Laphocladia and rugulopteryx, coming from the Red Sea and Japan respectively, are two new algae that now frequent the French coasts of the Mediterranean.
"Unfortunately, we can expect to discover more and more," says oceanographer Sandrine Ruitton, responsible for these two discoveries.
Ultimately, with global warming and the introduction of new species, “a total change in the composition of algae and completely bare rocks” in the Mediterranean is possible, underlines the researcher.
Laphocladia, a new algae originating from the Red Sea and the Indian Ocean was observed for the first time in the Port-Cros National Park (Var).
A discovery made by Sandrine Ruitton, teacher-researcher at the University of Aix-Marseille, attached to the Mediterranean Institute of Oceanology.
Last year, it was rugulopteryx, an alga native to Japan, and until then confined to the Etang de Thau, that she reported in the creeks of Marseille.
Successive discoveries that raise questions about the ecological dynamics of the Mediterranean.
Where does Laphocladia come from?
This algae, it has been nearly 100 years since it was introduced into the southern Mediterranean via the Suez Canal.
It comes from the Red Sea and the Indian Ocean.
The digging of the Suez Canal in 1869 allowed the passage of more than 650 species in the Mediterranean.
There are probably many more passing through, but some of them cannot acclimatize to the Mediterranean, so we don't even realize it.
And then, there are some that manage to settle and become naturalized... And among all these species, some pose problems and become invasive, that is to say there will be negative effects on the environment. or on economic activities, such as fishing.
Does it compete with Posidonia?
Is she threatening her?
To grow, it needs a support.
She can't grow on sand.
It can be rock, but also posidonia stems.
It is therefore a concern that we are considering.
In the south of the Mediterranean, it colonizes the mattes of Posidonia – the stems under the leaves – and covers them completely and enters into competition with Posidonia, modifying the associated fauna and favoring the imprisonment of sediments at soil level.
In addition, it contains many toxic molecules.
So it doesn't make herbivores want to eat it.
And that's why it's so successful, because there are no known organisms that consume it.
Last summer, in the creeks, rugulopteryx was observed...
It had never been observed in France in the open sea, but had been present in the Etang de Thau since the 2000s, introduced by Japanese oysters, and had never been invasive since the Etang de Thau is very cold in winter.
And then, it was there with 50 other species of Japanese algae, so it's a bit in its midst, with natural competition.
The Etang de Thau is a bit like the Sea of Japan in France.
Are these two seaweeds comparable?
So no.
They are completely different mechanisms.
Rugulopteryx is characteristic of cold temperate waters, therefore it is not at all related to global warming.
On the other hand, Laphocladia is clearly the introduction of a species via the opening of the Suez Canal and an extension linked to global warming.
For 30 years the Mediterranean has taken one degree on average in the zone of 0 to 20 meters.
It may not be much in absolute terms, but enough for new species to settle.
Can we hope that Laphocladia does not overwinter?
I do not think so.
Because we are currently in the middle of winter.
And when we dived in Port-Cros, the water was at 14 degrees and she looked in great shape.
We remember Taxifolia, which appeared in the 1990s. It threatened to take over Posidonia and there had been uprooting campaigns.
Eventually, she ended up disappearing on her own.
Can we expect the same from these algae?
Taxifolia is a species that does not reproduce sexually.
So we're not going to have a mixture of genes that will allow this species to have high-performance genetic characteristics.
We imagine that it is the same clone which allowed the colonization of the Mediterranean and that this clone is genetically exhausted.
The mixing of genes also allows species to adapt to new conditions and competition with other species.
We can also imagine that native species have developed competitive strategies that also limit its expansion.
Can we expect the same genetic exhaustion from the two new species?
It's a little bit the hope that we would have.
So for Laphocladia, we have not yet found a reproductive organ in the Mediterranean, but in the other regions, there are.
So she doesn't seem affected by this phenomenon.
For Rugulopteryx, no reproductive cells have yet been found and, moreover, even in Japan, there is very little sexual reproduction for this species.
It spreads mainly by cuttings and very rarely by reproduction.
So that's hope.
That of telling oneself that for 20 years, it will be in an expansion phase and that afterwards there will be a phase of stabilization or regression which will allow it to integrate into the native vegetation without having too many negative effects.
What could the French coasts look like in 20 years?
In 20 years, I don't think that will have changed much.
But if we look at the longer term, we have to see how the southern Mediterranean has already changed radically, in particular with the species from the Red Sea which acclimatise very well there.
For example, there is a herbivorous fish, the rabbitfish, which is native to the Red Sea and has adapted very well there.
So we don't have it on our shores yet.
But the day it will reach us, it will have a very strong impact on algae and marine vegetation, and we can imagine a total change in the composition of algae and completely bare rocks because the algae will be consumed by these fish .
Posidonia meadows, there too, if there are these herbivores arriving, risk regressing.
Two new species of algae in two years, is that an acceleration?
What's next you expect?
So we don't make such discoveries very often.
But unfortunately, we can expect to discover more and more.
Now we are waiting for Red Sea fish and especially rabbitfish.
It was sighted and netted by fishermen, but never settled.
We imagine that individuals go on an adventure a little to the north and arrive as far as ours, without spending the winter.
But we will one day.
There is also the lionfish, the lionfish, the flute fish… These are species that we can expect to see arriving soon.
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