We studied the fitness effects of animal personality by measuring activity

and its relation to survival in the marine isopod Idotea balthica. We asked

(1) whether activity could be considered to be a personality trait, (2)

whether this trait is connected to survival, and (3) whether personality

and survival exhibit sex differences. We found that activity fulfilled the

criteria of personality as individuals had consistent between-individual

differences over time and across situations. Consistent individual differences

in activity were associated with fitness as the survival probability of

active individuals was lower, but this did not depend on sex. Our results

demonstrate that personality exists in I. balthica and support recent suggestions

that the association between personality and life-history traits is a

central component in mediating animal personality.