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 relationsliip 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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