.
The astounding and record-breaking diving ability of sperm whales is well-documented. How a creature that regularly dives deeper than 1000m, and which has reached an incredible 2500m, while still avoiding HPNS, remains one of nature’s great mysteries.
It is particularly baffling as humans and other primates show nervous system disruption at only 150m. Scientists assume that evolutionary changes in the nervous system have allowed the sperm whale to attain such diving feats.

AS MANY DIVERS CAN CONFIRM, one of the most common pains from a dive comes in the form of squeeze.
And once again nature has provided the perfect solution for air-breathing marine animals, by filling sinuses with blood during dives and preventing the painful and dangerous expansion of air from which so many divers have suffered.
It’s not only reptiles and mammals that have their own in-built, mega-efficient scuba gear. Seabirds long ago caught up with evolutionary adaptations for an aquatic air-breathing existence.
Although spending their entire lives on land, their diving prowess is just as impressive. Using their powerful wings to fly under water, they overcome severe buoyancy problems.
Reaching a depth at which lungs are compressed, they can use less energy and oxygen. It appears that the deeper they go, the less energy they use. Again, it remains unclear just how seabirds are able to dive as they do, especially considering that they have low oxygen storage and high metabolic rates, all conditions that should naturally prevent such diving feats.
Without exception, these remarkable air-breathing marine animals have developed unique mechanisms to exploit the aquatic environment. They hold on to the need for air while maintaining an almost totally aquatic existence.
It’s clear that we still have limited knowledge of the mechanism involved in diving air-breathers.
I can only marvel at the sea snake gliding below me into the darkness as I come up for air. Its one breath will take it to a world more than 100m below me, and as I sit on the balcony two hours after my dive, watching a tropical sunset, it will only just have surfaced for one more breath.