The physiology of underwater diving is the physiological adaptations to diving of air-breathing vertebrates that have returned to the ocean from terrestrial lineages. They are a diverse group that include sea snakes, sea turtles, the marine iguana, saltwater crocodiles, penguins, pinnipeds, cetaceans, sea otters, manatees and dugongs. All known diving vertebrates dive to feed, and the extent of the diving in terms of depth and duration are influenced by feeding strategies, but also, in some cases, with predator avoidance. Diving behaviour is inextricably linked with the physiological adaptations for diving and often the behaviour leads to an investigation of the physiology that makes the behaviour possible, so they are considered together where possible. Most diving vertebrates make relatively short shallow dives. Sea snakes, crocodiles, and marine iguanas only dive in inshore waters and seldom dive deeper than . Some of these groups can make much deeper and longer dives. Emperor penguins regularly dive to depths of for 4 to 5 minutes, often dive for 8 to 12 minutes, and have a maximum endurance of about 22 minutes. Elephant seals stay at sea for between 2 and 8 months and dive continuously, spending 90% of their time underwater and averaging 20 minutes per dive with less than 3 minutes at the surface between dives. Their maximum dive duration is about 2 hours and they routinely feed at depths between , though they can exceed depths of . Beaked whales have been found to routinely dive to forage at depths between , and remain submerged for about 50 minutes. Their maximum recorded depth is , and the maximum duration is 85 minutes.
Air-breathing marine vertebrates that dive to feed must deal with the effects of pressure at depth, hypoxia during apnea, and the need to find and capture their food. Adaptations to diving can be associated with these three requirements. Adaptations to pressure must deal with the mechanical effects of pressure on gas-filled cavities, solubility changes of gases under pressure, and possible direct effects of pressure on the metabolism, while adaptations to breath-hold capacity include modifications to metabolism, perfusion, carbon dioxide tolerance, and oxygen storage capacity. Adaptations to find and capture food vary depending on the food, but deep-diving generally involves operating in a dark environment.
Diving vertebrates have increased the amount of oxygen stored in their internal tissues. This oxygen store has three components; oxygen contained in the air in the lungs, oxygen stored by haemoglobin in the blood, and by myoglobin, in muscle tissue, The muscle and blood of diving vertebrates have greater concentrations of haemoglobin and myoglobin than terrestrial animals. Myoglobin concentration in locomotor muscles of diving vertebrates is up to 30 times more than in terrestrial relatives. Haemoglobin is increased by both a relatively larger amount of blood and a larger proportion of red blood cells in the blood compared with terrestrial animals. The highest values are found in the mammals which dive deepest and longest.
Body size is a factor in diving ability. A larger body mass correlates to a relatively lower metabolic rate, while oxygen storage is directly proportional to body mass, so larger animals should be able to dive for longer, all other things being equal. Swimming efficiency also affects diving ability, as low drag and high propulsive efficiency requires less energy for the same dive. Burst and glide locomotion is also often used to minimise energy consumption, and may involve using positive or negative buoyancy to power part of the ascent or descent.
The responses seen in seals diving freely at sea are physiologically the same as those seen during forced dives in the laboratory. They are not specific to immersion in water, but are protective mechanisms against asphyxia which are common to all mammals but more effective and developed in seals. The extent to which these responses are expressed depends greatly on the seal's anticipation of dive duration. The regulation of bradycardia and vasoconstriction of the dive response in both mammals and diving ducks can be triggered by facial immersion, wetting of the nostrils and glottis, or stimulation of trigeminal and glossopharyngeal nerves. Animals cannot convert fats to glucose, and in many diving animals, carbohydrates are not readily available from the diet, nor stored in large quantities, so as they are essential for anaerobic metabolism, they could be a limiting factor.
Decompression sickness (DCS) is a disease associated with metabolically inert gas uptake at pressure, and its subsequent release into the tissues in the form of bubbles. Marine mammals were thought to be relatively immune to DCS due to anatomical, physiological and behavioural adaptations that reduce tissue loading with dissolved nitrogen during dives, but observations show that gas bubbles may form, and tissue injury may occur under certain circumstances. Decompression modelelling using measured dive profiles predict the possibility of high blood and tissue nitrogen tensions.
Range of performance
Strong allometric relationships have been observed between body mass and maximum diving depths and durations in marine mammals, birds and marine turtles. Exceptions can be seen in baleen whales, eared seals and flighted diving birds, which showed no correlation between size and maximum diving depth. Penguins showed the highest correlations of diving capacity with body mass, followed by toothed whales and true seals. Baleen whales show a correlation between mass and maximum dive duration. Comparisons between the groups show that alcids, penguins, and true seals are exceptional divers relative to their masses and that baleen whales dive to shallower depths and for shorter durations than would be expected from their size. Many of these variations can be explained by adaptation to ecological feeding niches exploited by the animals.
