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Density shapes patterns of survival and reproduction in hydromedusa Eleutheria dichotoma

Dańko, A., Schaible, R., Pijanowska, J., Dańko, M. J.

, 19 pages (2017). Unpublished.

Keywords: animal demography, demographic ageing, reproduction rate, senescence

Abstract

High population density is a feature found among many groups of hydromedusae. Crawling hydromedusae populations can grow very quickly because they can reproduce sexually and asexually. We investigate the effects of population density on the reproductive strategy and the survival of medusae Eleutheria dichotoma. We find that medusae maintained in different densities grew to roughly the same sizes at maturity, which suggests that density had a minor effect on individual growth rates. However, the effects of increased density on fitness were generally negative, as we observed a decreased budding rate, a decrease in the sizes of the sexually produced larvae, increased mortality among the medusae, an increased rate of aging, and decreased unobserved heterogeneity in mortality risks. Interestingly, the sexual reproduction rate was the highest in medium-density conditions, which suggests that resources were shifted from tissue maintenance to sexual reproduction. This is the first study that has investigated the role of density for fitness components of crawling hydromedusae. Our results help to explain the mechanisms of the formation and the persistence of aggregations of hydromedusae in small water reservoirs, as well as the estimation of the fitness costs of crowding for medusae.

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