Population dynamics and factors controlling somatic degrowth of the common jellyfish, Aurelia aurita, in a temperate semi-enclosed cove (Kertinge Nor, Denmark)

Abstract

Life expectancy of the scyphomedusa Aurelia aurita seems closely linked to seasonal shrinkage, or somatic degrowth, which occurs synchronously with sexual reproduction in temperate regions. In the present study, the mechanisms controlling body mass losses and subsequent disappearance of medusae were examined by following seasonal dynamics in population density, individual size, and sexual reproduction of A. aurita during 2013 and 2014 in the shallow, semi-enclosed Kertinge Nor, and the adjacent Great Belt, Denmark. After sexual maturation in early summer, medusae were characterized by a distinct phase of somatic degrowth, expressed by weight-specific individual growth rates of −0.5 to −1.4 % day−1, which was followed by the absence of A. aurita populations in both ecosystems during winter. The number of planula larvae per female (N L , ind.) was positively correlated with medusa diameter (d, mm), following the exponential relationship N L = 160.8 × e 0.029d . The percentage of body mass made up by planulae ranged from 6 to 11 % in Kertinge Nor and 10 to 33 % in Great Belt. ≤15 % of total size-specific body mass losses were due to the release of planulae in Kertinge Nor, suggesting shortage of zooplankton prey during autumn as main factor causing degrowth in A. aurita. Starving medusae produced significantly fewer oocytes and revealed increased mortality compared to well-fed individuals in supplementary laboratory experiments, indicating that degrowth controls the energetic investment into sexual reproduction. Seasonal variability in food supply, rather than energy allocation to reproduction, appears to shorten life spans of A. aurita medusae in temperate Danish waters.