Diving Physiology in Sea Turtles

A Green Sea Turtle Swims Effortlessly

   Recently, I was swimming in front of Hulihee’s Palace.  About twenty feet below me, a green sea turtle swam out from beneath a coral head and made his way to the surface.  Once there, he gulped air four times over about two minutes.  Following this relatively small bit  of breathing (during the same two minutes I breathed in and out no fewer than 20 times) he headed back down in his leisurely sea turtle fashion.

   This got me to thinking about sea turtles and their ability to stay submerged for long periods.  We, who are swimming on the surface and enjoying a regular supply of good old uncle oxygen, take for granted the relaxed beauty of a sea turtle as he swims nearby through the clear water, not on the surface taking a breath every few strokes.  So independent of breathing do turtles appear, that it is difficult to remember that these are not fish; turtles are air breathing vertebrates.  And so the question arises:  What is their secret?   How is it that a green sea turtle can dive 100 meters and stay submerged for three hours?

After Taking a few Breaths, the Turtle Dives

    As an anesthesiologist, I needed to know a fair amount about respiratory physiology.  This subject is not just lung volumes, but the characteristics of hemoglobin and acid base status.  As it turns out, sea turtles have lungs that are a bit more developed than most other reptiles, but not the equal of the fine alveolar system that we mammals possess.  And their hemoglobin is not that different from ours.  What turtles do better than mammals, is manage their carbon dioxide and pH and, most amazingly, tolerate incredible cerebral hypoxia.

    If you are denied a breath, your pO2 will fall to life threatening levels in 5 minutes.  Our brains are not terribly tolerant of a pO2 below 70 mm Hg.  At that point the neurons (brain cells) die. 

     On the other side of the equation, if you are able to provide pulmonary oxygen but keep the patient apneic (which, in a laboratory, is not quite as big a trick as you might think) the pCO2 will rise about 6 torr per minute.  For every 10 torr rise in PCO2, your pH drops by 0.1.  Mammalian life becomes difficult below a pH of 7.1.  
    

A Turtle Swims By Out On Paul Allen's Reef

   All of that may seem more than a little pedantic, but these are simple facts of mammalian respiratory physiology.  Here are some of the corresponding facts about sea turtle diving physiology.  In an unrestricted dive, loggerhead sea turtle pO2 dropped from 112 to 4 after only 25 minutes.  In that same period, the pCO2 increased only about 10 torr (to about 50 torr) and pH did not decrease by more than .1 to above 7.2.  The numbers associated with carbon dioxide and acid base status speak to an amazing buffering ability in the sea turtle blood.  (an incredible amount of carbon dioxide is converted to some other substance like bicarb).  Isn’t it amazing that a turtle can completely recharge this buffering system in a few breaths?  Or to look at it another way, if you were to carry around all the CO2 you discharge over three hours, you would need a 15 gallon bag (assuming that only 4% of your exhaled gas is carbon dioxide...a percent achieved at minimal exertion.)

A Green Sea Turtle in the Crystal Water of Ho'okena

Before moving on, it is important to note that researchers make a great deal of the unrestricted dive.  Not a lot of information on the biochemistry of sea turtle diving has been accumulated, because it is hard to accumulate data in a truly unrestricted turtle.  Sea turtles are remarkably adapted for diving, with their front legs transformed into hydrofoils and the sleek slope of their carapace.  But more than the physical characteristics, note the  leisurely, oxygen conserving manner in which they swim, which also produces far less carbon dioxide.  Though it seems so nonchalant, the unrestricted dive is critical to the turtle.  When in a tether, (a situation in which monitors may be more easily applied) the turtle struggles, oxygen consumption and the  PCO2 rise and the pH falls.  A struggling, or actively swimming, turtle needs to breath more often. 
    
    Most incredible to me, sea turtles tolerate an unbelievable amount of cerebral hypoxia.  Other turtles have an increased tolerance for brain hypoxia, but no other vertebrate comes close to the tolerance that sea turtles exhibit.  Intracerebral ATP (the biochemical battery of life) is preserved in the face of remarkably low arterial oxygen content.  There are articles on calcium and adenosine metabolism and the ability of the sea turtle to reduce its cerebral metabolic rate.  Medical researchers are very interested in how sea turtles defy the ravages of cerebral hypoxia and study these phenomenon in hopes that it might provide therapy for victims of

Protected Sea Turtles in Kona Spend More Time on the Beach

stroke and other brain injury.
 
     Here in Kona, we get used to observing green sea turtles at the surface.  It is my contention that us hau’olis have created a safe environment where the turtles can live at the surface or on the shore without danger of predation.  In the rest of the watery world, sea turtles spend no more than 6% of their time at the surface.  So when you are snorkeling out by the palace or at City of Refuge and you see a turtle swim out from a resting place near the bottom, just think a bit about what an amazing animal you are observing.   If you are lucky, you may see the turtle compensate for hours of underwater apnea with a few gulped breaths.