Foul a Boat! Distribute Your Genes!

The ocean is an incredible place with vast amounts of organisms living beneath the surface. There are millions of species that we can see and those that we cannot. Some of these organisms can also spread from one location to the next by attaching to the bottoms of ships and taking a ride to a new location. This is called boat fouling, a marine form of Uber, and it is exactly how the Star Sea Squirt made its way around the globe. In this study, we show how the sciences of genetics and marine history help us to understand the history and the current spread of this marine species. 

The star sea squirts whose scientific name is Botryllus schlosseri (Figure 1) are marine animals composed of many connected and identical individuals. One Star Sea Squirt consists of many individuals that share the same blood and nervous systems and a common body covering called tunica. This form of organism is called a colony. Star sea squirts are permanently attached to submerged objects, like the bottoms of boats, and are found in marinas and ports worldwide. By attaching to the bottoms of boats, they can hitch a ride to many different locations and spread their genes.

The star sea squirt is an invasive organism, one that is not native to an area. It can spread rapidly due to a lack of natural predators or controls in the new location and can even harm the environment. It originated in the Mediterranean Sea and Atlantic coasts of Europe but has since spread around the world. This power of invasion is due to the Sea squirts’ abilities to settle on the bottoms of ships and submerged parts of docks. The settling is known as ‘fouling’. When these ships stop to dock,  the star sea squirts can distribute their offspring into the water of the new environment. Thus, the star sea squirt’s worldwide distribution is because they can foul boats and travel the world. When vessels move through the seas, they bring the squirts with them.  

Over the past three decades, our research team, along with other teams, gathered samples from star sea squirt colonies living in marinas and ports. We wanted to know more about how these organisms spread around the world. It is important to note that the collection of animal and plant samples from around the world is a major activity in biological science. The samples allow scientists to study a variety of organisms’ body structures, genetic traits, ecology, and evolution. 

Our research team focused on samples from the Mediterranean Sea and European Atlantic Ocean coasts (Figure 2). 

These regions are believed to be the sites of origin of the Star Sea squirts. We wanted to learn more about how the Star Sea squirts were distributed along these coasts. To do this, we collected the genetic information, in the form of DNA, from star sea squirt samples. DNA is the complex molecule that contains all the genetic information necessary to build and maintain an organism. In other words, an organism’s genes are made up of DNA. All living things have genes that are made up of DNA within their body cells. Think of DNA as a software responsible for all the functions of the body. Every single organism has unique DNA in its body cells, allowing researchers to distinguish between the organisms. The DNA that was taken from the collected samples was used to draw information about the genetic differences between colonies and between populations of colonies. This genetic information helped us to get better knowledge about the distribution of the species. It also helped us understand the relationships between different distant populations and whether they exchange genes.  

To do this, we used two types of genetic identifiers known as genetic markers. These are short sequences of DNA that are found in the body cells of all living creatures. We used two types of markers: one type is named COI and the other type is called a microsatellite. These markers are short segments of DNA. The markers vary between individual organisms within a population because each organism has a different genetic makeup. This variability is even higher among different populations. This feature is helpful for studying the genetic differences between populations because it allows us to spot different populations based on their genetic differences.

In the present study, we analyzed 1008 COI and 995 microsatellite markers from star sea squirt colony samples. These samples were obtained from 64 Mediterranean Sea and European Atlantic sites (Figure 3). We looked for similarities and dissimilarities in the genetics of the animals between these different sites.

We wanted to assess how similar or different the populations of star sea squirts from the different sites were. When organisms are genetically similar, it is believed that they share a common ancestor and might be relatives. When organisms are genetically different, we believe they are not related and likely do not share a common ancestor, while genetic similarity means that they do share a common ancestor. A common ancestor is an organism or species that lived long long ago from which two or more distinct groups came. For instance, cats and tigers have a common ancestor as well as humans and gorillas.  

We found significant genetic differences between the eastern and western Mediterranean Sea sites and also differences between these sites and European Atlantic sites. These findings encouraged us to look for possible reasons for these results. When the star sea squirt colonies are carried by fouled ships, the genetic information that is found in their DNA is transferred with them. This DNA is passed on to their offspring and is disseminated along the shipping routes used by the fouled vessels. These genes are passed down the generations for many years. Accordingly, we hypothesized that the present-day genetic distribution of the colonies reflects past different shipping lanes. 

In order to test our hypothesis, we examined data from previous studies detailing shipping frequency, directions, and main ports across the Mediterranean Sea and European Atlantic coasts. Additionally, we gathered information about the Mediterranean Sea and the Atlantic Ocean to understand why the ancient mariners had chosen particular sea routes. This information included wind directions and sea currents for over 3000 years! The results of this historical survey revealed that humans have sailed the Mediterranean and European Atlantic waters since the 9th century BCE - that’s almost 3000 years ago! Centuries of humans traveling the oceans have established complex coastal and cross-seas navigation networks.

Ships traveled along the same, primarily coastal, sea routes for centuries. This was due to constraints like wave and wind directions as well as limitations of navigation tools and ship design. Trips lasted for weeks and months. Ships stopped along the route for water and food replenishment, rest, and extended port-docking periods. These conditions were perfect for the settlement and persistence of fouling marine organisms on submerged surfaces, like the star sea squirts. Once the ship stopped, the star sea squirts were able to hop on and make their home on the bottom of the boat. They then traveled with the sailors to the next stop. On the other hand, star sea squirts that already fouled the boat could release their offspring into the water. These offspring could now foul other boats and so on.

Based on this historical seafaring data, we identified three major Mediterranean/Atlantic Europe maritime routes and their central ports that served most of the marine transportation (Figure 4). The Eastern Mediterranean route (yellow) towards Greece and further East had Venice as its central port. The Western Mediterranean route (green) towards Southern France, Spain, and the Strait of Gibraltar used Genoa as its central port. The Northern route (red) towards western Europe, the English Isles and Scandinavia centered in the port of Gibraltar. A fourth route circled the Italian peninsula between Venice and Genoa (purple), connecting the eastern and western Mediterranean routes.

We studied and analyzed the distribution of the star sea squirts genetic markers in our samples along each of these four historical seafaring routes. We found genetic differences between the routes due to different variants and different frequencies of the genetic markers in each of the routes. These results show that global maritime traffic significantly contributes to the transfer of marine organisms and their DNA. Long-term seafaring routes can establish a ‘genetic memory’ of historical sea voyages in ancient populations of marine organisms. This genetic memory may persist for centuries into present-day population genetics.

By using genetic markers, we found a link to past sea-voyages on current population genetics of a fouling organism, the star sea squirt. Centuries of European seafaring have influenced the current seascape genetics of the star sea squirt. Next time you visit a marina and see growing organisms on the submerged parts of a boat, think about the ways their ancestors traveled around the world. 

Written By: Eitan Reem

Academic Editor: Neuroscientist

Non-Academic Editor: Local High Schooler

Original paper

• Title: Historical navigation routes in European waters leave their footprint on the contemporary seascape genetics of a colonial urochordate. 

• Authors: Eitan Reem, Jacob Douek and Baruch Rinkevich.

• Journal: Scientific Reports. 

• Date published: 04 November 2023

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