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 fj.) 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. Rhoads 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 growth 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 
