Publication Abstract

The rise of the invasive ctenophore Mnemiopsis Leidyi

The rise of the invasive ctenophore Mnemiopsis Leidyi

I. Gardiner, D. Speirs, L. Kelly, S* Pitois and J van der Molen*

The ctenophore Mnemiopsis leidyi, commonly referred to as the sea walnut, is a native of the Western Atlantic and is listed among the world’s 100 worst invaders, largely due to the devastation caused by its explosive population growth after accidental introduction via ballast waters into the Black Sea in the early 1980s. It subsequentlyappeared in the Caspian, Aegean, Azov, Marmara,North, Baltic, Skagerrak, and  diterranean Seas.From 2005 it has been recorded in the southern North Sea in coastal waters from northern France to southern Denmark. In some regions the its population density has remained low, but in the Black Sea a series of population outbreaks caused the collapse of anchovy stocks as a result of direct predation on the fish larvae and the reduction of other zooplankton on which both Mnemiopsis and the fish larvae feed. There is therefore continued concern about whether such outbreaks, with their associated economic harm to fisheries, are possible in the North Sea and if so under what conditions. However, forecasting potential Mnemiopsis outbreaks requires constructing an explicitly spatial population dynamic model which contains the key elements of its life-history in relation to important environmental drivers such as  emperature, salinity, and ocean currents. As a precursor to this challenge we here review the current state of knowledge of the ecology of this species, and summarize the quantitative information on demographic parameters. A key process is that temperature affects the development rate through the  egg, cydipped, transitional, and adult lobate stages.Fecundity varies with ctenophore size and is also sensitive to temperature, salinity, and food availability. Mnemiopsis is a self-fertilising hermaphrodite, which along with high egg production rates and fast development time may be a key feature in its capacity to reach high abundances from very low initial densities. There are no specialised overwintering stages, but feeding and reproduction stop as temperatures fall. This means that overall the demography seems to be dominated by source-sink dynamics in which population persistence requires regions with relatively low advection, such as coastal or estuarine regions. This suggests that driving population dynamic models by physical transport derived from ocean circulation models will be an essential component in developing forecasts.

I. Gardiner, D. Speirs, L. Kelly, S* Pitois and J van der Molen*

Marine Alliance for Science and Technology for Scotland: Annual Science Meeting, Heriot-Watt University Conference Centre, Edinburgh