dead shell, it is also possible to relate the season of death to absolute age at 

 death. 



The age at sexual maturity and season of reproduction can be determined 

 by relating the position of spawning breaks to absolute age and seasonal 

 pattern of growth. An illustration of the usefulness of growth patterns in 

 determining age and season of reproduction is given by Rhoads and Pannella 

 (32). They examined a population of Gemma gemma from an intertidal muddy 

 sand flat on Long Island Sound. Summer growth patterns in G. gemma 

 consisted of thick increments (7-25 ii) and were readily distinguished from 

 winter ones which were thin (1-3 ju). A period of decreased growth was seen in 

 shell sections and was interpreted by them as reflecting reproductive events 

 which occurred at the beginning of summer deposition. These thin increments, 

 if related to spawning, should be associated with a spawning break in the shell 

 margin. Rlioads and Pannella (32) determined that the periods of highest stress 

 and mortality were different for juvenile and mature bivalves. Specimens 3.2 

 mm (generally less than 6 months old) died with greatest frequency from 

 summer to mid-autumn. Older individuals died primarily in late fall or early 

 winter. 



Ontogenetic Records of Environmental Change. 



In addition to episodic and periodic events, variations in environmental 

 parameters including food supply, the type of substratum, salinity, oxygen 

 content, turbidity, agitation, temperature, and population density can 

 influence growth of bivalves. Hallam (15) reviews these various environmental 

 parameters as causes of stunting and dwarfing in living and fossil marine 

 benthic invertebrates. Several studies conducted within the past few years have 

 used microscopic grov^h increments within shells to define the effects of 

 various environmental perturbations on bivalve growth. Rhoads and Pannella 

 (32), for example, through careful examination of both acetate peels and thin 

 sections, have demonstrated that examination of both acetate peels and thin 

 sections, have demonstrated that Mercenaria mercenaria grows faster in sandy 

 sediments than in mud when other variables are eliminated. Farrow (13) used 

 microstructural growth increments within the shell of Cerastoderma edule to 

 illustrate that dense populations of the cockles had a much shorter growing 

 season than sparse populations. An inverse relationship between individual size 

 and population density of cockles was also noted. In a subsequent study, 

 Farrow (14) used growth increments within the outer shell layer of this species 

 to demonstrate that individuals living high in the intertidal zone were stunted. 

 The higher shore cockles were situated near the high water mark, and, 

 consequently, were aerially exposed for several days during neap tides. 

 Following neap tide deceleration, there was a resumption of vigorous growth. 

 Many of the high intertidal cockles were some two-thirds the size of individuals 

 lower in the intertidal zone, where growth was more continuous. 



165 



