320
Marine Mammal Physiology: Requisites for Ocean Living
territory (Francis and Heath 1991; Bejarano et al. 2008). The high incidence of adult female
poisonings during this critical time period is especially concerning from an epidemiologic
standpoint when considering the potential role of domoic acid in abortion and adverse
birthing outcomes in California sea lions (Goldstein et al. 2009).
14.3.3 Other
Other marine mammal clades exhibit a wide range of responses to biotoxins, and in particular those associated with HABs. An unexplained die-off of Alaskan sea otters (Enhydra
lutris) at Kodiak Islands, AK in 1987 may have been associated with saxitoxin, after investigators reported finding concentrations of PSP toxin in blue mussels (Mytilus edulis) 50
times above the upper threshold for human consumption in Alaska (>5800 μg/100 g
mussel as compared to the threshold of 80 μg/100 g mussel, Degange and Vacca 1989).
In a controlled setting, sea otters appear to reduce their feeding rates when presented
with clams containing high concentrations (226 ± 96 μg/g STX) versus clams with lower
concentrations (37 ± 9 μg/g STX) of saxitoxin. In this study, the otters also avoided the
siphons, gills, and pericardial tissue of clams, which contain higher concentrations of the
toxins compared to the other clam tissues (60%–80% of total STX sequestered by clam).
The method of presumed chemodetection is unknown, however, the authors speculate
that the otters may be responding to a tingling sensation reported in humans exposed
to PST, or they may have STX-specific gustatory receptors such as those found in fish
(Yamamori et al. 1988).
14.4 Toxic oxygen: Adaptations to anaerobic diving
While cellular and physiologic adaptations to diving will be discussed elsewhere in this
volume, it is worth noting that some of the most potent poisons that marine mammals are
exposed to come not from environmental exposure but from their own physiology. As airbreathing mammals that forage underwater, marine mammals have developed a unique
physiology that allows them to cope with extended periods of anaerobic activity.
Molecular oxygen (O 2 ) is often thought of as an incontestably wholesome chemical,
given its necessity for cellular respiration and the swift and severe consequences that arise
in the absence thereof. However, the atom O is quite damaging in many forms that are produced via normal physiologic processes and/or as a consequence of some toxins. An array
of O by-products are known to cause damage to macromolecules, some changes being irreversible. In particular, the apneic behavior of many marine mammals (diving, sleeping)
creates risk of a number of oxygen-related injuries such as ischemia–reperfusion injury
and the generation of reactive oxygen species (ROS). Voluntary breath-holds in northern
elephant seal pups have been shown to increase the activity of hypoxanthine (which generates ROS upon reperfusion) in plasma, but without observable effects to biomarkers of
oxidative stress (4-hydroxynonenal and 8-iso prostaglandin F2α) (Vázquez-Medina et al.
2006). Similarly, basal concentrations of CO and carboxyhemoglobin (COHb) in Weddell
seals and northern elephant seals are several times higher than in human smokers (Pugh
1959; Tift et al. 2014). Both Tift et al. (2014) and Vázquez-Medina et al. (2006) suggest that
endogenous production of CO and XO confers protective effects against oxidative damage
caused by apnea. Carbon monoxide (CO) is widely considered to be a toxic gas and has been
known to cause hypoxia and death in humans due to its affinity for heme protein which
prevents oxygen from appropriately binding to hemoglobin, lowering blood O 2 saturation
levels. However, CO is produced endogenously, largely through the breakdown of heme,
Marine Mammal Physiology: Requisites for Ocean Living
territory (Francis and Heath 1991; Bejarano et al. 2008). The high incidence of adult female
poisonings during this critical time period is especially concerning from an epidemiologic
standpoint when considering the potential role of domoic acid in abortion and adverse
birthing outcomes in California sea lions (Goldstein et al. 2009).
14.3.3 Other
Other marine mammal clades exhibit a wide range of responses to biotoxins, and in particular those associated with HABs. An unexplained die-off of Alaskan sea otters (Enhydra
lutris) at Kodiak Islands, AK in 1987 may have been associated with saxitoxin, after investigators reported finding concentrations of PSP toxin in blue mussels (Mytilus edulis) 50
times above the upper threshold for human consumption in Alaska (>5800 μg/100 g
mussel as compared to the threshold of 80 μg/100 g mussel, Degange and Vacca 1989).
In a controlled setting, sea otters appear to reduce their feeding rates when presented
with clams containing high concentrations (226 ± 96 μg/g STX) versus clams with lower
concentrations (37 ± 9 μg/g STX) of saxitoxin. In this study, the otters also avoided the
siphons, gills, and pericardial tissue of clams, which contain higher concentrations of the
toxins compared to the other clam tissues (60%–80% of total STX sequestered by clam).
The method of presumed chemodetection is unknown, however, the authors speculate
that the otters may be responding to a tingling sensation reported in humans exposed
to PST, or they may have STX-specific gustatory receptors such as those found in fish
(Yamamori et al. 1988).
14.4 Toxic oxygen: Adaptations to anaerobic diving
While cellular and physiologic adaptations to diving will be discussed elsewhere in this
volume, it is worth noting that some of the most potent poisons that marine mammals are
exposed to come not from environmental exposure but from their own physiology. As airbreathing mammals that forage underwater, marine mammals have developed a unique
physiology that allows them to cope with extended periods of anaerobic activity.
Molecular oxygen (O 2 ) is often thought of as an incontestably wholesome chemical,
given its necessity for cellular respiration and the swift and severe consequences that arise
in the absence thereof. However, the atom O is quite damaging in many forms that are produced via normal physiologic processes and/or as a consequence of some toxins. An array
of O by-products are known to cause damage to macromolecules, some changes being irreversible. In particular, the apneic behavior of many marine mammals (diving, sleeping)
creates risk of a number of oxygen-related injuries such as ischemia–reperfusion injury
and the generation of reactive oxygen species (ROS). Voluntary breath-holds in northern
elephant seal pups have been shown to increase the activity of hypoxanthine (which generates ROS upon reperfusion) in plasma, but without observable effects to biomarkers of
oxidative stress (4-hydroxynonenal and 8-iso prostaglandin F2α) (Vázquez-Medina et al.
2006). Similarly, basal concentrations of CO and carboxyhemoglobin (COHb) in Weddell
seals and northern elephant seals are several times higher than in human smokers (Pugh
1959; Tift et al. 2014). Both Tift et al. (2014) and Vázquez-Medina et al. (2006) suggest that
endogenous production of CO and XO confers protective effects against oxidative damage
caused by apnea. Carbon monoxide (CO) is widely considered to be a toxic gas and has been
known to cause hypoxia and death in humans due to its affinity for heme protein which
prevents oxygen from appropriately binding to hemoglobin, lowering blood O 2 saturation
levels. However, CO is produced endogenously, largely through the breakdown of heme,