+ Oxygen stores, routine and maximum depths and durations for some diving vertebrates Species Bodymass(kg) Totalstore(ml/kg) %Lungs %Blood %Muscle Routinedepth(m) Maximumdepth(m) Routineduration(min) Maximumduration(min) Human 70 20 24 57 15 5 133 0.25 6 Weddell seal 400 87 5 66 29 200 700 15 93 Elephant seal 400 97 4 71 25 500 1500 25 120 California sealion 100 40 21 45 34 40 275 2.5 10 Bottlenose dolphin 200 36 34 27 39 - 535 - - Sperm whale 10000 77 10 58 34 500 2035 40 75 Leatherback turtle - - - - - - 1280 3-8 >70
Marine mammals
Marine mammals adaptation to deep and long duration breath-hold diving involves more efficient use of lungs that are proportionately smaller than those of terrestrial animals of similar size. The adaptations to the lungs allow more efficient extraction of oxygen from inhaled air, and a higher exchange rate of air of up to 90% of each breath. Their blood chemistry extracts more oxygen and faster due to high red blood cell count, and high concentrations of myoglobin in the muscles stores more oxygen for availability during a dive. They also have a relatively high tolerance to carbon dioxide which builds up during breath-hold, and lactic acid, produced by anaerobic muscle work. The lungs and ribs are collapsible, allowing them to collapse without damage under the pressure of great depths They do not have air-filled sinuses in the facial bones.
There are differences in the diving strategies of marine mammals, which vary both with depth range and taxonomic grouping. Some of these remain inadequately explained. Some, such as Cuvier's beaked whale, routinely exceed their aerobic dive limit, making a relatively long recovery period necessary, while others, like elephant seals, appear to require very little recovery time between dives to similar depth, indicating that they tend to remain within their aerobic limits on almost all dives.
Many aquatic mammals such as seals and whales dive after full or partial exhalation, which would reduce the amount of nitrogen available to saturate the tissues by 80 to 90%. Aquatic mammals are also less sensitive to low alveolar oxygen concentrations and high carbon dioxide concentrations than purely terrestrial mammals. Seals, whales and porpoises have slower respiratory rates and larger tidal volume to total lung capacity ratio than land animals which gives them a large exchange of gas during each breath and compensates for low respiratory rate. This allows greater utilisation of available oxygen and reduced energy expenditure. In seals, bradycardia of the diving reflex reduces heart rate to about 10% of the resting level at the start of a dive.
Deep diving mammals do not rely on increased lung volume to increase oxygen stores. The whales with long and deep diving capabilities have relatively small lung volumes which collapse during the dive, and seals dive following partial exhalation with a similar effect. Short duration diving mammals have lung volumes similar to their terrestrial equivalents and dive with full lungs, using the contents as an oxygen store. The oxygen affinity of the blood is related to lung volume. Where the lungs do not represent an oxygen store, the oxygen affinity is low to maximise the unloading of oxygen and to maintain a high tissue oxygen tension. Where the lungs are utilised as an oxygen store, the affinity is high and maximises uptake of oxygen from the alveolar volume.
Adaptation of oxygen storage capacity of blood and muscle in diving mammals is an important factor in their diving endurance, and ranges from roughly equivalent to terrestrial mammals to nearly ten times as much, in proportion to the duration of dives and the metabolic demand during dives.
Swimming adaptations of drag reduction by hydrodynamically streamlined body forms and efficient swimming actions and appendages reduce the amount of energy expended in the diving, hunting and surfacing activity.
Heat loss is controlled by reducing the surface to volume ratio, and thick insulating layers of blubber and/or fur, which also help with streamlining for reduced drag. Exposed areas with relatively high circulation may use a rete mirabile counterflow heat exchange system of blood vessels to reduce heat loss.
Marine mammals use sound to communicate underwater, and many species use echolocation to navigate and locate prey. Pinnipeds and fissipeds have facial whiskers capable of locating prey by detecting vibrations in the water.
The hearts of marine mammals are typical of mammals. The heart is a slightly lower percentage of body mass in the great whales compared to pinnipeds and smaller cetaceans. Chamber size, stroke volume, resting cardiac output and heart rate are also in the general mammalian range, but the hearts of diving mammals are dorso-ventrally flattened, with enlarged right ventricular chambers, and, in some species, the thickness of the right ventricular wall may be increased. The dorso-ventral flattening prevents chest compression from compromising filling, and thicker walls may compensate for the effects of increased vascular resistance and lung collapse during chest compression.
Dense innervation of arteries in seals by sympathetic nerves may be part of a system for maintaining vasoconstriction of the dive response independent of local metabolite induced vasodilation. Venous capacitance is highly developed, especially in phocid seals and whales, and includes a large hepatic sinus and posterior vena cava, and is thought to be related to the large blood volume of the animals. The relatively large spleen also injects extremely high hematocrit blood into the hepatic sinus during dives and is a significant storage organ for red blood cells.