Divers from the Alive Foundation and Project Sèpia have discovered new settlements, two years after they reported the presence of a very dense population in Cala Montgó.
Caulerpa cylindracea is a tropical seaweed native to the Indian Ocean that makes bright green stems that grow parallel to the ground and take root in each part. These stolons can grow up to 1 cm a day and can overgrow any organism on the seabed, making it difficult for them to survive (calcareous algae, sponges, bryozoans, corals and gorgonians).
It was detected in 2008 for the first time in Catalonia, on the Garraf coast, and since then it has been spreading along the Catalan coast. On the Costa Brava, it was cited in 2012 and, subsequently, it has been detected at several specific points. But so far it has not shown the aggressive behavior that has been observed in other parts of the Mediterranean.
Their dispersal is facilitated by ship anchors and fishing gear, which tear off fragments and inadvertently transplant them. But also by sea currents, which are a mode of transport for their spores, when they are released into the open sea and which can fall and settle in any corner of the coast.
In Cala Montgó, a Caulerpa half a square meter growth was detected in 2022 at a depth of 18 meters, on a precoraligenous rocky bottom in an area exposed to currents and where no boats usually anchor. This August 2024 this population was observed again with a dense network of stolons overgrowing the calcareous algae that build the sea bottom and affecting other benthic organisms.
But this year, a second settlement of Caulerpa has also been detected, about 20 meters from the previous one. This new observation is a of 15 meters depth on a rocky bottom with a precoraligenrocky bottom with some photophilous algae, rarely affected by ship anchors. At this point, a possible predation of the sphaerechinus granularis on Caulerpa cylindracea has been observed. But we must be cautious, as this statement will need to be corroborated with laboratory experiments.
And finally, this year a third population of Caulerpa cylindracea has also been discovered at a depth of 9 meters growing on a bed of scree with dead matte and photophilous algae, next to the Posidonia oceanica meadow. Its stolons cover the different benthic organisms that live there. Here it is common for pleasure boats to drop anchor and plow the bottom with anchors and chains, so it is likely that this situation has favored the establishment of the alga in this particular enclave.
The biologist who monitors the Caulerpa, Boris Weitzmann, remembers that “in the sea it is almost impossible to stop the expansion of these invasive algae, but we can act to avoid dispersing it more quickly. For example, we can avoid throwing the anchor in the places where the invasive algae grows so as not to break off fragments that are dispersed and fixed in new places.”
Weitzmann has worked at the CEAB-CSIC for 10 years monitoring invasive species in the Catalan sea and has been collaborating with the Montgrí Natural Park, Medes and Baix Ter Islands and the Cap de Creus National Park for 14 years.
The surveys were carried out thanks to the support of Patricia Besem, Bep Pallí and Andreu Pujades.
