Physiology of underwater diving
[2] 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.
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.
[7] 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.
[14] 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.
[16] During deep dives, any remaining air in their bodies is stored in the bronchioles and trachea, which prevents them from experiencing decompression sickness, oxygen toxicity and nitrogen narcosis.
These tissue masses, which contain extensive contorted spirals of arteries and thin-walled veins, act as blood reservoirs that increase oxygen stores for use during diving.
The nitrogen loads may still build up to some extent over several consecutive dives, but this is greatly reduced in comparison with a human diver continuously breathing under pressure.
The vasoconstriction causes a large increase in resistance to flow and is compensated by a proportional reduction of heart rate to maintain a suitable blood pressure sufficient to provide the reduced circulation.
[3] The heart rate in seals may drop as low as 4 to 6 beats per minute to balance central arterial blood pressure with the large increase in peripheral vascular resistance.
The studies show that most major organs, including kidneys, liver, gut, skeletal muscle, and heart, have severely reduced circulation, while the brain gets most of the residual blood supply.
Otariids have proportionately much larger foreflippers and pectoral muscles than phocids and have the ability to turn their hind limbs forward and walk on all fours, making them far more manoeuvrable on land.
Tagging studies by Hooker and Baird, (1999) show that the northern bottlenose whale, Hyperoodon ampullatus, is capable of diving to depths more than 1500 m with durations of over an hour.
When swimming, baleen whales use their forelimb flippers in a wing-like manner similar to penguins and sea turtles for locomotion and steering, while using their tail fluke to propel themselves forward through repeated vertical motion.
[50]: 1140 Because of their great size, right whales are not flexible or agile like dolphins, and none can move their neck because of the fused cervical vertebrae; this sacrifices speed for stability in the water.
Rorquals need to build speed to feed, and have several adaptions for reducing drag, including a streamlined body; a small dorsal fin, relative to its size; and lack of external ears or hair.
[52][53] While feeding, the rorqual mouth expands by stretching the throat pleats to a volume that can be bigger than the resting whale itself;[54] The mandible is connected to the skull by dense fibres and cartilage (fibrocartilage), allowing the jaw to swing open at almost a 90° angle.
[64]: 131–146 Baleen whale have been observed seeking out highly specific areas within the local environment in order to forage at the highest density prey aggregations.
The Sirenia currently comprise the families Dugongidae (the dugong and, historically, Steller's sea cow) and Trichechidae (manatees) with a total of four species.
They have among the densest bones in the animal kingdom, which may be used as ballast, counteracting the buoyancy effect of their blubber and help keep sirenians suspended slightly below the water's surface.
The lungs of sirenians are unlobed;[77] they, along with the diaphragm, extend the entire length of the vertebral column, which help them control their buoyancy and reduce tipping in the water.
Perfusion of organs during bradycardia and peripheral vasoconstriction in forced submersions of ducks has shown similar findings to seals, confirming redistribution of blood flow to essentially the brain, heart, and adrenal glands.
Birds that dive deeper tend to trap less air in the plumage, reducing their potential buoyancy, but this also represents a loss of thermal insulation, which can be compensated by subcutaneous fat, which increases body mass and thereby the energy cost of the flight.
Penguins avoid this problem by having lost the power of flight, and are the densest of birds, with solid bones, short, closely packed feathers, and a substantial layer of subcutaneous fat, reducing diving effort expended against buoyancy.
After the mass extinction at the end of the Cretaceous period, marine reptiles were less numerous, but there was still a high variety of species in the early Cenozoic, such as "true" sea turtles, bothremydids,[100] palaeophiid snakes, a few choristoderes such as Simoedosaurus and dyrosaurid crocodylomorphs.
The heads of the humeri and femora of many fossils show necrosis of the bone tissue, caused by nitrogen bubble formation due to a too rapid ascent after deep diving.
However, this does not provide sufficient information to deduce a depth with any accuracy, as the damage could have been caused by a few very deep dives, or by a large number of relatively shallow exposures.
The vertebrae show no such damage: they may have been protected by a superior blood supply, made possible by the arteries entering the bone through the two foramina subcentralia, large openings in their undersides.
The relatively low body temperature is conjectured to help reduce the risk of bubble formation by providing a higher solubility of nitrogen in the blood.
[115] Okuyama et al. (2014) found that green turtles Chelonia mydas maximised their submerged time, but changed their dive strategy depending on whether they were resting or foraging.
Termination of a shallow dive relatively early if no food is encountered could be energy efficient over long periods for animals that habitually spend more time submerged and only surface briefly to exchange gas, which is the case with turtles.
