Effect of crowding on growth and reproduction of Simocephalus vetulus O.F. Müller

Abstract

Simocephalus vetulus was reared at five different densities, 1, 20, 40, 60 and 80 ind. (20 ml)^-1, to examine the effects of population density on growth and reproduction under a sufficient food supply using a flow-through technique. The carapace length of S. vetulus older than six-days, and the cumulative number of neonates produced decreased with increasing population density over 40 ind. (20 ml)^-1. This result demonstrates that both growth and reproduction of the animals are adversely affected by crowding even under a sufficient food supply, except for juvenile growth. Although the reduction of growth rate in adult stages could be interpreted as resulting from a decrease in available food by depression of feeding rate due to crowding, reduction of the clutch size was simply caused by reduction of their parent size as population density increased. Maturation age of the animals was the same at each density, but body size at maturation declined with increasing density. This response may be different from that of food limited animals, because delayed maturation determining body weight has been shown in food-limited conditions. Additionally, body size and weight of the neonates released at the highest density were greater than those at the lowest one. Reproductive investment was also greater at the highest density. This suggests that the crowded animals may allocate more energy to reproduction, then reduce their own body size, and finally produce large offspring, to provide for severe future food conditions due to crowding.