summer growth rate, while growth of the cockle, Clinocardium nuttalli, which 
inhabited a neighboring mud flat, was slowed by as much as a factor of 19. 
In addition to rhythms based on periodic environmental fluctuations, 
biological rhythms, such as breeding periods, are also reflected in growth 
increment clustering. “Breaks” due to spawning events are less severe than 
winter breaks. They are preceded by little or no slowed growth, and recovery is 
more rapid than after winter breaks. In most bivalves, spawning occurs during 
the summer, sometimes more than once a year. In Mercenaria mercenaria , 
reproductive breaks do not occur until the second year of growth (32). 
Semiperiodic and Random Events 
Depositional breaks in bivalves also result from semiperiodic or random 
events such as storms, unseasonable temperatures, attacks by predators, and 
environmental pollution disturbances. The irregular nature of such events 
makes them easily separable from periodic or cyclical shell-secretion rhythms 
(18, 30, 32). 
Storm breaks, a common feature, may have different characteristics 
depending on the severity of the storm and depth at which the bivalves live 
below the water surface. In any case, these breaks appear suddenly and are 
followed by a rapid return to increments of pre-storm width (9). Shuster (33) 
noted that during storms, silt became trapped between the mantle and the shell 
in My a arenaria and was subsequently incorporated into the shell. Trapped silt 
within shell indentations formed by storm breaks has also been observed in 
Mercenaria mercenaria (9, 18). 
The Season, Age, and Frequency of Reproduction and Death 
Growth patterns can supply detailed information on the age of individuals 
at time of death and their season of death. The age at death will be: 
2 
where Ad is age at death and N s and N w are, respectively, the number of 
summer and winter bands in the shell (32). The season of death is determined 
by relating the position of the last increment at the margin of the shell to the 
seasonal growth pattern. For example, a margin preceded by a complete 
summer depositional record represents death in early fall. A margin which 
follows a long period of winter growth represents late winter or early spring 
death. Often, the shell margin is preceded by a few days of growth slowdown, 
and comparison of several individuals may be necessary in order to determine if 
the slowing down in growth represents a moribund condition prior to death, or 
is related to seasonal changes (32). By counting the number of increments in a 
164 
