ASSESSING LIFESPAN BIAS IN THE FOSSIL RECORD USING CHIONE ELEVATA AND MERCENARIA MERCENARIA FROM A TIDAL FLAT IN NORTH CAROLINA
This study assesses the occurrence of lifespan bias by comparing the live and dead abundance of two shallow, infaunal bivalves – Chione elevata and Mercenaria mercenaria – that co-occur in tidal flats in Middle Marsh, Beaufort, North Carolina. Chione elevata commonly lives 3-4 years in North Carolina, while M. mercenaria can reach 25 years old. Preliminary data suggest that living C. elevata are less numerous than living M. mercenaria, but are more numerous than M. mercenaria in the death assemblage. This discrepancy suggests that lifespan bias might be altering the relative abundance of these two species, and further analysis is necessary.
Samples were taken from 0.25 m2 x 0.2 m plots, sieved at 5 mm in the field, and all M. mercenaria and C. elevata specimens were collected. Collection is ongoing. Intial data from 256 specimens show that mean and median height are 13.22 mm and 10.00 mm, respectively, for C. elevata and 28.82 mm and 14.27 mm, respectively for M. mercenaria. The difference in height among live specimens is more pronounced. Mean height of live M. mercenaria and C. elevata are 46.28 mm and 14.81 mm, respectively; medians are 57.11 and 11.57, respectively. The large difference in sizes serves as a proxy for the difference in average ages of these species until increased sample sizes allow age-frequency distributions to be plotted.
The ratio of live to dead C. elevata (1:37.75) is much less than for M. mercenaria (1:8.18). The ratio of live C. elevata to live M. mercenaria is 1:2.75; the ratio of dead C. elevata to dead M. mercenaria is 1:0.60. Chione elevata is overrepresented in the death assemblage compared to the live community. These discrepancies, if borne out with additional data and analysis of the age frequency distribution of each species, would be consistent with the effect of lifespan bias.