Behavioral factors influencing movement, dispersion and mortality in Macrobrachium rosenbergii

Abstract

The purpose of this study is to examine the relationship between social behavior in M. rosenbergii and the utilization of resources. This shrimp is commercially aquacultured, and much of its life history is known. However, the behavioral elements influencing growth and contributing to mortality are little understood. Territoriality was examined through laboratory experiments and field observations. In the laboratory, single animals were placed singularly in large elliptical tanks that contained one shelter. One of the animals, originally in a tank by itself, was designated as an immigrant and introduced into another tank housing a single animal. The resident retained its shelter on the day of introduction in each of 18 trials. The prior resident advantage disappeared within two days and was replaced by a social system of size-determined dominance. Thus prawns can be considered territorial but confinement and repeated encounters shift the social system from territoriality to dominance. When two animals were introduced simultaneously, dominance was established by the largest prawn. Movement within a commercial pond was examined in the field through telemetry. Small sonic tags were placed upon the carapace of five males and four females. The movements of each animal were monitored for eight days. During that eight day period the day to day change in position was recorded. Recordings also were made at specific times throughout two nights. The results indicated that males had a stationary home range and females had a shifting home range. A field sampling study was undertaken in two commercial ponds to obtain background data on the population of prawns within the ponds. The populations within the two ponds were similar in density, sex ratio, and average animal size. The ponds differed in productivity, variability in mud depth, and degree in which numbers of prawns were correlated with mud depth. In one pond there was no correlation between mud depth and number of animals in any particular molt state. In the other pond, near ecdysis prawns (molt states A, B, and D3) were positively associated with mud depth while molt state C and D0-2 animals were negatively associated with mud depth. The pond with the highest productivity also was the most variable in mud depth, and the animals in this pond appeared to be segregating by molt state. The results from the field sampling indicated that prawns may be segregating by molt state into different microhabitats. The results of a two-way ANOVA indicated that molt state A, B, and D3 animals, when confined with molt state C (intermolt) prawns, were susceptible to behaviorally mediated death. Further testing, in large circular tanks, indicated that shrimp near ecdysis avoided intermolts by moving into suboptimal microhabitats. Animals near ecdysis were often killed by conspecifics, but it is unlikely that M. rosenbergii kill each other for food. Data on cannibalism in crustacea must be reexamined in light of the following cost/benefit minimum risk hypothesis: intermolt animals minimize their risk by killing near ecdysis prawns, and, at the same time, maximize their benefits by eliminating a competitor. Cannibalism or killing in M. rosenbergii is an extreme form of competition.