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The Hitchhiker’s Guide for Hostile Species

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First-of-its-kind study shows how invasive marine species survive under surprising environmental conditions.

The phenomenon of marine animals invading distant regions endangers local marine environments and their resident species. A new study from Tel Aviv University included a pioneering experiment simulating the changing environmental conditions encountered en route by marine animals ‘hitching a ride’ by clinging to the bottom of container ships, traveling with the ship to distant regions around the globe. In this study, researchers demonstrate that suitable regulation can decrease this phenomenon and prevent potential invaders from reaching new habitats.

The study was led by research student Doron Bereza under the supervision of Prof. Noa Shenkar of the School of Zoology at the George S. Wise Faculty of Life Sciences and the Steinhardt Museum of Natural History at Tel Aviv University. The paper was published in the prestigious journal Science of the Total Environment

 

“At any given moment, thousands of marine creatures travel from one location to another by marine vessels.”

 

Harmful to Local Species

The experiment demonstrated that the animals’ ability to survive the arduous journey depends on factors like the type of vessel and the route it navigates, as well as the changing temperature and salinity of seawater.

According to the researchers, the routes of vessels of different sizes are determined mainly by technical limitations of infrastructures at different ports, as well as economic trends in the shipping industry. This results in unique geographic routes that create completely different sets of environmental and other challenges for creatures attaching themselves to these vessels. 

“At any given moment, thousands of marine creatures travel from one location to another by marine vessels,” says Prof. Shenkar. “They do this in two different ways: in the ballast water – seawater taken on by the vessel for stabilizing, or by clinging to the ship’s hull. The problem of invasive species transferred by ballast is addressed by legislation, but the ‘hitchhikers’ clinging to the ships are not – and thus numerous species are transferred from place to place along international trade routes.”

 

Prof. Noa Shenkar

An experiment conducted by research student Doron Bereza, together with Prof. Shenkar, examined the survivability of two species of ‘ascidians’ [marine invertebrates, or cold-blooded animals with no backbone], known to be harmful, on a journey that follows a typical trade route – from Southeast Asia to Northern Europe. Ascidians attach to hard surfaces such as rocks, breakwaters, and ship hulls. There are hundreds of species of ascidians, and the rise in global trade enables some opportunistic species to disperse over great distances, sometimes establishing themselves as invasive species and harming both marine infrastructures and local species in their new habitats.

Doron Bereza: “We focused on two species of ascidians that are common in the Mediterranean, including Israel, and are known to be transferred by ships. I created a comprehensive database, comprising info from about 200 container ships, and used it to build a route representing the trade routes of two different types of container ships – giant vessels, over 395m in length, vs. ‘regular’ container ships that can be served by the infrastructures of more harbors. In addition, I collected data about changes in seawater temperatures and salinity, as well as chlorophyl concentrations, as a measure for the availability of food on the voyage and at the different ports along the way.”

 

“We were surprised to discover that one tropical ascidian species survived the entire journey to Rotterdam. This does not mean that the creatures enjoyed their trip, but the fact is that they did survive, and just a few individuals are sufficient for launching an invasive population in the new territory.” 

 

Making their Trip Unbearable

In the second stage of the study, the researchers exposed both species of ascidians to similar conditions in the lab. Bereza: “We discovered that survivability was significantly impacted by several factors: environmental conditions, the type of vessel, and traits of the animal itself. Under extreme conditions, found in some eastern ports, such as a combination of high temperatures and low salinity, one species died out completely, while no mortality was observed in the other species.”

“In real life, even when routes are generally similar, these ports are not visited by ships over a certain size, for lack of suitable infrastructures. Thus, we concluded that docking at ports with different extremes in conditions can significantly diminish the survival chances of specific species clinging to the ships. Additional experiments of this kind, specifically addressing groups of marine animals that pose a threat, can lead to effective regulatory measures for preventing the conveyance of species.”

Prof. Shenkar adds: “We were surprised to discover that one tropical ascidian species survived the entire journey to Rotterdam. This does not mean that the creatures enjoyed their trip, but the fact is that they did survive, and just a few individuals are sufficient for launching an invasive population in the new territory. Moreover, global warming is expected to enable tropical species to thrive in water that is still too cold at present. The fact that the environmental conditions in some ports on the way proved deadly to almost all members of a certain species, suggests that such locations may be utilized as environmental barriers to prevent the spreading of invasive species.”

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