Tricks of the Trade

 

How is it that diving mammals so much better adapted to deep-sea diving than we are?

The answer to this question lies in both in the differences in shape between humans and specialised divers, but also the in internal differences in physiology.

External Adaptations

All the best divers are very streamlined, almost spindle-shaped. In fact, the shape of a penguin is so nearly resistance-free that it creates as close to the lowest amount of drag that it is possible to create. On top of the spindle-shape, diving mammals have little hair, or very short hair, and the feathers of diving birds are modified in such a way that they glide nearly friction-free through the water.

The hind limbs of aquatic animals are either shortened or do not exist at all, and what would be forelimbs on terrestrial mammals are modified to become flippers or fins. Diving birds use their wings as flippers.

 

A gentoo penguin underwater - its streamline shape is adapted to reduce friction underater ("drag") to a minumum, therefore conserving energy during diving.

Internal Adaptations

Many of the most important adaptations have taken place internally and are related to the way diving mammals distribute and store oxygen. One modification that all deep divers have in common, and is perhaps the hallmark characteristic that sets them apart from all land forms, is the distribution and concentration of the protein MYOGLOBIN.

Myoglobin is found mainly in muscle, and its main function is to bind with oxygen. In some diving mammals, most notably those which feed whilst submerged, concentrations of muscular myoglobin can range from 3-10 times as high as those found in terrestrial mammals. This suggests that these animals store more oxygen than non-divers and this relates to their ability to dive…

However it is not only differences in myoglobin concentrations which enable divers to exploit the deep blue… there are a vast number of physiological adaptations which work in unison enabling divers to forage for prey where terrestrial animals cannot. These are the “tricks of the trade” for deep-sea divers and have been grouped here as:

  • cardiac adaptations
  • muscle physiology
  • oxygen stores
  • respiratory adaptations.

Cardiac adaptations

The cardiac response to diving has been of great interest to researchers over the years and a number of studies have been conducted investigating bradycardia, consequent reduction in cardiac output and peripheral vasoconstriction. One such study on the cardiac response of grey seals by Thompson and Fedak (1992) illustrated the extent and effect of bradycardia in diving mammals. In grey seals, the heart rate measured at the surface was high, rhythmic and related to size. As the seals began to dive, heart rate dropped and bradycardia was maintained throughout the dive regardless of any changes in swim speed. While diving, heart beat was arrhythmic and there were long pauses between beats. Pauses were often followed by two or more beats in close succession. The number of beats per dive increased with dive duration until it reached 7 minutes. All dives of 7 minutes or longer averaged a total of 220 beats, as recorded in the study.

The specific regulatory mechanism of bradycardia remains unknown. It could be due to conscious control, but it is more likely to be regulated by the seal’s activity pattern or by blood deoxygenenation levels (Thompson and Fedak 1992).

Tachycardia is exhibited prior to surfacing upon ascent in many diving species. It has been notably measured in phocid seals. It has been suggested that its purpose is to increase circulation to oxygen depleted skeletal muscles and organs in order to remove waste products such as carbon dioxide from the blood. The consequence is faster oxygen uptake at the surface and a decrease in time required for recovery between dives (Thompson and Fedak, 1992). Some researchers have suggested that tachycardia could be triggered by pressure changes upon ascent, for example in Weddell seals (Kooyman et al., 1980)

One team of researchers investigated the effects of using drugs to prevent both bradycardia and tachycardia in harbour seals during diving bouts (Elliot et al., 2002). The treatments did not affect percentage dive times, suggesting that cardiovascular dive response is not a requirement for short dives. It was hypothesised that the response was an adaptation utilised to conserve oxygen during longer dives in extreme situations.

Blood in diving species such as the elephant seal also shows adaptations for extended diving: the ability to carry more haemoglobin and, larger red blood cells and is less viscous than the blood of terrestrial mammals. (Wickham et al. 1989)

The figure illustrates heart rates of Weddell seals in response to different durations of dives. Figures A and D display heart rate for the beginning and end of a 3.3 min shallow dive respectively. The arrows show the points when the seal both submersed and surfaced. Figure E shows a northern elephant seal "dived and compressed to 4 atm absolute in a compression chamber". The black bar marks the start of decompression (the releasing of pressure). Bradycardia is clearly seen post-submersion, and tachycardia prior to surfacing. Source: adapted from Kooyman and Campbell, 1972.

