Supplementary Table 1:
Marine mammal anatomical, biochemical, physiological and behavioural adaptations for diving. (Some
adaptations have been observed, others are hypothesised.)
Adaptations
Species
Description
Source
Anatomical
Flexible chest
All
Highly compressible chest facilitates compression collapse of
alveoli.
[1-3]
Modified lung
structure
facilitating alveolar
collapse
All
Stiffened upper airways and lack of smaller respiratory bronchii
compared to terrestrial mammals. Thin-walled compliant alveoli
collapse under increasing hydrostatic pressure causing a graded
decrease in the amount of respiratory gases absorbed by the blood
stream as the depth of diving increases. Effective gas exchange
between lungs and blood ceases when all alveoli are collapsed.
Reinforced lung structure may also facilitate high ventilation rates
at surface.
[3-5]
Bronchial
sphincters
Cetaceans Presence of a series of bronchial sphincter muscles found in the
terminal segments of the airways. Function is largely unknown, but
is hypothesised to relate to management of lung air.
[3, 6, 7]
Large aortic bulb
Phocids
Elastic and bulbous ascending aorta hypothesised to play a role in
maintaining arterial pressure during the long diastolic intervals of
diving bradycardia. A larger bulb is found in deeper divers. (There
is some suggestion of a similar bulb in mysticete whales.)
[8, 9]
Enlarged spleen
and hepatic sinus
Phocids
The enlarged spleen acts as a reservoir for red blood cells, and is
used in concert with the hepatic sinus to meter blood allowing
temporary increases in hematocrit during periods of apnoea and
diving. The release of red blood cells responds to increases in
epinephrine. The size of the spleen is correlated with diving
capacity, i.e. deep diving phocids have the largest spleens.
[10-13]
Vena caval
sphincters
High brain
capillary density
Retia mirabilia
Pinnipeds, Muscle sphincter located around thoracic caudal vena cava, just
Some
cranial to the diaphragm and large hepatic sinus (phocids).
cetaceans. Innervated by branch of right phrenic nerve. Assumed to be a
mechanism to regulate venous return to the heart.
Phocids
A higher capillary density facilitates increased O2 conductance to
neural tissue.
Cetaceans, In cetaceans, these are a series of vascular networks of densely
Sirenians looped arteries primarily located from base of brain case, along and
within the vertebral column, and retro-pleurally lining the ventral
aspect of the rib arches. It is the only path of arterial blood to the
brain in adult cetaceans. Possible functions of the retia include as a
windkessel for brain blood flow, allowing intrathoracic and
vascular engorgement to mitigate ‘lung squeeze’, or as a filter for
arterial gas emboli preventing DCS (neuroprotective effect).
[14-17]
[18, 19]
[4, 16,
20-22]
Supplementary Table 1 (cont)
Adaptations
Species
Description
Source
Anatomical (cont)
Large epidural
Cetaceans, Extensive epidural venous plexuses in cetaceans and sinuses in
venous sinuses and Phocids phocids may act as a blood store. In cetaceans, the small plexiform
plexuses
structure could help trap gas and/or fat emboli before they occlude
small veins and arteries in the CNS. Juxtaposition to central
nervous system may allow regional heterothermy of brain and
spinal cord.
[14, 17,
23-25]
Smaller lung size
[4, 26]
Elaborate
plexiform veins in
and around air
spaces
DeepThe relatively large lung size in delphinids and phocoenids
diving
indicates a reliance on lung stores for oxygen in relatively shortcetaceans duration shallow diving species. Deep diving species rely on
greater blood and muscle oxygen stores (haemoglobin and
myoglobin) to meet oxygen demands and have therefore retained a
relatively small comparative lung size.
Odontocete
cetaceans
and
phocids
Investment of odontocete pterygoid and peribullar sinuses with
elaborate plexiform veins. More elaborate and voluminous sinus
vasculature in deep divers (e.g. physeterids, kogiids, ziphiids)
compared to shallow-diving delphinids. Phocids and deep diving
odontocetes also have extensive thoracic venous vasculature. As
air volume in sinuses and lung cavities reduces under pressure,
these structures may allow blood volume to replace diminishing air
volume. Possibility of gas/nitrogen exchange at sinuses is
unknown.
[14, 2731]
Peripheral
vasoconstriction
and selective
ischemia
All
A marked reduction in peripheral blood flow and variable reduction
to organs during diving reduces oxygen delivery to tissues with
relatively lower metabolic demands. Variable responses have been
observed during natural diving with the magnitude of
vasoconstriction being dependent on the dive duration.
