Although all organisms with either an exoskeleton or endoskeleton can 
potentially provide ontogenetic data, bivalve molluscs are the most universally 
used group for obtaining these types of information for three reasons: 
(1) Most members of the Class Bivalvia are preservable, and are common 
faunal elements in both recent and fossil assemblages. Many species are 
present in areas impacted by pollution and are represented in both 
early and late stages of ecological successions following seafloor 
disturbance. 
(2) Preparation of the shell for obtaining ontogenetic information is easily 
done. This involves sectioning or fracturing the shell along a plane 
passing from the oldest part of the shell, the umbo, to the growing edge 
along the maximum axis of growth (30, 32). Coiled or otherwise 
torqued shells (e.g. gastropods) make this technique impossible with 
present methods. 
(3) Most research relating shell parameters to environmental conditions is 
based on bivalves. 
Data from Living and Dead Molluscs 
The relationship of a species to its environment has been conceptualized in 
the niche model (28). The species of interest is able to grow and reproduce as 
long as the organisms’ functional range (biospace) is not exceeded by the 
ambient environment. Not all parts of the realized biospace promote equal 
growth or fecundity. Different combinations of niche parameters will be 
manifested in changed rates of growth, survivorship, or reproductive success. 
All of these manifestations are capable of being preserved within the shell. We 
therefore have a record of an organism’s responses to changing niche conditions 
preserved in shells of individuals, composing either the living or death 
assemblage. 
Suboptimal niche conditions can be thought of as ecological stress. 
Ecological stress is responsible for most, if not all, growth patterns within 
individual shells. In this regard, an ecological stress, such as a pollution event, 
can be assigned dimensions in both space and time. These dimensions are 
important when considering species appropriate for establishing after-the-fact 
relationships. Ideally, the spatial distribution of a species should overlap and 
extend beyond the affected area. Populations falling outside of the polluted 
area can be used as control or reference populations. If the pollution event is 
lethal to part or all members of the population occupying the affected seafloor, 
after-the-fact study will include a comparison of both living and death 
assemblages. If the effect is sublethal, living assemblages alone will be used. 
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