311
Chapter fourteen: Toxicology and poisons
between toxin-producing organisms and marine mammals cannot be said to exist in static
isolation; rather, the spatial, temporal, and geographic range of both these groups of organisms are influenced by environmental conditions and are constantly shifting. With climate
change altering the seasonal and geographic ranges of (among others) biotoxin-producing
harmful algal blooms (HABs), climate variability is an important consideration in the poison exposure paradigm (Moore et al. 2008).
In addition to traditional poisons, marine mammals must also cope with other stressors that result from their unique and often highly specialized physiology, behavior, and
diet that are inextricably linked. All marine mammals are air-breathing and, with a few
exceptions, forage underwater. During dives, marine mammals deplete their on-board
oxygen supplies (hypoxia) which promotes the formation of acutely toxic by-products such
as lactic acid, hydrogen ions, and radical oxygen species (Berta et al. 2006). While a more
thorough review of the adaptations of diving physiology are presented elsewhere in this
textbook (Chapters 2 through 4), it is important to consider that due to their rapid bursts of
apneic activity marine mammals must tolerate anaerobic cellular conditions and the generation of a number of toxic by-products using a battery of defense mechanisms, usually
antioxidant in nature. These processes inherently alter the response of diving mammals
to other toxicants.
While marine mammals display tolerance and resistance to a number of poisons,
some physiologic adaptations (populations) and/or acclimations (individuals) of marine
mammals to diving may have predisposed some species to be more susceptible to certain
chemicals, while the opposite may occur in different host species or chemical classes. For
instance, being endothermic mammals in often cold water conditions, most marine mammals have developed thick layers of blubber for insulation and energy stores. However,
this blubber layer also tends to accumulate lipophilic compounds such as polychlorinated
biphenyls (PCBs) as well as limit the amount of partitionable room for water-soluble compounds since most blubber has low percent water composition (Hoekstra et al. 2002; Van
Dolah et al. 2003). Furthermore, during dives blood is directed to the heart and brain to
increase oxygen delivery to these highly aerobic tissues (Chapter 2). However, in doing so
poisons in the bloodstream are similarly delivered at a greater magnitude per unit of time
and tissue mass to these sensitive tissues, thus limiting their biotransformation and elimination in so-called filtering organs such as the liver or kidneys (Geraci et al. 1989).
It is critical to not extrapolate with respect to a chemical’s adverse effect on marine
mammals as this is a non-monophyletic group with a large diversity of lineages and only
grouped together based on generalized habitat relationships (i.e., lives in or feeds in the
ocean). In the same vein, it is imprudent to generalize regarding the physiology or lifehistory characteristics of marine mammals, however, many marine mammal clades share
traits either conserved from previous common ancestors or co-evolved that allow us to
make comparisons. Mammals have evolved complex and often elegant detoxification and
sequestering mechanisms for poisons, and adaptive machinery that is present in marine
mammals is often found across mammalian lineages. While particular xenobiotic biotransformation pathways may not be unique to marine mammals, it is important to discuss
them in the context of adaptation since marine mammals are exposed to different classes
of poisons and at varying concentrations than their terrestrial counterparts. As an example, consider the detoxification and sequestration process of a potent neurotoxin, monomethylmercury (MeHg + ), to an insoluble crystal complex with selenium (Se) and other
components which occurs in the livers of marine mammals (Koeman et al. 1975; LailsonBrito et al. 2012). While this process is also known to occur in some terrestrial animals,
the extent of MeHg + exposure in fish-consuming mammals in the marine environment is
Chapter fourteen: Toxicology and poisons
between toxin-producing organisms and marine mammals cannot be said to exist in static
isolation; rather, the spatial, temporal, and geographic range of both these groups of organisms are influenced by environmental conditions and are constantly shifting. With climate
change altering the seasonal and geographic ranges of (among others) biotoxin-producing
harmful algal blooms (HABs), climate variability is an important consideration in the poison exposure paradigm (Moore et al. 2008).
In addition to traditional poisons, marine mammals must also cope with other stressors that result from their unique and often highly specialized physiology, behavior, and
diet that are inextricably linked. All marine mammals are air-breathing and, with a few
exceptions, forage underwater. During dives, marine mammals deplete their on-board
oxygen supplies (hypoxia) which promotes the formation of acutely toxic by-products such
as lactic acid, hydrogen ions, and radical oxygen species (Berta et al. 2006). While a more
thorough review of the adaptations of diving physiology are presented elsewhere in this
textbook (Chapters 2 through 4), it is important to consider that due to their rapid bursts of
apneic activity marine mammals must tolerate anaerobic cellular conditions and the generation of a number of toxic by-products using a battery of defense mechanisms, usually
antioxidant in nature. These processes inherently alter the response of diving mammals
to other toxicants.
While marine mammals display tolerance and resistance to a number of poisons,
some physiologic adaptations (populations) and/or acclimations (individuals) of marine
mammals to diving may have predisposed some species to be more susceptible to certain
chemicals, while the opposite may occur in different host species or chemical classes. For
instance, being endothermic mammals in often cold water conditions, most marine mammals have developed thick layers of blubber for insulation and energy stores. However,
this blubber layer also tends to accumulate lipophilic compounds such as polychlorinated
biphenyls (PCBs) as well as limit the amount of partitionable room for water-soluble compounds since most blubber has low percent water composition (Hoekstra et al. 2002; Van
Dolah et al. 2003). Furthermore, during dives blood is directed to the heart and brain to
increase oxygen delivery to these highly aerobic tissues (Chapter 2). However, in doing so
poisons in the bloodstream are similarly delivered at a greater magnitude per unit of time
and tissue mass to these sensitive tissues, thus limiting their biotransformation and elimination in so-called filtering organs such as the liver or kidneys (Geraci et al. 1989).
It is critical to not extrapolate with respect to a chemical’s adverse effect on marine
mammals as this is a non-monophyletic group with a large diversity of lineages and only
grouped together based on generalized habitat relationships (i.e., lives in or feeds in the
ocean). In the same vein, it is imprudent to generalize regarding the physiology or lifehistory characteristics of marine mammals, however, many marine mammal clades share
traits either conserved from previous common ancestors or co-evolved that allow us to
make comparisons. Mammals have evolved complex and often elegant detoxification and
sequestering mechanisms for poisons, and adaptive machinery that is present in marine
mammals is often found across mammalian lineages. While particular xenobiotic biotransformation pathways may not be unique to marine mammals, it is important to discuss
them in the context of adaptation since marine mammals are exposed to different classes
of poisons and at varying concentrations than their terrestrial counterparts. As an example, consider the detoxification and sequestration process of a potent neurotoxin, monomethylmercury (MeHg + ), to an insoluble crystal complex with selenium (Se) and other
components which occurs in the livers of marine mammals (Koeman et al. 1975; LailsonBrito et al. 2012). While this process is also known to occur in some terrestrial animals,
the extent of MeHg + exposure in fish-consuming mammals in the marine environment is