Muscle Physiology

The need for conservative use of oxygen during dives has led to certain adaptations of the muscles in diving species to hypoxia. Animals which live at high altitudes on land have had to adapte to similar hypoxic conditions. However, terrestrial mammals have adapted by increasing cardiac output and ventilation, whereas divers have to deal with a fixed oxygen supply and therefore adaptations differ accordingly.

A study by Reed et al. (1994) found that phocid seals relied more heavily than shallower divers (such as otariid seals) on fatty acids as a fuel source. This was similar to the fuel-requirements of high-altitude terrestrials such as the llama and alpaca, although phocids relied more heavily on fatty acids as a source of energy. Fatty acids release more energy in the form of adenosine triphosphate (ATP) than carbohydrates. So it is logical that an animal looking to conserve oxygen would rely heavily on fatty acids as an energy source.

Myoglobin levels were also found to be greater in divers than in terrestrial animals. Myoglobin binds oxygen reversibly, drawing oxygen away from the blood since it has a greater affinity for oxygen than haemoglobin at certain pressures (see graph below).

This graph illustrates that at the same partial pressure of oxygen (take this to mean 'concentration' of oxygen, on the x axis) myoglobin has a greater level of saturation than haemoglobin. Source: http://www.cliffsnotes.com/study_guide/Hemoglobin-and-Myoglobin.topicArticleId-24998,articleId-24965.html

Myoglobin, therefore, acts as a temporary oxygen store and allows divers to continue aerobic respiration whilst diving. The heart and the brain, which require oxygen to function, are able to utilise the stores of oxygen maintained by the myoglobin. Myoglobin concentrations in skeletal muscles of harbour seals have been measured at 10 times the level of muscles in similar-sized dogs (Kanatous et al. 2001). In fact, elevated myoglobin concentration is a common characteristic of all diving mammals and birds. Total myoglobin stores in the body increase as diving abilities increases (Kooyman and Ponganis 1998b).

Oxygen Stores

One of the reasons deep divers are able to reach such astounding depths and remain submerged for longs periods is their ability to maintain aerobic respiration for extended durations. One way this is accomplished is by utilising an enlarged spleen which act as reservoirs of red blood cells. In 1992, Ponganis and his coleagues found that the Weddell seal spleen contained many smooth muscle cells which were densely innervated and controlled by the sympathetic nervous system. Contraction of the spleen resulted in rapid release of red blood cells into the circulatory system. A year earlier, a study by Thornton et al. (2001) on the volume of the spleen during the dives of the northern elephant seal showed that contractions caused the spleen to be 1/5 of its resting volume after only 3 minutes of the dive, after which the size of the spleen remained constant.

This graph shows the recorded spleen volumes of northern elephant seals while they were resting (min 0) and diving (min 1-7). After 3 minutes, volume does not decrease significantly. Source: adapted from Thornton et al. (2001).

However, haematocrit levels do not peak until between 15 to 25 minutes after contraction of the spleen. This may appear to be illogical, but it can be explained by the role of the hepatic sinus. It is hypothesised that the hepatic sinus, which acts as a venous reservoir, could delay the release of red blood cells and could consequently prevent the blood from becoming too thick under certain conditions. As a result, as Thornton wrote, “the elegant system of storage, transfer, and metering of red blood cells provided by the spleen/sinus interaction” increases heamatocrit levels during low-oxygen conditions (e.g. an extended dive) but reduces haematocrit levels when oxygen is readily available in order to reduce the risk of blood becoming too thick and clotting.

Respiratory adaptations

For diving mammals, lungs have the potential to be problematic because they are more efficient stores of nitrogen than oxygen. As a result, marine mammals have evolved the most structurally modified lungs of any mammalian group (Kooyman & Ponganis 1998b). For more information, see the page on this website entitled: “The Bends

In order to prevent decompression sickness and nitrogen narcosis, mechanisms have evolved to isolate the lung gases in areas of the respiratory system where gaseous exchange does not take place. Airways of phocid seals are designed for graded collapse during diving. The airways are more rigid than those of terrestrial mammals, strengthened by connective tissue and smooth muscle, enabling gas from the alveoli to be trapped in the rigid airways, thus preventing gas exchange during submersion (Kooyman & Sinnet 1982).

**NEXT PAGE** The Bends: dangers of deep-sea diving

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