Sea Grapes

Source: One Species at a Time/Ari Daniel - January 8, 2018 in Radio

Sea Grapes
Photo: James Steamer

Sea grapes may sound like something Poseidon would snack on, and not a killer algae. Yet Caulerpa racemosa var. cylindracea poses a serious threat to marine life. Spread by the bilge water of boats, this fast-growing alga is quick to take root, squeezing out native species. But there is one spot in the Mediterranean where cylindracea hasn’t yet taken over, and biologists like Juan Manuel Ruiz Fernández are trying to discover why.


Ari: From the Encyclopedia of Life, this is: One Species at a Time. I’m Ari Daniel.

This is a story of plants duking it out. We’ll start in southwestern Australia… where you can find a kind of algae called Caulerpa racemosa variety cylindracea. It also goes by the easier-to-pronounce name “sea grapes” because of the rounded side-shoots that dot the little branches. Cylindracea evolved off Australia, and it’s part of a web of creatures that compete, but coexist. It gets eaten by urchins and fish. It fights other species of algae for light and turf. Sometimes it wins, and sometimes it loses. All these species, and many others, keep each other in check.

Now, cylindracea isn’t like pond algae. It doesn’t grow on the water’s surface. Instead, it anchors itself to the sea bottom, and to rocks. But cylindracea didn’t stay put in Australia. Back in the early 90s, it showed up in the Mediterranean Sea, off Libya.

Fernández: What exactly happened nobody knows.

Ari: But chances are, says Juan Manuel Ruiz Fernández – a biologist with the Spanish Institute of Oceanography, it hitched a ride on a ship traveling from Australia to the Mediterranean…in the ballast waters. The theory is that a ship took on seawater in Australia and sucked in some cylindracea by accident. Once the ship got to the Mediterranean – somewhere close to Libya – it discharged that water. And the cylindracea just slipped out into the Med.

Now, it’s important to know what the Mediterranean looked like before cylindracea got here in the 90s.

Fernández: The native algae in the Mediterranean Sea are brown and red algaes.

Ari: In addition, there are these lush meadows of Posidonia – a green sea grass that rings the Mediterranean. Posidonia roots itself in the sea bottom and grows only where light can penetrate – within 150 feet of shore. Ruiz Fernández has gotten to see Posidonia up close.

Fernández: It’s really nice – you can see the sea grass move from one side to other, following the movement of the waves. It’s a feeling of peace.

Ari: Ruiz Fernández talks about Posidonia like a kind of underwater forest. It provides shelter for countless fish and invertebrates, it helps prevent erosion and keep the water clear, and it stores carbon. The sea grass leaves even change their colors depending on the season.

Ok, so that’s the picture of the Mediterranean before 1990. Thick meadows of Posidonia. Carpets of brown and red algae. It’s what things looked like when cylindracea – the green algae from Australia – showed up on the scene in the early 90s.

And it wasn’t long before cylindracea was on the move again – this time within the Mediterranean, which turned out to be a perfect new home for the algae. The temperature, the sunlight, the water – everything was just right for cylindracea.

Fernández: The algae have filaments that grow very fast.

Ari: As the filaments were slowly creeping along the seabed and over rocks, boat anchors and fishing nets inadvertently dragged cylindracea from one Mediterranean port to the next. And gradually, day by day, cylindracea fanned out, coating vast stretches of the Med. This algae has become one of the most aggressive invaders this part of the world has ever seen. By 2000 – in less than 10 years – cylindracea was flourishing off the coasts of 10 countries besides Libya, and the big Mediterranean islands.

The reason for cylindracea’s conquest is that usually when it comes up against one of the native brown or red algaes, it wins. Cylindracea just grows faster … so it’s not even a contest.

Fernández: And this is the problem. The invasive algae almost in every place.

Ari: Cylindracea’s settled much of the rocky terrain, but there is one place where it can’t seem to get a grip.

Fernández: The sea grass meadows.

Ari: The Posidonia sea grass meadows, which continue to thrive in the shallow, sandy waters along the Mediterranean coast.

Fernández: The hypothesis is that inside sea grass meadows, the algae don’t have enough light to grow and survive.

Ari: So it sounds like this invasive algae, it’s changing things – I mean it’s not causing an entire crash of the Mediterranean ecosystem.

Fernández: We don’t really know if this will be good or bad. This may be a new Mediterranean landscape with which we must live.

Ari: It’ll take time to know whether the changes will be subtle or severe. And it’s this uncertainty that’s fueling Ruiz Fernández. He’s set up a network of stations off the Spanish coast to monitor the presence and growth of cylindracea. His ultimate goal is to find a way to contain it.

Fernández: If we can protect sea grass meadows in the Mediterranean, we can control the spread of the invasive algae in every place.

Ari: For now, there’s no real winner in this struggle between these plants. And Ruiz Fernández is working to make sure there aren’t any big losers either. The seafloor isn’t something most of us get to experience everyday. But its daily rhythms and balance are critical to the health of life both above and below the surface.


We’ve just created a Google Earth tour to illustrate this story of the invasive sea grapes. Take a look at eol.org.

Our series, One Species at a Time, is produced by Atlantic Public Media in Woods Hole, Massachusetts. I’m Ari Daniel Shapiro.


The Encyclopedia of Life has gathered information from all over the world to create the largest repository of species known to science. Their mission is to increase awareness and understanding of living nature through an Encyclopedia of Life that gathers, generates, and shares knowledge in an open, freely accessible and trusted digital resource.

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