Studies on the Life Cycle of Campeloma rufum, A Fresh-Water Snail

(1) Campeloma rufum in the Salt Fork River at Homer Park, Illinois, has been subjected to field and laboratory analysis of population samples taken at irregular intervals over a period of four years. (2) Collections of active snails have been taken at all seasons of the year, including winter. (3) Contrary to all recorded observations, separate sexes do not occur in C. rufum in the habitat under consideration. (4) More than 1,500 specimens representing every season of the year have been examined critically, but neither males nor protandrous hermaphrodites with any sort of copulatory organs have been found. All available evidence points to parthenogenesis for this species. (5) This is the first recorded instance of lack of males in the genus Campeloma. (6) Shells of uterine young range in height from about 1.5 to 4.5 mm. (7) Parturition is most active in May but extends through summer to about September 1. (8) Field collections and adult snails kept in laboratory aquaria over winter give evidence that young are carried in the uterus through the entire winter without any being liberated. (9) The new generation born in May has a mode of shell height of 4 mm. By September, these have reached about 12 mm. By the following May, at the age of one year, these have reached about 22 mm. (10) The smallest bearing individuals are about 20 mm in shell height. (11) In a number of collections a second mode of distribution occurs at about 30 mm. These individuals seem to be two years of age. (12) Forty mm is the maximum for Campeloma rufum in the habitat under consideration. These largest individuals are interpreted as probably three years of age.