Sentinel Species - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 159

Sentinel species have been sought in all environments. Numerous authors (e.g., Rand
1985; NRC 1991; White 2004) have drawn up lists of the characteristics considered to be
of greatest importance in these species, but there have been different interpretations, in
particular, according to the definition used for a sentinel species and the kind of exposure
(in laboratory or in field). The most usually quoted desirable characteristics are discussed
below:
• Their sedentarity in the site studied, so that the condition of the individuals can be
directly correlated to the level of pollution at that site. Animals moving over long
distances or visiting the studied sites only occasionally cannot specifically reflect
the level of contamination at the site of capture. In this respect, sessile or weakly
mobile animals are thus preferred.
• Their ease of identiication, capture, or collection all year round, requiring a good
knowledge of the ecology of the species and its adaptations to the spatiotemporal
variations of the environmental factors in estuarine and coastal zones (tempera-
ture, salinity) or the nature of any reproductive migrations.
• A suficient population size so that any impact of regular collections on popula-
tion structure and density is negligible. Furthermore, the size of the population
available for collection conditions the speed of collecting, thereby affecting the
feasibility and the cost of the monitoring program.
• A wide and known distribution range that not only allows comparisons between
various sites, but also increases the probability of finding sites that are as weakly
impacted as possible by the pollution. However, since it is nearly impossible to find
totally cosmopolitan species, closely related species, whether of local or other ori-
gin, can be used. So, in the historic example of the Mussel Watch, various species
of mussels (genera Mytilus and Perna ) or oysters (genera Crassostrea and Saccostrea )
have been preferred. Other closely related taxa have also been used to supplement
mussels and oysters, such as various species of zebra mussels (genus Dreissena )
in the region of the Great Lakes, or another bivalve, Chama sinuata , in the Florida
Keys (NOAA 2005). Similarly, oysters ( Crassostrea gigas ) and mussels ( Mytilus spp.)
have been used since 1979 in the framework of the ROCCH for the monitoring
of French coasts, whereas similar programs have been set up in numerous other
coastal areas (Chapter 15).
• A longevity of several years and a resistance to pollutants in the environment,
allowing individual specimens to be in contact with contaminants over long
enough periods for any changes in biomarkers to be measurable. This persistence
of individuals in the environment also allows changes in parameters to be studied
over an extended period, and possibly to reveal any reversibility of ecotoxicologi-
cal damage if measures have been taken to reduce the level of contaminant expo-
sure. However, the majority of the relevant species have a much lower longevity
than humans, and the disorders caused by pollutants appear more quickly, with-
out the latency periods observed in longer-lived animals.
• Dose-effect and cause-effect relationships must be consistently established.
Primarily, the species has to present a clear response (effect) to a measurable
increase in the level of contaminant (dose). The sentinel species, although resis-
tant to contaminants, does nevertheless have to show sufficient sensitivity to
these substances that dysfunctions are apparent, and that these are measurable by