315
Chapter fourteen: Toxicology and poisons
high trophic levels (strict predators). Many studies have reported concentrations of some
poisons positively correlated with age, although these relationships are often complicated
by growth/dilution dynamics and reproductive status (Dietz et al. 1996; Knott et al. 2011).
For instance, Ross et al. (2004) found increasing concentrations of persistent organic pollutants (POPs, i.e., PCBs, PCDEs) in older male harbor seals, but found decreasing concentrations in females associated with parity and lactation (i.e., transfer to pup). The effect of
reproduction on poison accumulation and offloading in marine mammals is an important
driver of observed concentrations in adult females and represents a significant exposure
route in utero and to nursing pups. These dynamics will be discussed further in subsequent sections.
Some lipophilic poisons can increase in concentration with increasing trophic level
due to biomagnification (van de Vijver et al. 2003; Coelho et al 2013, numerous others).
Since many marine mammals are strict carnivores, they often assume high trophic positions, especially among marine mammal species that prey upon other marine mammals
such as killer whales (Saulitis et al. 2000), polar bears (Gormezano and Rockwell 2013),
leopard seals (Boveng et al. 1998), and walruses (Lowry and Fay 1984). Trophic transfer of
poisons is often complicated by varying abilities of taxonomic groups, or even individual
species to sequester or biotransform certain poisons (Tanabe et al. 1988). In some instances,
biotransformation at certain trophic positions can greatly alter the potency of a particular
poison. For instance, ciguatoxin, which is a potent neurologic biotoxin associated with the
dinoflagellate Gambierdiscus toxicus, enters lower trophic positions as gambiertoxins which
are produced by G. toxicus. As the gambiertoxins are biotransformed by herbivorous and
predatory fish, they are oxidized into more potent ciguatoxins (Lewis and Holmes 1993;
Van Egmond et al. 2004).
14.2.1 Maternal transfer: Early exposure to poisons
14.2.1.1 Placental mammals
For many species, especially placental mammals, one of the most sensitive cohorts to poisoning is the fetus and neonate (transplacental and lactational transfer). As opposed to
oviparous animals, in which embryos do not develop within the mother, marine mammal
fetuses are exposed to maternal toxicants both in utero (for compounds which can cross the
placental barrier; transplacental) as well as during nursing (trans-mammary). For instance
methylmercury is transferred to the developing fetus through the placenta, and while
studies of embryonic development in marine mammals are rare, methylmercury has been
shown to impair development and cause adverse health outcomes in fish eating humans
and wildlife (Grandjean et al. 1994; Debes et al. 2006; Johansson et al. 2007; Tonk et al. 2010).
Some of these studies and criteria (oral intake and tissue concentrations) have been used to
extrapolate potential adverse effects in marine mammals.
Interestingly, one of the main excretory routes for methylmercury, besides excretion
in urine and bile, is through hair growth (Clarkson and Magos 2006). Mammalian hair is
composed of large amounts of the protein keratin, which contains a large number of cysteine residues which form the disulfide bridges necessary for its rigidity. Methylmercury
binds tightly to the sulfur-containing cysteine residues, and thus hair concentrations of
methylmercury are often several-fold higher than blood or other tissues (e.g., Rea et al.
2013). Both pinniped and cetacean fetuses develop hair in utero (lanugo), which in pinnipeds can persist for several weeks post-partum before it is shed and a new coat develops.
Most mammals, including humans, display signs of lanugo development in utero, which
is likely a conserved trait harkening back to common ancestors which maintained full
Chapter fourteen: Toxicology and poisons
high trophic levels (strict predators). Many studies have reported concentrations of some
poisons positively correlated with age, although these relationships are often complicated
by growth/dilution dynamics and reproductive status (Dietz et al. 1996; Knott et al. 2011).
For instance, Ross et al. (2004) found increasing concentrations of persistent organic pollutants (POPs, i.e., PCBs, PCDEs) in older male harbor seals, but found decreasing concentrations in females associated with parity and lactation (i.e., transfer to pup). The effect of
reproduction on poison accumulation and offloading in marine mammals is an important
driver of observed concentrations in adult females and represents a significant exposure
route in utero and to nursing pups. These dynamics will be discussed further in subsequent sections.
Some lipophilic poisons can increase in concentration with increasing trophic level
due to biomagnification (van de Vijver et al. 2003; Coelho et al 2013, numerous others).
Since many marine mammals are strict carnivores, they often assume high trophic positions, especially among marine mammal species that prey upon other marine mammals
such as killer whales (Saulitis et al. 2000), polar bears (Gormezano and Rockwell 2013),
leopard seals (Boveng et al. 1998), and walruses (Lowry and Fay 1984). Trophic transfer of
poisons is often complicated by varying abilities of taxonomic groups, or even individual
species to sequester or biotransform certain poisons (Tanabe et al. 1988). In some instances,
biotransformation at certain trophic positions can greatly alter the potency of a particular
poison. For instance, ciguatoxin, which is a potent neurologic biotoxin associated with the
dinoflagellate Gambierdiscus toxicus, enters lower trophic positions as gambiertoxins which
are produced by G. toxicus. As the gambiertoxins are biotransformed by herbivorous and
predatory fish, they are oxidized into more potent ciguatoxins (Lewis and Holmes 1993;
Van Egmond et al. 2004).
14.2.1 Maternal transfer: Early exposure to poisons
14.2.1.1 Placental mammals
For many species, especially placental mammals, one of the most sensitive cohorts to poisoning is the fetus and neonate (transplacental and lactational transfer). As opposed to
oviparous animals, in which embryos do not develop within the mother, marine mammal
fetuses are exposed to maternal toxicants both in utero (for compounds which can cross the
placental barrier; transplacental) as well as during nursing (trans-mammary). For instance
methylmercury is transferred to the developing fetus through the placenta, and while
studies of embryonic development in marine mammals are rare, methylmercury has been
shown to impair development and cause adverse health outcomes in fish eating humans
and wildlife (Grandjean et al. 1994; Debes et al. 2006; Johansson et al. 2007; Tonk et al. 2010).
Some of these studies and criteria (oral intake and tissue concentrations) have been used to
extrapolate potential adverse effects in marine mammals.
Interestingly, one of the main excretory routes for methylmercury, besides excretion
in urine and bile, is through hair growth (Clarkson and Magos 2006). Mammalian hair is
composed of large amounts of the protein keratin, which contains a large number of cysteine residues which form the disulfide bridges necessary for its rigidity. Methylmercury
binds tightly to the sulfur-containing cysteine residues, and thus hair concentrations of
methylmercury are often several-fold higher than blood or other tissues (e.g., Rea et al.
2013). Both pinniped and cetacean fetuses develop hair in utero (lanugo), which in pinnipeds can persist for several weeks post-partum before it is shed and a new coat develops.
Most mammals, including humans, display signs of lanugo development in utero, which
is likely a conserved trait harkening back to common ancestors which maintained full