Physiological and Genetic Responses to Environmental Stress
165
Because sea turtles are long-lived animals, the cumulative effect of various
stressors is likely to be great. Because sea turtles spend discrete portions of their
life in a variety of marine habitats, they are vulnerable at multiple life stages: as
eggs on the beach, in the open ocean gyres, as juveniles in nearshore waters, and
as adults migrating between feeding and nesting grounds. Thus, turtles may be
exposed to a greater variety of environmental stressors than less migratory animals,
with presumably different vulnerabilities at each stage. However, their exposure
to a particular stressor may be limited by the length of that life history stage. For
example, fibropapilloma disease appears to affect primarily juvenile green turtles
of 40–90 cm carapace length (Ehrhart, 1991), but is rare in nesting adults. Exposure
to weathered oil has significant health effects on swimming turtles (Lutcavage
et al., 1995), but in one study demonstrated little impact on egg survival. Fresh
oil, on the other hand, significantly affected egg survival (Fritts and McGehee,
1981). Vulnerability to certain stressors will also vary by ecological niche, i.e.,
polychlorobiphenyl (PCB) and dichlorodiphenyldichloroethylene (DDE) accumulations are consistently higher in loggerhead turtle tissues and eggs than in those
of green turtles (George, 1997; Clark and Krynitsky, 1980), presumably because
of dietary differences. Clark and Krynitsky (1980) also reported that DDE and
PCB loads in both loggerhead and green turtle eggs were significantly lower than
in bird eggs taken from the same location (Merritt Island, FL) and lower than
contaminant levels in eggs from Everglades (FL) crocodiles. They speculated that
adult turtles nesting on Merritt Island lived and fed in areas less contaminated
than did the residential bird and Everglades crocodile populations.
Natural stressors include thermal stress (heat stress, cold stunning), seasonal or
temperature-related changes in immune function, and the presence of disease, parasites, or epiphytes. Even these natural physiological stressors may, of course, be
impacted or exaggerated by anthropogenic factors. For example, physiological
responses to natural diving are significantly different from those produced by the
forced submergence of trawl entanglement (Lutcavage et al., 1997), and animals
with a depressed immune system related to pollutant levels would be more vulnerable
to parasites and disease.
Anthropogenic stressors may have either direct or indirect impacts on sea turtle
health. Direct impacts include such problems as oil spills, latex or plastic ingestion,
fishing line entanglement, and the presence of persistent pesticides, hormone disrupting pollutants, and heavy metals. Indirect effects occur primarily through habitat
degradation: eutrophication, the contribution of pollutants to toxic algal blooms, and
collapse of the food web.
Inappropriate sea turtle behavior can put them at particular risk. For example,
it appears that unlike marine mammals, adult sea turtles show no avoidance behavior
when they encounter an oil slick (Odell and MacMurray, 1986); they also indiscriminately ingest tar balls and plastics (Lutz, 1990), and hatchlings congregate in ocean
rift zones where floating debris concentrate. Their breathing pattern of large tidal
volumes and rapid inhalation before diving will result in the most direct and effective
exposure to petroleum vapors (the most toxic part of oil spills), as well as biotoxin
aerosols resulting from dinoflagellate blooms.
1123 book.book Page 165 Monday, November 11, 2002 11:11 AM
165
Because sea turtles are long-lived animals, the cumulative effect of various
stressors is likely to be great. Because sea turtles spend discrete portions of their
life in a variety of marine habitats, they are vulnerable at multiple life stages: as
eggs on the beach, in the open ocean gyres, as juveniles in nearshore waters, and
as adults migrating between feeding and nesting grounds. Thus, turtles may be
exposed to a greater variety of environmental stressors than less migratory animals,
with presumably different vulnerabilities at each stage. However, their exposure
to a particular stressor may be limited by the length of that life history stage. For
example, fibropapilloma disease appears to affect primarily juvenile green turtles
of 40–90 cm carapace length (Ehrhart, 1991), but is rare in nesting adults. Exposure
to weathered oil has significant health effects on swimming turtles (Lutcavage
et al., 1995), but in one study demonstrated little impact on egg survival. Fresh
oil, on the other hand, significantly affected egg survival (Fritts and McGehee,
1981). Vulnerability to certain stressors will also vary by ecological niche, i.e.,
polychlorobiphenyl (PCB) and dichlorodiphenyldichloroethylene (DDE) accumulations are consistently higher in loggerhead turtle tissues and eggs than in those
of green turtles (George, 1997; Clark and Krynitsky, 1980), presumably because
of dietary differences. Clark and Krynitsky (1980) also reported that DDE and
PCB loads in both loggerhead and green turtle eggs were significantly lower than
in bird eggs taken from the same location (Merritt Island, FL) and lower than
contaminant levels in eggs from Everglades (FL) crocodiles. They speculated that
adult turtles nesting on Merritt Island lived and fed in areas less contaminated
than did the residential bird and Everglades crocodile populations.
Natural stressors include thermal stress (heat stress, cold stunning), seasonal or
temperature-related changes in immune function, and the presence of disease, parasites, or epiphytes. Even these natural physiological stressors may, of course, be
impacted or exaggerated by anthropogenic factors. For example, physiological
responses to natural diving are significantly different from those produced by the
forced submergence of trawl entanglement (Lutcavage et al., 1997), and animals
with a depressed immune system related to pollutant levels would be more vulnerable
to parasites and disease.
Anthropogenic stressors may have either direct or indirect impacts on sea turtle
health. Direct impacts include such problems as oil spills, latex or plastic ingestion,
fishing line entanglement, and the presence of persistent pesticides, hormone disrupting pollutants, and heavy metals. Indirect effects occur primarily through habitat
degradation: eutrophication, the contribution of pollutants to toxic algal blooms, and
collapse of the food web.
Inappropriate sea turtle behavior can put them at particular risk. For example,
it appears that unlike marine mammals, adult sea turtles show no avoidance behavior
when they encounter an oil slick (Odell and MacMurray, 1986); they also indiscriminately ingest tar balls and plastics (Lutz, 1990), and hatchlings congregate in ocean
rift zones where floating debris concentrate. Their breathing pattern of large tidal
volumes and rapid inhalation before diving will result in the most direct and effective
exposure to petroleum vapors (the most toxic part of oil spills), as well as biotoxin
aerosols resulting from dinoflagellate blooms.
1123 book.book Page 165 Monday, November 11, 2002 11:11 AM