296
Marine Mammal Physiology: Requisites for Ocean Living
Certain marine mammals go even further to keep the ecosystem healthy, and their
presence plays a critical role in maintaining the ecological community, far beyond
what would be expected for their population size. Named a keystone species, sea otters
(Enhydra lutris) are a well-known example. Their high metabolism requires them to eat
a large amount of invertebrates each day, including mussels, clams, and urchins. Their
presence keeps the sea urchin population in check. In places where sea otters are no
longer a part of the community, sea urchins quickly overpopulate and they destroy
entire kelp forests by eating the base of kelp fronds. The entire ecosystem of the kelp
forest, and all of the organisms that are a part of it, rely on the sea otters to keep this
balance in place.
These animals are highly specialized to their environment, and if there are any
changes to that environment, or to the animals themselves, they may be predisposed to
disease. Humans are changing the environment—degradation and development of the
coast allows for run-off of contaminants, infectious organisms, or nutrients that may trigger a toxic algal bloom. New industrial chemicals are biomagnified in the food chain, and
end up being stored in marine mammal blubber—just as environmental concentrations of
old, outlawed chemicals such as DDT are starting to decrease.
Marine mammals have a variety of stressors to contend with in this changing environment. In this chapter, we will explore how the physiological adaptations that they have
developed to live in a marine environment can either protect or predispose them to disease.
13.2 Cetaceans
13.2.1 Physiologic adaptation: Respiratory system
Members of the order Cetacea are one of the most radically adapted groups to the aquatic
environment. They are obligate aquatic species and cannot survive outside of the water
due to extreme anatomical and physiological changes. Adaptations to the respiratory and
dermal system necessary for aquatic life also play a significant role in maintaining health
in these species.
The respiratory system of cetaceans is highly adapted for swimming and diving. Their
nares are positioned at the top of the head to allow for inspiration at the water’s surface.
The lungs of cetaceans are highly compliant, which help protect them from decompression sickness during repeated deep diving. Within the lung there is very little lymphoid
tissue and the bronchial epithelium and glands contain almost no mucoid-producing
cells, which typically play a role in trapping foreign particles (Goudappel and Slijper 1958).
The alveoli contain no epithelial lining, which typically provides a last layer of protection against invaders. This directly exposes the capillary network to respiratory gases,
allowing for rapid exchange of oxygen and carbon dioxide (Haynes and Laurie 1937). It is
hypothesized that these protective anatomical respiratory structures are unnecessary to
cetaceans that live in a mostly dust-free environment. However, this leaves the cetacean
respiratory system susceptible to insult from a variety of external factors and prone to
developing pneumonia. Bottlenose dolphins (Tursiops truncatus) exposed to volatile hydrocarbons following the Deepwater Horizon oil spill in the Gulf of Mexico in 2010 were five
times more likely to have moderate to severe lung disease than counterparts at a reference
site in Florida (Schwacke et al. 2014).
Immediately following an oil spill, volatile hydrocarbon vapors are released from the
oil. Cetaceans appear to be able to detect oil slicks on water but do not necessarily avoid
them (Gubbay and Earll 2000). Therefore, dolphins swimming through an oiled area may
Marine Mammal Physiology: Requisites for Ocean Living
Certain marine mammals go even further to keep the ecosystem healthy, and their
presence plays a critical role in maintaining the ecological community, far beyond
what would be expected for their population size. Named a keystone species, sea otters
(Enhydra lutris) are a well-known example. Their high metabolism requires them to eat
a large amount of invertebrates each day, including mussels, clams, and urchins. Their
presence keeps the sea urchin population in check. In places where sea otters are no
longer a part of the community, sea urchins quickly overpopulate and they destroy
entire kelp forests by eating the base of kelp fronds. The entire ecosystem of the kelp
forest, and all of the organisms that are a part of it, rely on the sea otters to keep this
balance in place.
These animals are highly specialized to their environment, and if there are any
changes to that environment, or to the animals themselves, they may be predisposed to
disease. Humans are changing the environment—degradation and development of the
coast allows for run-off of contaminants, infectious organisms, or nutrients that may trigger a toxic algal bloom. New industrial chemicals are biomagnified in the food chain, and
end up being stored in marine mammal blubber—just as environmental concentrations of
old, outlawed chemicals such as DDT are starting to decrease.
Marine mammals have a variety of stressors to contend with in this changing environment. In this chapter, we will explore how the physiological adaptations that they have
developed to live in a marine environment can either protect or predispose them to disease.
13.2 Cetaceans
13.2.1 Physiologic adaptation: Respiratory system
Members of the order Cetacea are one of the most radically adapted groups to the aquatic
environment. They are obligate aquatic species and cannot survive outside of the water
due to extreme anatomical and physiological changes. Adaptations to the respiratory and
dermal system necessary for aquatic life also play a significant role in maintaining health
in these species.
The respiratory system of cetaceans is highly adapted for swimming and diving. Their
nares are positioned at the top of the head to allow for inspiration at the water’s surface.
The lungs of cetaceans are highly compliant, which help protect them from decompression sickness during repeated deep diving. Within the lung there is very little lymphoid
tissue and the bronchial epithelium and glands contain almost no mucoid-producing
cells, which typically play a role in trapping foreign particles (Goudappel and Slijper 1958).
The alveoli contain no epithelial lining, which typically provides a last layer of protection against invaders. This directly exposes the capillary network to respiratory gases,
allowing for rapid exchange of oxygen and carbon dioxide (Haynes and Laurie 1937). It is
hypothesized that these protective anatomical respiratory structures are unnecessary to
cetaceans that live in a mostly dust-free environment. However, this leaves the cetacean
respiratory system susceptible to insult from a variety of external factors and prone to
developing pneumonia. Bottlenose dolphins (Tursiops truncatus) exposed to volatile hydrocarbons following the Deepwater Horizon oil spill in the Gulf of Mexico in 2010 were five
times more likely to have moderate to severe lung disease than counterparts at a reference
site in Florida (Schwacke et al. 2014).
Immediately following an oil spill, volatile hydrocarbon vapors are released from the
oil. Cetaceans appear to be able to detect oil slicks on water but do not necessarily avoid
them (Gubbay and Earll 2000). Therefore, dolphins swimming through an oiled area may