300
Marine Mammal Physiology: Requisites for Ocean Living
13.5 Sirenians
13.5.1 Physiologic adaptation: Gastrointestinal tract
Sirenians are an order of marine mammals that include manatees and dugongs, and they
are the only herbivorous marine mammals. They feed almost exclusively on sea grasses and
other water plants. Similar to terrestrial herbivores such as horses, their gastrointestinal tract
is specially formatted to break down plant material, which is done in an elongated large
intestine. In addition to the structures similarly found in the horse, manatees have several
specialized glands in their stomach that are suspected to regulate water–salt balance as they
forage for plants in both saltwater and freshwater (Reynolds and Rommel 1996).
Because of their specialized diet, manatees are susceptible to diseases that may be associated with the plants they eat. Biotoxins are toxic compounds that are produced by living
organisms, and marine algal blooms can produce a variety of biotoxins. In Florida, where
one endangered species of manatee lives, more than 70 potentially harmful algal species
have been identified, producing toxins such as brevetoxin, domoic acid, okadaic acid, and
saxitoxin (Abbott et al. 2009). As an example, brevetoxin (produced by the diatom Karenia
brevis) is lipophilic, and will concentrate in sea grass blades and rhizomes, and in the epiphytes living on the surface of the grass. As manatees ingest the sea grass they receive
a high dose of the toxin, which causes listlessness and neurological signs (O’Shea et al.
1991). In addition, if the toxin is inhaled, pneumonia and bronchitis may develop. While
some manatees have been rehabilitated and released into the wild, large toxic blooms have
caused fatalities of more than 100 individuals in a single event (Bossart et al. 1998).
Sirenians are not the only marine mammals to be affected by harmful algal blooms
(HABs). Toxin-producing HABs are increasing worldwide. One of the toxins that has been
most heavily studied in marine mammals is domoic acid, produced by the diatom Pseudonitzschia sp. While domoic acid is found in oceans across the world, frequent blooms off
the coast of California cause widespread morbidity and mortality in the California sea
lions there. Domoic acid specifically targets glutamate receptors, where it acts as an excitotoxin, causing massive cellular depolarization and death (Olney et al. 1979). There is a
high concentration of glutamate receptors in the hippocampus, a region of the brain that is
responsible for spatial processing and memory.
In humans, a syndrome called Amnesic Shellfish Poisoning results from ingestion of
domoic acid-tainted shellfish, characterized by memory loss, seizures, and death. In sea
lions that have consumed domoic acid-tainted fish, similar signs are seen, and seizures,
disorientation, and death result from hippocampal necrosis. Interestingly, clinical signs
are seen with both acute intoxication as well as chronic, sub-lethal exposure (Goldstein
et al. 2008). Chronically exposed animals exhibit complex partial seizures, atrophy of the
hippocampus, and involvement of other brain regions—all similar to temporal lobe epilepsy as seen in humans (Figure 13.3).
Biotoxins pose a large risk to marine mammal health. The specializations in prey
selection, and the relatively small geographic range that pinnipeds and sirenians use to
forage make them susceptible to large algal blooms, and the toxins they produce.
13.6 Polar bears
13.6.1 Physiologic adaptation: Prey sources
Polar bears are apex predators, feeding mainly on marine mammals such as ringed seals
(Phoca hispida), bearded seals (Erignathus barbatus), and hooded seals (Cystophora cristata).
Marine Mammal Physiology: Requisites for Ocean Living
13.5 Sirenians
13.5.1 Physiologic adaptation: Gastrointestinal tract
Sirenians are an order of marine mammals that include manatees and dugongs, and they
are the only herbivorous marine mammals. They feed almost exclusively on sea grasses and
other water plants. Similar to terrestrial herbivores such as horses, their gastrointestinal tract
is specially formatted to break down plant material, which is done in an elongated large
intestine. In addition to the structures similarly found in the horse, manatees have several
specialized glands in their stomach that are suspected to regulate water–salt balance as they
forage for plants in both saltwater and freshwater (Reynolds and Rommel 1996).
Because of their specialized diet, manatees are susceptible to diseases that may be associated with the plants they eat. Biotoxins are toxic compounds that are produced by living
organisms, and marine algal blooms can produce a variety of biotoxins. In Florida, where
one endangered species of manatee lives, more than 70 potentially harmful algal species
have been identified, producing toxins such as brevetoxin, domoic acid, okadaic acid, and
saxitoxin (Abbott et al. 2009). As an example, brevetoxin (produced by the diatom Karenia
brevis) is lipophilic, and will concentrate in sea grass blades and rhizomes, and in the epiphytes living on the surface of the grass. As manatees ingest the sea grass they receive
a high dose of the toxin, which causes listlessness and neurological signs (O’Shea et al.
1991). In addition, if the toxin is inhaled, pneumonia and bronchitis may develop. While
some manatees have been rehabilitated and released into the wild, large toxic blooms have
caused fatalities of more than 100 individuals in a single event (Bossart et al. 1998).
Sirenians are not the only marine mammals to be affected by harmful algal blooms
(HABs). Toxin-producing HABs are increasing worldwide. One of the toxins that has been
most heavily studied in marine mammals is domoic acid, produced by the diatom Pseudonitzschia sp. While domoic acid is found in oceans across the world, frequent blooms off
the coast of California cause widespread morbidity and mortality in the California sea
lions there. Domoic acid specifically targets glutamate receptors, where it acts as an excitotoxin, causing massive cellular depolarization and death (Olney et al. 1979). There is a
high concentration of glutamate receptors in the hippocampus, a region of the brain that is
responsible for spatial processing and memory.
In humans, a syndrome called Amnesic Shellfish Poisoning results from ingestion of
domoic acid-tainted shellfish, characterized by memory loss, seizures, and death. In sea
lions that have consumed domoic acid-tainted fish, similar signs are seen, and seizures,
disorientation, and death result from hippocampal necrosis. Interestingly, clinical signs
are seen with both acute intoxication as well as chronic, sub-lethal exposure (Goldstein
et al. 2008). Chronically exposed animals exhibit complex partial seizures, atrophy of the
hippocampus, and involvement of other brain regions—all similar to temporal lobe epilepsy as seen in humans (Figure 13.3).
Biotoxins pose a large risk to marine mammal health. The specializations in prey
selection, and the relatively small geographic range that pinnipeds and sirenians use to
forage make them susceptible to large algal blooms, and the toxins they produce.
13.6 Polar bears
13.6.1 Physiologic adaptation: Prey sources
Polar bears are apex predators, feeding mainly on marine mammals such as ringed seals
(Phoca hispida), bearded seals (Erignathus barbatus), and hooded seals (Cystophora cristata).