Biomarkers of Defense, Tolerance, and Ecological Consequences - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 61

Moreover, cadmium-tolerant F3 and F4 specimens were less resistant to heat, even at tem-
peratures naturally observed in summer in their habitat. Without exposure to cadmium,
F7 specimens displayed numerous life history traits that were negatively influenced by
tolerance in comparison to control specimens. Likewise, although they were more toler-
ant of acute Cu contamination, larvae of Catostomus commersoni spawned from adults liv-
ing in contaminated lakes were hatched at smaller size, grew less, and showed a lower
survival rate than those spawned from adults living in a comparatively healthy habitat
(Munkittrick and Dixon 1988). The authors hypothesized that this altered condition could
be caused by the cost of synthesis of protective proteins. The literature quoted by Xie and
Klerks (2004) also shows a lowered fecundity in cadmium-tolerant Drosophila , a shorter
lifetime in mercury-resistant fish, and a longer period of growth in insecticide-resistant
mosquitoes. Investigation of defense systems in fish (lower level of cadmium intake in F3
and F4 offspring, production of MT observed in F8 offspring) has led the authors to con-
sider that the deteriorating life history traits (see Table 3.3) could be caused by a change in
energy allocation.
Metal concentrations in the environment, apart from anthropogenic sources, vary
depending on the geographical area and specific location within this area, according to
season and water supply from feeder watercourses. Organisms are typically able to main-
tain intracellular concentrations of essential metals in the range of optimal concentrations
thanks to homeostasis, regardless of external concentrations. Various observations have
been published that demonstrate that when this homeostasis occurs, animals are not sub-
jected to stress (Van Tilborg and Van Assche 1998). These observations no longer apply
when considering metals with no vital functions or to xenobiotics, but the dose-response
relationship generally has a sigmoid shape and the no observed effect level (NOEL) can be
accepted as a valid concept for many contaminants. In several cases, there is apparently
no significant physiological cost for various insects and acarid mites resistant to pesti-
cides (quoted by Xie and Klerks 2004). In two isopod crustaceans able to survive in an
area impacted by smelting works, it has been shown that differing strategies were imple-
mented, involving (in Oniscus asellus ) or not involving (in Porcellio scaber ) an energy cost
(Schill and Köhler 2004). In polychaete worms, Nereis diversicolor , laboratory exposure
to silver or copper induces a higher production of mucus in individuals adapted in the
field to chronic metal pollution compared to individuals from a control site (Mouneyrac
et al. 2003). Metal stress also induces mucus secretion in mussels ( Mytilus edulis ) or fish
( Oncorhynchus mykiss ) (see Wicklum and Davies 1996). In freshwater invertebrates, mucus
secretion contributes significantly to the energy budget, representing 13% to 32% of
absorbed energy (see Wicklum and Davies 1996). In the marine gastropod Patella vulgata ,
mucus production accounts for 23% of the energy acquired through food ingestion (Davies
et al. 1990). This cost of mucus production in a gastropod is more important than the total
cost of locomotion in a reptile or a mammal of the same size (Denny 1980, in Leung et al.
2000). Consequently, these authors consider that mucus production in cadmium-exposed
gastropods Nucella lapillus is linked to changes observed at the level of energy metabolism
(decreased rate of oxygen consumption and glycogen concentration).
As regards the cost of tolerance, the NOEL is all the lower when chemical stress com-
bines with nonchemical stress, more particularly with those stressors affecting energy
metabolism—that is, temperature and, to a greater extent, food availability as discussed
above. Thus, in the rainbow trout ( Oncorhynchus mykiss ), long-term exposure (100 days)
to low concentrations of metals (3 μg Cd L −1 , 75 μg Cu L −1 , or 250 μg Zn L −1 ) involves three
types of successive responses: damage, repair, and acclimatization. When the rainbow
trout can get enough food, there was no effect of metal exposure on growth, but copper