Predation by feeding-current foraging medusae can have detrimental effects on prey populations. Understanding the mechanics that control prey selection and ingestion rates with different types of prey enables us to better predict the predatory impact of these medusae. We quantified the outcomes of each post-entrainment stage of the feeding process in multiple scyphozoan jellyfish species to understand how post-entrainment feeding events influence feeding patterns. Using 3-dimensional video, we observed and quantified the fate of both passive and actively swimming prey that were entrained in the feeding current of 5 different scyphomedusan species belonging to the orders Semaeostomeae and Rhizostomeae. Less than 65% of entrained prey contacted the capture surfaces (termed contact efficiency) of the semaeostome medusae, while the rhizostome medusae came into contact with less than 35% of the prey entrained in the feeding current. However, when contacted, prey were very likely to be ingested (> 90%) by all species examined. These results suggest that prey capture by oblate medusae appears to be largely limited by the probability that prey entrained in the feeding current will contact a capture surface. As a passive process, this contact stage of the feeding process is directly affected by the morphology of the contact surfaces. The importance of the contact stage of the feeding process suggests that differences in prey selection patterns observed among oblate medusan taxa are likely dominated by the morphology of contact surfaces as opposed to traits which influence the other stages of the feeding process, i.e. bell shape and nematocysts.

No abstract

Global change has led to manifold changes of marine ecosystems and biodiversity world-wide. While it has been shown that certain jellyfish and comb jelly species have increased regionally, it remains to be investigated if this is a general trend or localized phenomenon. Especially for the economically important Baltic Sea, which is characterized by an extreme physical environmental gradient, this question has not been addressed to date. Here we present a detailed account of the gelatinous macro-zooplankton community including their physiological tolerance towards abiotic conditions and resulting distribution ranges in the Baltic. We show that the arrival and establishment of non-indigenous species has led to a rising importance of jellyfish and comb jellies in the Baltic. This accounts for the comb jelly Mnemiopsis leidyi, which was first observed in Northern Europe in 2005, as well as for the hydromedusae Blackfordia virginica, first sighted in 2014. Both species have been shown to attain high population densities with pronounced grazing impact in other invasive regions. Given the current and anticipated changes of the physical environment of the Baltic Sea, especially ongoing warming, amplification of their impact can be expected.