Ctenophores in the Baltic Sea are potentially contributing to the microbial loop

Gelatinous zooplankton have thought to have increased in abundance around the world due to climate and global change. Mertensia ovum was first reported in the Baltic Sea in 2007 and represents the only ctenophore species currently known to be permanently established there. However, the organisms living in the Baltic have much a smaller maximum size, 8mm, compared to arctic waters, 75mm. This could be due to the physiological adaptations that had to occur to enable the ctenophore to survive in a temperate climate and brackish waters.

In aquatic environments, the body size of the predator and the prey are linked to foraging success. Therefore it is unlikely that the Baltic population feed mainly on large copepods like the arctic population. Ctenophores capture prey with their tentacles and branching tentillae. The tentacles of individuals from the Baltic population only stretches up to twice its length and only possess a few branching tentillae. In comparison, the arctic populations have tentacles several times their body length and have hundreds of branching tentillae, enabling them to catch larger prey. Predation on mesozooplankton can cause injury in small-sized tentaculate ctenophores, such as in the Baltic population. From these observations, Majaneva et al., (2014) hypothesized that M. ovum in the Baltic Sea mainly consume micro-plankton and even smaller prey, such as the picocyanobacterium, Synechococcus spp.

Laboratory feeding experiments were carried out as well as a molecular gut analysis of the ctenophores. To quantify the abundance of Synechococcus spp. in samples a real-time PCR assay was completed along with a cell count in the culture using a haemocytometer. Additional water samples were also taken to examine the co-occurrence of M. ovum and its potential prey.

From these experiments they concluded that M. ovum showed a clear prey preference as the copepodites and copepod nauplii were hardly eaten compared to the smaller prey, ciliates and picocyanobacterium. It was also discovered that the quantity of Synechococcus spp. in the stomach cavity was increased dramatically with prey density, suggesting that the ctenophores from the natural environment are feeding on the picocyanobacteria close to their maximum limit. This indicates that the bacterio- and microplankton prey are highly important in the diet of this ctenophore. Patterns in the vertical and seasonal overlap of M. ovum and its prey also supports the theory that picocyanobacteria have a high importance in the diet of these organisms. These results imply that Baltic ctenophores are contributing to the coupling between the microbial loop and higher consumers in pelagic food webs.

I think that this paper was really interesting, however more information on the ecological implications of the ctenophore on the microbial loop is needed and how it affects higher consumers in pelagic food webs.

Majaneva, S., Setala, O., Gorokhova, E. and Lehtiniemi (2014) Feeding of the Arctic ctenophore Mertensia ovum in the Baltic Sea: evidence of the use of microbial prey. Journal of Plankton Research. 36:91-103