Seasonal variations in growth and reproduction of the earthworm Perionyx excavatus (Oligochaeta: Megascolecidae)

 One-week-old juveniles of Perionyx excavatus were cultured in urine-free cow manure for 16 weeks in three different seasons to find out the influence of environmental factors on growth and reproduction. The biomass of worms increased gradually until the 16th week in all seasons. There was no significant (F=2.51) variation in biomass between the three seasons; however, the growth rate varied significantly (F=50.49, P<0.01). During the pre-clitellar period, the growth rate (mg/g) increased steeply (1013.17±41.33 in the monsoon season, 728.64±15.5 in winter and 463.16±22.96 in summer) until the worms attained maturity (5–8 weeks) and from then onwards gradually decreased after the commencement of cocoon production. Worms attained maturity when the mean weight of each worm ranged from 167.46 mg to 197.13 mg. The cumulative number of cocoons varied significantly (F=26.07, P<0.01) with season. Similarly, the rate of cocoon production also varied significantly (F=62.06, P<0.01). The average rate of cocoon production was 7.23, 0.99 and 0.53/worm/week in the monsoon season, winter and summer, respectively. By week 16, the ratio of weight gain to the initial weight of 1-week-old juveniles was 1 : 43.13 in the monsoon season, 1 : 30.49 in the winter and 1 : 23.69 in the summer. The maximum mass gain, growth rate, and cocoon production and earliest attainment of maturity were recorded during the monsoon season, and were attributed to the lowest range of fluctuating temperatures and high humidity compared with during winter and summer. The growth rate of worms in all seasons was inversely proportional to the number of cocoons produced. Further, the higher the rate of cocoon production, the lower the weight of individual cocoons.