For years, scientists have been wondering how marine animals with limited mobility nonetheless manage to end up thousands of kilometers from their natural habitats. An Israeli study published this week shows that they use a special form of transport: the digestive system of fishes, in which they survive until they leave it for their new home.
The study, published in the journal “Limnology and Oceanography Letters,” was conducted by Tamar Guy-Haim of the University of Haifa and the Israel Oceanographic and Limnological Research Institute, Orit Hyams-Kaphzan and Ahuva Almogi-Labin of the Geological Survey of Israel, Erez Yeruham of the Israel Oceanographic and Limnological Research Institute and James T. Carlton of Williams College in Massachusetts.
The study focused on one-celled organisms known as foraminifera, which are found everywhere in the sea, from the shallowest water to the deepest. About 70 species of foraminifera have traveled from the Red Sea via the Suez Canal to the Mediterranean Sea, where they have become invasive species pushing out the locals. But scientists didn’t know how they managed to get there, given that they can move at the rate of only a few millimeters per hour. At that rate, it would take them around 4,000 years to traverse the 164 kilometers of the canal.
One theory was that they arrived in the ballast water used to stabilize ships. But checks of ballast water aboard ships that docked in Israel found no foraminifera.
The researchers therefore investigated another theory: that they arrived in fish which had swallowed them inadvertently. This is a known means of transport, but until now, it has only been confirmed in rivers and lakes rather than in the open sea.
To test this theory, the researchers examined two species of invasive rabbitfish that reached the Mediterranean via the Suez Canal: the marbled spinefoot (Siganus rivulatus) and the dusky spinefoot (Siganus luridus). Both fish eat marine vegetation, and in the process they sometimes ingest small animals.
The researchers caught a few dozen of these fish and examined their fecal pellets. Inside, they found 11 different species of foraminifera that were migrants to the Mediterranean.
The researchers also examined dead fish preserved in collections at the Hebrew University of Jerusalem. These fish had been caught over the span of several decades, and the researchers found invasive species of foraminifera in their bodies as well.
One of these species has become extremely common, comprising 90 percent of the foraminifera found on rocks in the Mediterranean. A map of all the places where the invasive rabbitfish and this species of foraminifera are found and the times at which they appeared shows a large overlap between both the route the two life forms traveled and the timing of their westward movement into the Mediterranean.
“The organisms we found can survive for several days in a fish’s digestive system,” Guy-Haim said. “We still don’t have an explanation for how they manage to survive, and that’s an issue we plan to investigate.”
But the new study clearly shows the variety of ways in which invasive species can reach the Mediterranean, and therefore raises concerns about the spread of such species, to the detriment of local life forms. The researchers believe the method of transport they identified in the Mediterranean is common elsewhere in the world as well.