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Marine Mammal Physiology: Requisites for Ocean Living
7.2.2.1 Gastrointestinal tract—The entrance for water
The intestines are critical for promoting net water gain in marine mammals because they
likely meet a significant portion of their water requirements from pre-formed water in their
diets when not fasting (Irving et al. 1935; Smith 1936; Fetcher 1939; Pilson 1970; Depocas
et al. 1971; Ortiz et al. 1978; Hui 1981). Water is, generally, absorbed in the small and large
intestines. The intestine, especially the small intestine, is longer in cetaceans and pinnipeds than those in terrestrial carnivores (Helm 1983; Goodman-Lowe et al. 2001; Williams
et al. 2001). Therefore, elongation of the small intestine may be advantageous for water
absorption. Generally, water is absorbed through cell junction of the absorptive enterocyte
(the paracellular pathway) in terrestrial mammals (Barrett 2005). However, it was reported
recently that the water channel aquaporin-1 (AQP1) is located at the apical membrane of
the absorptive enterocyte in the small intestine of bottlenose dolphin (Tursiops truncatus)
unlike terrestrial mammals (Suzuki 2010). The distribution of AQP1 along the apical membrane potentially contributes to the rapid and effective absorption of water from the lumen
into the body across the cellular membrane (the transcellular pathway). These anatomical
and histological features of the intestine may facilitate the efficient absorption of water.
In addition to the demand to conserve and obtain free water, marine mammals in a
hyperosmotic habitats need to avoid excessive Na + ingestion, either from their prey directly
or indirectly during the act of eating. Cetaceans possess a very muscular sphincter at the
entrance of the esophagus that is well developed and forms a very narrow (3–5 mm) opening leading to the stomach that may function to only allow prey to pass without much
seawater (Harrison et al. 1970). Reidenberg (2007) suggested that the tongue of cetaceans
may function in squeezing water out of the mouth, minimizing ingestion of saltwater.
7.2.2.2 Kidney—The principal regulator
The kidneys are responsible, among other functions, for the regulation of water and salt
balance. Electrolytes, water, and certain metabolic by-products in the plasma are filtered
by the kidneys through a series of processes that constitute selective filtration, secretion,
and excretion. While the processes are consistent throughout mammals, the anatomy of
the kidneys in marine mammals is distinctive from terrestrial mammals. The kidneys
of cetaceans and pinnipeds are reniculated and comprised of hundreds to thousands of
reniculi, which contain divided cortical tissue and a single medullary pyramid, generally
inserted in a single calyx and bundled within the kidney capsule (Ommanney 1932; Bester
1975; Hedges et al. 1979; Vardy and Bryden 1981; Stoskopf and Herbert 1990) (Figure 7.2).
In manatees, the kidney is superficially lobulated (Hill and Reynolds 1989) and is elongated and unlobulated in the dugong (Dugong dugon) (Batrawi 1953, 1957; Marsh et al. 1978).
Similar to cetaceans and pinnipeds, sea otters possess a lobulated kidney and possess
slightly greater urine concentrating ability than river otters (Hoover and Tyler 1986). The
kidneys in marine mammals are relatively larger than in terrestrial mammals (Beuchat
1996) and may be an adaptation for a deep-diving lifestyle and/or a direct consequence of
an enlargement of their body mass, independent of any osmoregulatory burden (Pfeiffer
1997; Maluf and Gassman 1998; Ortiz 2001).
The kidneys consist of a cortex and medulla. Plasma is filtered in the glomeruli in the
cortex, and the original urine flows to the proximal tube in the cortex, where electrolytes
and organic nutrients including urea are reabsorbed. Subsequently, the urine enters the
descending thin loop of Henle in the medulla, where necessary molecules are absorbed
and then goes up in the loop of Henle connecting to the collecting tubules in the cortex.
Several collecting tubules converge into the collecting duct where water is finally absorbed,
Marine Mammal Physiology: Requisites for Ocean Living
7.2.2.1 Gastrointestinal tract—The entrance for water
The intestines are critical for promoting net water gain in marine mammals because they
likely meet a significant portion of their water requirements from pre-formed water in their
diets when not fasting (Irving et al. 1935; Smith 1936; Fetcher 1939; Pilson 1970; Depocas
et al. 1971; Ortiz et al. 1978; Hui 1981). Water is, generally, absorbed in the small and large
intestines. The intestine, especially the small intestine, is longer in cetaceans and pinnipeds than those in terrestrial carnivores (Helm 1983; Goodman-Lowe et al. 2001; Williams
et al. 2001). Therefore, elongation of the small intestine may be advantageous for water
absorption. Generally, water is absorbed through cell junction of the absorptive enterocyte
(the paracellular pathway) in terrestrial mammals (Barrett 2005). However, it was reported
recently that the water channel aquaporin-1 (AQP1) is located at the apical membrane of
the absorptive enterocyte in the small intestine of bottlenose dolphin (Tursiops truncatus)
unlike terrestrial mammals (Suzuki 2010). The distribution of AQP1 along the apical membrane potentially contributes to the rapid and effective absorption of water from the lumen
into the body across the cellular membrane (the transcellular pathway). These anatomical
and histological features of the intestine may facilitate the efficient absorption of water.
In addition to the demand to conserve and obtain free water, marine mammals in a
hyperosmotic habitats need to avoid excessive Na + ingestion, either from their prey directly
or indirectly during the act of eating. Cetaceans possess a very muscular sphincter at the
entrance of the esophagus that is well developed and forms a very narrow (3–5 mm) opening leading to the stomach that may function to only allow prey to pass without much
seawater (Harrison et al. 1970). Reidenberg (2007) suggested that the tongue of cetaceans
may function in squeezing water out of the mouth, minimizing ingestion of saltwater.
7.2.2.2 Kidney—The principal regulator
The kidneys are responsible, among other functions, for the regulation of water and salt
balance. Electrolytes, water, and certain metabolic by-products in the plasma are filtered
by the kidneys through a series of processes that constitute selective filtration, secretion,
and excretion. While the processes are consistent throughout mammals, the anatomy of
the kidneys in marine mammals is distinctive from terrestrial mammals. The kidneys
of cetaceans and pinnipeds are reniculated and comprised of hundreds to thousands of
reniculi, which contain divided cortical tissue and a single medullary pyramid, generally
inserted in a single calyx and bundled within the kidney capsule (Ommanney 1932; Bester
1975; Hedges et al. 1979; Vardy and Bryden 1981; Stoskopf and Herbert 1990) (Figure 7.2).
In manatees, the kidney is superficially lobulated (Hill and Reynolds 1989) and is elongated and unlobulated in the dugong (Dugong dugon) (Batrawi 1953, 1957; Marsh et al. 1978).
Similar to cetaceans and pinnipeds, sea otters possess a lobulated kidney and possess
slightly greater urine concentrating ability than river otters (Hoover and Tyler 1986). The
kidneys in marine mammals are relatively larger than in terrestrial mammals (Beuchat
1996) and may be an adaptation for a deep-diving lifestyle and/or a direct consequence of
an enlargement of their body mass, independent of any osmoregulatory burden (Pfeiffer
1997; Maluf and Gassman 1998; Ortiz 2001).
The kidneys consist of a cortex and medulla. Plasma is filtered in the glomeruli in the
cortex, and the original urine flows to the proximal tube in the cortex, where electrolytes
and organic nutrients including urea are reabsorbed. Subsequently, the urine enters the
descending thin loop of Henle in the medulla, where necessary molecules are absorbed
and then goes up in the loop of Henle connecting to the collecting tubules in the cortex.
Several collecting tubules converge into the collecting duct where water is finally absorbed,