Jellyfish traverse the sea by ship
March 2006, CIESM News

The anthropogenic introduction of exotic species may threaten native marine biodiversity by altering foodwebs, altering the balance of coastal systems, or displacing endemic species. While damage and preventive control are pricey, predicting the introduction of an exotic species remains a challenge. To begin with, knowledge of natural dispersal patterns, species diversity and biogeography is often insufficient to distinguish between a natural and an exotic population. Now, scientists can use molecular genetic techniques in association with biophysical global ocean models to identify the origin of multiple introductions worldwide.

The jellyfish Aurelia has been the object of such a study * . By incorporating representative life-history characteristics of Aurelia into a computer model the limits of natural dispersion of this species could be investigated. The model, in particular based on the ocean circulation and sea temperature patterns, is supplemented by a high-resolution bathymetry data set to improve the modelled migration. After conducting the computer simulation of the possible movement of this marine species and combining with genetic data, the only logical mode of transportations appears to be ship transport.

Ships take in water to stabilise and, once in the destination port ** , discharge or exchange this ballast water. In this way, but also as aggregating fouling on the hulls, ocean-going vessels carry a number of species on their routes from harbour to harbour. This issue is the focus of one CIESM programme PORTAL and is repeatedly discussed in CIESM forums – see for example CIESM Monograph No  20. Thanks to maritime traffic, marine organisms like the medusa Aurelia voyage across the seven seas.

Various species of Aurelia (16 species have been identified in the study - showing, in passing, that marine biodiversity is higher than previously thought) have disjunct distributions, a likely characteristic of introduced species, and could not have migrated naturally to all the corners of the world where they are presently found. For instance, Aurelia aurita, commonly called the moon jellyfish for its pale, disk-like appearance, is endemic in the North Atlantic and disjunct in the Black Sea like the introduced ctenophore Mnemiopsis while Aurelia sp.8 occurs on both sides of the Suez Canal, i.e. has a “Lessepsian” distribution (see also CIESM Monograph No. 20).

Moreover, through the last centuries, the appearance of Aurelia in many locations coincided with historical data of increased shipping. Only in rare cases did the model identify routes of natural dispersal, for example a limb of an ocean gyre or an eddy, evidencing multiple introduction.

For additional CIESM information related to this topic:

- CIESM program PORTAL (PORT surveys of ALien organisms introduced by ships)

- CIESM Atlas of exotic species in the Mediteranean

- CIESM Monograph No. 20: “Alien marine organisms introduced by ships in the Mediterranean and Black seas.” (downloadable)

- Archives of CIESM congresses

- CIESM News Room Links: Jellyfish

* Michael N. Dawson, Alex Sen Gupta, and Matthews H. England: Coupled biophysical global ocean model and molecular genetic analyses identify multiple introductions of cryptogenic species. Proceedings of the National Academy of Science, Vol. 102, No. 34, 11968-11973.

** Currently, most of the strategies to deal with problems posed by ballast water in ports and terminals focus on minimizing transport and recommend open ocean ballast water exchange. However, only few countries have adopted such recommendations and requirements from IMO (the United Nations International Maritime Organization).