[32, 33]
Bradycardia
All
Reduction in heart rate driven by circulatory adjustment with heart
rate following vascular resistance to maintain constant blood
pressure. Dramatic reduction in heart rate was initially observed in
forced dive studies, but has also been observed during natural
diving. Heart rates decline as a function of dive duration. Abrupt
bradycardia observed for phocid seals, more gradual bradycardia
for otariids and cetaceans. Modified by conscious control and by
exercise.
[4, 3442]
Hypometabolism
All
The post-submersion rate of O2 consumption appears insufficient to
maintain a normal metabolic rate during submersion. This suggests
a hypometabolic response to diving, which has been verified
experimentally for trained sea lions and grey seals diving
voluntarily.
[43, 44]
Physiological
Regional
heterothermy
Pinnipeds, Regional reduction in body temperature may contribute to
Cetaceans? hypometabolism by reducing the oxygen consumption demands of
cooled tissues, including possibly the brain.
[45-48]
Supplementary Table 1 (cont)
Adaptations
Species
Description
Source
Physiological (cont)
Hypoxic tolerance
All
Hypometabolism and cell protection achieved through co-ordinated
system-level reorganisation involving reductions in oxygen
delivery to organs, partial shutdown of some organs, and complex
reconfiguration at cellular and molecular levels.
[18, 49,
50]
Pulmonary shunt
?
Intrapulmonary arteriovenous pathways linked to exercise and
hypoxia in humans ([51, 52]). Blood bypasses functional alveoli
(i.e., there is ventilation without perfusion) reducing pulmonary gas
exchange.
[53]
Anaerobic
extension of dives
beyond aerobic
dive limit
?
Potential to use anaerobic metabolism to extend dive duration.
Some deep divers repeatedly dive beyond the calculated aerobic
dive limit.
[54-58]
Biochemical
High myoglobin
concentration,
aerobic enzyme
capacities and
mitochondrial
volume density
All
Increased total body oxygen stores provided by higher myoglobin
concentrations. Concentrations highest in areas of muscle which
provide greatest force (and have greatest oxygen requirements).
Increased oxidative capacity provided by higher aerobic enzyme
capacities and mitochondrial densities found in swimming muscles,
which appear to be adapted for aerobic lipid metabolism under
diving hypoxia. The increased capacity is estimated to be as high as
20 times greater than that of terrestrial mammals.
[59-64]
High resident
neural globin
concentration
All
Neural haemoglobins, neuroglobins and cytoglobins facilitate
oxygen transfer into neural tissues, and may protect against reactive
oxygen and nitrogen groups. Increased concentrations are observed
for fast-swimmers and divers relative to terrestrial species.
[65]
Lung surfactant
production
Pinnipeds Increased surfactant production caused by transient
Cetaceans? mechanostimulation (pressure), facilitates reinflation of the
collapsed lung during ascent.
[66, 67]
Hypocoagulable
blood (lacking
several clotting
factors)
Cetaceans Cetaceans may lack a number of clotting factors common to
terrestrial mammals. This may serve several functions, including
improved microcirculation during dive-induced bradycardia. It may
also reduce venous thrombosis, which is thought to be an important
mechanism in human DCS).
[68]
Tolerance to
oxygen free
radicals
Pinnipeds The cycles of regional ischemia and prompt post-dive reperfusion
raise the potential for production of reactive oxygen species (ROS)
and associated oxidative damage. Seals show high tolerance to
ROS via enzyme superoxide dismutase (SOD) formation and large
antioxidant capacity.
[69-72]
Supplementary Table 1 (cont)
Adaptations
Species
Description
Source
Phocids
Phocid seals exhale before diving, reducing the diving lung volume
and facilitating a shallower depth of lung collapse.
[2, 4]
Behavioural
Reduce initial lung
volume (exhale
prior to diving)
Modification of
swimming gait
All
Modifying swimming gait to glide downward or upward during
descent or ascent of dives is possible due to buoyancy changes at
depth. The process of gliding conserves energy and increases the
aerobic dive duration by reducing the need for myogenic
propulsion.
[55, 7376]
Constrained diving
patterns
All
A number of constraints on feasible patterns of diving have been
hypothesized as a means to reduce N2 saturation.
[77-79]
Consecutive dives
crush bubble nuclei
All
It has been hypothesized that rapid compression (descent) may
crush pre-existing gas nuclei and thereby reduce the risk of bubble
formation during decompression.
[80, 81]
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