The Mole Crab Emerita talpoida (Say): A Case of Changing Life History Pattern

Mole crabs of the genus Emerita are successful colonizers of exposed sandy beaches, in spite of the severity of the physical environment. An adult population of Emerita talpoida (Say) was studied for 2 yr to determine possible changes of its life history pattern as a consequence of its physically severe habitat. Adult populations were sampled in four localities on Bogue Banks, North Carolina, USA, monthly during 1972 and in March, June and September 1973. Size distribution–in spite of some transient differences in the abundance of the larger individuals–was similar at all localities in each month of the year, and underwent dramatic simultaneous changes in spring and summer. The changes in spring resulted from rapid growth of overwintering animals. Changes in summer were produced by the recruitment of new individuals and the death of many large ones. Although size distribution was similar at all localities, significant differences in abundance were found in each month for all sizes except the smallest, suggesting that recruitment was simultaneous and of equal magnitude all along Bogue Banks. In spring, ovigerous females were larger, and carried a greater number of larger eggs. However in summer females were more abundant and produced several clutches. Most recruitment occurred in September as a result of the summer spawning. A greater proportion of females was observed in spring and fall. The spring peak was caused by death of the larger males. The fall peak was caused by a large recruitment of females (63%). When males died in spring, they were about half the size of the largest females observed. No increase in the number of medium—size females was observed in spring. In both sexes the mortality rates were constant, but greater for males, resulting in a shorter life expectancy for the latter. Thus it appears unlikely that sex reversal in males occurs in Emerita talpoida. Survival curves indicate that if sex reversal occurs in this species it would happen at a constant rate in all age intervals. Sex reversal was not observed under laboratory conditions. Life table parameters calculated from data for 1972 indicate that the population has expanded in the recent past, assuming egg production to be independent of density. This result, along with the appearance of larger females in 1973 (which were less abundant in 1972) suggests that in 1972 the population was recovering from mortality probably caused by two hurricanes in the fall of 1971.