polyphemus size and age. The analysis of cohorts demonstrated that C. fornicata in Pleasant Bay can be divided into 5 size cohorts (Fig. 1A), with C. fornicata of approximately 4–6 mm in length appearing to represent the most recent spatfall. The cohorts differed in abundance, reflecting different magnitudes of recruitment from year to year. Growth rates in this study did not decrease with increased size and age (Fig. 1B). This may be due to low numbers of larger (50 mm 1) and older C. fornicata. Published data of sizes and ages (1, 2) match those found in this study, and thus confirm the conversion from size to age of the C. fornicata. The largest C. fornicata found resident on a horseshoe crab was 58 mm. This size C. fornicata could be from 8–11 years old (2). There was no evident relationship between maximum length and age of C. fornicata and size of the host horseshoe crabs (Fig. 1C). Male horseshoe crabs were consistently smaller than females, but in both sexes the length and presumed age of C. fornicata varied greatly, and was independent of the size of the crab. It is not possible to establish a strong relationship between true horseshoe crab length and the length of the C. fornicata upon it. At most the data of Figure 1C support that a minimum age can be calculated by adding the maximum C. fornicata length on a given horseshoe crab to the minimum age of horseshoe crabs at maturity. Using 9 years as the age at maturity (4), the crabs in this study were from 12 to 17 years old. This study was funded by the Woods Hole Marine Science Consortium and a grant from the Friends of Pleasant Bay.