Diversity and Physiological Tolerance of Native and Invasive Jellyfish/Ctenophores along the Extreme Salinity Gradient of the Baltic Sea

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.

Neural System and Receptor Diversity in the ctenophore Beroe abyssicola

Although, neuro-sensory systems might evolve independently in ctenophores, very little is known about their neural organization. Most of the ctenophores are pelagic and deep-water species and cannot be bred in the laboratory. Thus, it is not surprising that neuroanatomical data are available for only one genus within the group - Pleurobrachia. Here, using immunohistochemistry and scanning electron microscopy, we describe the organization of two distinct neural subsystems (subepithelial and mesogleal) and the structure of different receptor types in the comb jelly Beroe abyssicola - the voracious predator from North Pacific. A complex subepithelial neural network of Beroe, with five receptor types, covers the entire body surface and expands deeply into the pharynx. Three types of mesogleal neurons are comparable to the cydippid Pleurobrachia. The predatory lifestyle of Beroe is supported by the extensive development of ciliated and muscular structures including the presence of giant muscles and feeding macrocilia. The obtained neuroanatomy atlas provides unique examples of lineage-specific innovations within these enigmatic marine animals, and remarkable complexity of sensory and effector systems in this clade of basal Metazoa.Graphical AbstractAlthough, neuro-sensory systems might evolve independently in ctenophores, very little is known about their neuroanatomy. Here, using immunohistochemistry and scanning electron microscopy, we describe the organization of two neural systems and five different receptor types in the comb jelly Beroe abyssicola - the voracious predator from North Pacific. The predatory lifestyle of Beroe is supported by the extensive development of ciliated, muscular, and nervous systems including the presence of giant muscles and exceptional feeding macrocilia. The obtained neuroanatomy atlas provides unique examples of lineage-specific innovations within this enigmatic group of marine animals.