Population ecology of Muggiaea atlantica (Cnidaria, Siphonophora) in the Western English Channel

Recent observations suggest that the siphonophore Muggiaea atlantica is expanding its geographical distribution. The mechanisms behind this expansion remain unclear due to our limited knowledge of the species' ecology. We modelled the functional relationship between the 2 main life-cycle stages of M. atlantica over a 5 yr period (2009-2013) in the Western English Channel. Our aims were to determine the key features of the species' population dynamics and the influence of local environmental conditions on its population development. Our results highlighted a strong coupling between the timing of specific environmental conditions and the development of the M. atlantica population, thereby explaining interannual differences in the phenology of its blooms. Population development commenced with the initiation of eudoxid production by the overwintering polygastric stages. This reproductive event was linked to the onset of a spring temperature threshold, suggesting a critical basal limit of 10 degrees C for eudoxid production. Interannual variability in the timing of this threshold modulated the degree of mismatch between the developing M. atlantica population and the availability of copepod prey. Unusually cold conditions in the spring of 2010 and 2013 limited the capacity for M. atlantica to initiate eudoxid production leading to poor trophic phasing and the production of single autumn cohorts. In contrast, warmer conditions during spring 2009, 2011, and 2012 facilitated earlier population development, optimal trophic phasing and the production of both summer and autumn cohorts. These findings represent an important addition to our understanding of the ecology of M. atlantica in the Northeast Atlantic.