324
Marine Mammal Physiology: Requisites for Ocean Living
concentrations of Cd appear not to have fluctuated strongly in Northern latitudes (this is
in contrast to Hg which has shown an increase since post-industrialization) (Fitzgerald
et al. 2005). However, given the large variation seen in pilot whales, it is likely that there
are other physiologic and molecular modulators of Cd bound to MT, both as total amount
and proportion of total MT. These include the presence of other metal ions in diet which
could compete for MT binding, the induction of hormone systems which are known to
impact MT synthesis, or seasonal variation in physiology which could drive transport or
sequestration of heavy metals (i.e., nursing, lipid mobilization, etc.).
14.5.2 Pinnipeds
In addition to Hg and Se, some pinnipeds have high concentrations of Cd (Dietz et al. 1996,
1998). Unlike other heavy metals such as lead and mercury, which have shown a marked
increase in the Arctic since the industrial revolution, Cd levels appear to have been consistent since the fifteenth century based on hair samples from Greenland (Hansen et al. 1989).
Using Greenland ringed seals (Phoca hispida) as an example, Dietz et al. (1998) reported that
based on previous studies cadmium levels found in the kidneys of ringed seals exceeded
thresholds associated with renal damage in other mammals (200 μg/g ww, as determined
by WHO 1992). In the kidneys with very high concentrations of cadmium (up to 726 μg/g
Cd ww in the kidney cortex), Dietz et al. (1998) found no significant differences in renal
structure or necrosis was observed compared to the kidneys with low (1.63–5.19 μg/g Cd)
and intermediate (86.5–91.3 μg/g Cd) concentrations. The concentrations of Cd in the livers
of bowhead whales (Balaena mysticetus) have been also found that are associated with toxic
thresholds in domestic animals (Woshner et al. 2001). Rosa et al. (2008) found moderate to
severe thickening of the Bownan’s capsule in the kidneys of bowhead whales associated
with age and Cd concentrations, however, there was no evidence of renal dysfunction. This
supports the conclusion of Dietz et al. (1998) that given historically high Cd concentrations in
the marine environment (as compared to terrestrial, especially Arctic regions) marine mammals may have evolved to tolerate or detoxify relatively high concentrations of Cd.
Besides genetic adaptation of phenotypic plasticity, there is another mechanism which
could help explain how pinnipeds, and indeed other species of marine mammals, appear to
tolerate concentrations of heavy metals often without gross injury. A proposed mechanism
for Cd-induced hepatotoxicity is oxidative stress (Shaikh et al. 1999), which occurs when
MT synthesis cannot effectively sequester free Cd. Methylmercury is known to similarly
generate oxidative stress, although often in different tissues (for review see Farina et al.
2011). The more infamous heavy metals of concern in the marine environment do not exist
in isolation—marine mammals are exposed to these metals through a variety of routes
but generally diet, which for high trophic level marine mammals such as pinnipeds and
odontecetes means a diet of fish, invertebrates, and possibly other marine mammals (for
instance orcas and walruses consume seals and sea lions). Other nutritional components of
a fish-based diet, such as selenium (Se), may counter some of the potential negative effects
of heavy metal intoxication in general. While the mercury–selenium relationship is well
publicized and is discussed elsewhere in this chapter, selenium is also a key component
of the glutathione-peroxidase (GPx) family of enzymes. Proteins with selenium containing
active sites (selenoproteins) often contain selenocysteine active sites, such as human GPx1,
in which selenium takes the place of sulfur in the cysteine residue forming a selanol group.
Interestingly, the selenocysteine amino acid is not coded for by the standard genetic alphabet, rather, it is synthesized into the growing polypeptide via the traditional stop codon
TGA (UGA in mRNA) (Chambers et al. 1986; Böck et al. 1991). The translation of the stop
Marine Mammal Physiology: Requisites for Ocean Living
concentrations of Cd appear not to have fluctuated strongly in Northern latitudes (this is
in contrast to Hg which has shown an increase since post-industrialization) (Fitzgerald
et al. 2005). However, given the large variation seen in pilot whales, it is likely that there
are other physiologic and molecular modulators of Cd bound to MT, both as total amount
and proportion of total MT. These include the presence of other metal ions in diet which
could compete for MT binding, the induction of hormone systems which are known to
impact MT synthesis, or seasonal variation in physiology which could drive transport or
sequestration of heavy metals (i.e., nursing, lipid mobilization, etc.).
14.5.2 Pinnipeds
In addition to Hg and Se, some pinnipeds have high concentrations of Cd (Dietz et al. 1996,
1998). Unlike other heavy metals such as lead and mercury, which have shown a marked
increase in the Arctic since the industrial revolution, Cd levels appear to have been consistent since the fifteenth century based on hair samples from Greenland (Hansen et al. 1989).
Using Greenland ringed seals (Phoca hispida) as an example, Dietz et al. (1998) reported that
based on previous studies cadmium levels found in the kidneys of ringed seals exceeded
thresholds associated with renal damage in other mammals (200 μg/g ww, as determined
by WHO 1992). In the kidneys with very high concentrations of cadmium (up to 726 μg/g
Cd ww in the kidney cortex), Dietz et al. (1998) found no significant differences in renal
structure or necrosis was observed compared to the kidneys with low (1.63–5.19 μg/g Cd)
and intermediate (86.5–91.3 μg/g Cd) concentrations. The concentrations of Cd in the livers
of bowhead whales (Balaena mysticetus) have been also found that are associated with toxic
thresholds in domestic animals (Woshner et al. 2001). Rosa et al. (2008) found moderate to
severe thickening of the Bownan’s capsule in the kidneys of bowhead whales associated
with age and Cd concentrations, however, there was no evidence of renal dysfunction. This
supports the conclusion of Dietz et al. (1998) that given historically high Cd concentrations in
the marine environment (as compared to terrestrial, especially Arctic regions) marine mammals may have evolved to tolerate or detoxify relatively high concentrations of Cd.
Besides genetic adaptation of phenotypic plasticity, there is another mechanism which
could help explain how pinnipeds, and indeed other species of marine mammals, appear to
tolerate concentrations of heavy metals often without gross injury. A proposed mechanism
for Cd-induced hepatotoxicity is oxidative stress (Shaikh et al. 1999), which occurs when
MT synthesis cannot effectively sequester free Cd. Methylmercury is known to similarly
generate oxidative stress, although often in different tissues (for review see Farina et al.
2011). The more infamous heavy metals of concern in the marine environment do not exist
in isolation—marine mammals are exposed to these metals through a variety of routes
but generally diet, which for high trophic level marine mammals such as pinnipeds and
odontecetes means a diet of fish, invertebrates, and possibly other marine mammals (for
instance orcas and walruses consume seals and sea lions). Other nutritional components of
a fish-based diet, such as selenium (Se), may counter some of the potential negative effects
of heavy metal intoxication in general. While the mercury–selenium relationship is well
publicized and is discussed elsewhere in this chapter, selenium is also a key component
of the glutathione-peroxidase (GPx) family of enzymes. Proteins with selenium containing
active sites (selenoproteins) often contain selenocysteine active sites, such as human GPx1,
in which selenium takes the place of sulfur in the cysteine residue forming a selanol group.
Interestingly, the selenocysteine amino acid is not coded for by the standard genetic alphabet, rather, it is synthesized into the growing polypeptide via the traditional stop codon
TGA (UGA in mRNA) (Chambers et al. 1986; Böck et al. 1991). The translation of the stop