Increased guarding duration reduces growth and offspring number in females of the skeleton shrimp Caprella penantis

Abstract

The reproductive strategy of one sex often imposes fitness costs on the other. In crustaceans, precopulatory mate guarding often imposes reproductive costs on females. In iteroparous species with indeterminate growth, such costs may affect not only current reproduction but also growth and lifetime fecundity. We investigated the cost imposed by male mate guarding on the long-term reproductive fitness of females in the skeleton shrimp Caprella penantis. We kept skeleton shrimps for 50 days in three different sex ratio treatments: male biased, even and female biased. Five female fitness components were recorded: survival rate, growth rate, number of clutches, clutch interval and average number of juveniles. Occurrence and duration of guarding increased in the male-biased treatment, indicating that males responded to changes in mate availability through increased guarding. Females under the male-biased sex ratio showed reduced growth rate owing to the longer guarding duration and were consequently smaller. No significant difference between treatments was found in the number of and interval between clutches, but, compared with the even sex ratio treatment, females in the male-biased treatment produced fewer juveniles on average than those in the female-biased treatment. These results indicate that extended guarding duration imposes long-term costs on female fitness in the form of decreased growth and low offspring production.