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Marine Mammal Physiology: Requisites for Ocean Living
Little is known, for instance, about the way diseases are transmitted and persist in
a marine environment. Marine morbilliviruses, including PDV and CeMV, have caused
explosive outbreaks of disease characterized by high mortality specifically in the Atlantic
Ocean. In the Pacific, morbilliviruses have been documented in marine mammals, but have
not yet caused widespread mortality (Goldstein et al. 2009). Further research is needed to
understand why there is such extensive variation in host susceptibility to marine morbilliviruses, and whether this is driven primarily by host differences, or geographic differences in the viral strains (Grenfell et al. 2014).
Many questions remain about leptospirosis, the zoonotic bacterial infection that
causes kidney damage in California sea lions. Cyclical outbreaks of disease are seen, but
it is unknown whether the sea lion population continues to become re-infected by an
unknown terrestrial reservoir host, or if the disease persists in the sea lion population
itself (Lloyd-Smith et al. 2007). Widespread sampling of the population during and outside
of epidemics is currently underway to better understand the disease.
The role that biotoxins play in marine mammal disease is continuing to be elucidated.
Sampling of stranded animals’ body fluids and tissues for the detection of various biotoxins
often produces a positive result, but the significance of this positive test may not be clear. The
level of toxin required to cause disease varies with type of toxin and is unknown for nearly all
biotoxins. Because these are naturally occurring algal species, animals frequently ingest low
levels of the toxins in their prey, and positive tests do not necessarily mean that intoxication
is the cause of illness or death. A suite of clinical signs in live animals, or certain pathological
findings in dead animals, may strongly suggest that intoxication has occurred, especially if the
animal strands in a location and time in which a known algal bloom is occurring. However,
more research is needed to correlate clinical findings with biotoxin concentrations, in order to
determine the concentrations necessary to cause either acute or chronic disease.
As the environment changes from climate change and human pressures, we have seen
marine mammals adapt in a variety of ways. northern elephant seals (Mirounga angustirostris) are expanding their range as their population grows, resting on beaches where they
had never been documented before. The critically endangered Hawaiian monk seal
(Neomonachus schauinslandi) reproduces largely on sandy atolls, which are disappearing
with sea level rise, forcing them to choose new sites. And in years when fish stocks move
offshore, California sea lion dams are forced to travel further from rookery sites to feed—
meaning they may not produce as much milk to nurse a pup.
It is unknown exactly what diseases will be favored as marine mammals change their
behavior. Increased concentrations of animals may allow an introduced disease to spread
through a naïve population quickly. Animals already stressed with fewer food resources
may be susceptible to common pathogens. Certain species may actually benefit from
these changes, if their behavior changes result in less interaction with other individuals.
Scientists must respond to a changing environment by altering the questions they ask—in
order to understand how diseases affect marine mammals in this new world.
Glossary
Cetacean: A marine mammal of the order Cetacea, including whales, dolphins, and
porpoises.
ELISA: Enzyme-linked immunosorbent assay.
Keystone species: A species that has a disproportionately large effect on its environment relative to its abundance, such that if it were removed, the ecosystem would
change drastically.
Marine Mammal Physiology: Requisites for Ocean Living
Little is known, for instance, about the way diseases are transmitted and persist in
a marine environment. Marine morbilliviruses, including PDV and CeMV, have caused
explosive outbreaks of disease characterized by high mortality specifically in the Atlantic
Ocean. In the Pacific, morbilliviruses have been documented in marine mammals, but have
not yet caused widespread mortality (Goldstein et al. 2009). Further research is needed to
understand why there is such extensive variation in host susceptibility to marine morbilliviruses, and whether this is driven primarily by host differences, or geographic differences in the viral strains (Grenfell et al. 2014).
Many questions remain about leptospirosis, the zoonotic bacterial infection that
causes kidney damage in California sea lions. Cyclical outbreaks of disease are seen, but
it is unknown whether the sea lion population continues to become re-infected by an
unknown terrestrial reservoir host, or if the disease persists in the sea lion population
itself (Lloyd-Smith et al. 2007). Widespread sampling of the population during and outside
of epidemics is currently underway to better understand the disease.
The role that biotoxins play in marine mammal disease is continuing to be elucidated.
Sampling of stranded animals’ body fluids and tissues for the detection of various biotoxins
often produces a positive result, but the significance of this positive test may not be clear. The
level of toxin required to cause disease varies with type of toxin and is unknown for nearly all
biotoxins. Because these are naturally occurring algal species, animals frequently ingest low
levels of the toxins in their prey, and positive tests do not necessarily mean that intoxication
is the cause of illness or death. A suite of clinical signs in live animals, or certain pathological
findings in dead animals, may strongly suggest that intoxication has occurred, especially if the
animal strands in a location and time in which a known algal bloom is occurring. However,
more research is needed to correlate clinical findings with biotoxin concentrations, in order to
determine the concentrations necessary to cause either acute or chronic disease.
As the environment changes from climate change and human pressures, we have seen
marine mammals adapt in a variety of ways. northern elephant seals (Mirounga angustirostris) are expanding their range as their population grows, resting on beaches where they
had never been documented before. The critically endangered Hawaiian monk seal
(Neomonachus schauinslandi) reproduces largely on sandy atolls, which are disappearing
with sea level rise, forcing them to choose new sites. And in years when fish stocks move
offshore, California sea lion dams are forced to travel further from rookery sites to feed—
meaning they may not produce as much milk to nurse a pup.
It is unknown exactly what diseases will be favored as marine mammals change their
behavior. Increased concentrations of animals may allow an introduced disease to spread
through a naïve population quickly. Animals already stressed with fewer food resources
may be susceptible to common pathogens. Certain species may actually benefit from
these changes, if their behavior changes result in less interaction with other individuals.
Scientists must respond to a changing environment by altering the questions they ask—in
order to understand how diseases affect marine mammals in this new world.
Glossary
Cetacean: A marine mammal of the order Cetacea, including whales, dolphins, and
porpoises.
ELISA: Enzyme-linked immunosorbent assay.
Keystone species: A species that has a disproportionately large effect on its environment relative to its abundance, such that if it were removed, the ecosystem would
change drastically.