The waters off the Andalusian coast , especially near Granada and the Strait of Gibraltar , are undergoing a silent transformation. What were once dense meadows of Posidonia seagrass and kelp forests typical of the Mediterranean are now being replaced by almost continuous blankets of a brown macroalga that arrived from the western Pacific.
This species, identified as Rugulopteryx okamurae , has already established itself as one of the worst invasive species in the Andalusian marine environment. A series of recent studies conducted by Andalusian universities confirms that its success is based on a clear strategy: it spreads primarily through asexual and vegetative reproduction , forming veritable armies of clones capable of rapidly colonizing new seabeds.
An Andalusian project to decipher the invasion of the Asian algae
These results are the result of coordinated work by specialists in Botany, Plant Physiology, Geology, and Ecology from the University of Málaga (UMA), together with the Department of Botany at the University of Granada (UGR). The project is funded by the Regional Ministry of Universities, Research, and Innovation of the Andalusian Regional Government, ERDF funds, and supported by the Biodiversity Foundation of the Ministry for Ecological Transition.
The team set out to understand how this exotic macroalga manages to maintain and expand its populations in an environment that is not its own. To do this, they focused their efforts on an already invaded territory: a Posidonia oceanica seagrass meadow on the coast of Granada , a marine ecosystem specially protected by European regulations due to its enormous biodiversity.
The results have been published in the scientific journal Marine Pollution Bulletin under the title "Reproductive performance and propagule pressure: Key drivers of Rugulopteryx okamurae (Ochrophyta, Dictyotales) invasion of a Posidonia oceanica meadow", where it is detailed how asexual reproduction has become the key to this invasion.
Ceuta 2015: the beginning of a dazzling expansion
The recent history of Rugulopteryx okamurae in our waters begins in Ceuta, in 2015. It was there that the researcher from the University of Malaga, María Altamirano, first detected the presence of this macroalga from the western Pacific, the area where it originates.
Since that first observation, the spread has been rapid and continuous. In less than a decade, the species has colonized large areas of the Andalusian coast , especially in the Strait of Gibraltar and the Granada coast, displacing much of the native marine flora. Its dense mats compete with Posidonia seagrass and other native algae, profoundly altering the underwater landscape that many divers knew just a few years ago.
This change has more than just ecological consequences . The massive accumulation of biomass on beaches and in ports creates a logistical and economic problem: local authorities are forced to organize costly removal and collection campaigns , while small-scale fishing and other professional activities are hampered by the presence of large masses of algae in nets and gear.
The combination of environmental and socioeconomic impacts has led the scientific community and administrations to consider Rugulopteryx okamurae as the most problematic invasive species in the Andalusian marine environment at the moment.
A year following the algae in a Posidonia meadow
To understand what lies behind this capacity for expansion, the team from the University of Malaga (UMA) and the University of Granada (UGR) chose as their natural laboratory a population of Rugulopteryx okamurae invading a Posidonia seagrass meadow in Cala de Cambriles , on the coast of Granada. This area harbors one of the most valuable underwater ecosystems on the Andalusian coast, making it ideal for studying the clash between the invasive species and the protected marine vegetation.
For a full year, a team of scientific divers conducted regular dives every two months to collect samples of the algae without causing further damage to the Posidonia oceanica seagrass meadow. The protocol involved carefully removing representative fragments of the invasive population, which were then analyzed in the laboratory.
Once at the scientific facilities, the researchers examined the thalli —the structural part of the algae, equivalent to the plant's "body"—and classified them into different size categories. The goal was to determine the proportion of thalli with reproductive structures and how many reproductive units each individual produced at different times of the year.
This detailed monitoring made it possible to quantify the intensity with which the algae multiplies and estimate its real potential for expansion in a habitat already occupied by a key species such as posidonia.
Asexual reproduction: clones, fragments, and spores
In its native range in the western Pacific, Rugulopteryx okamurae was known to reproduce both sexually and asexually . Gametes, tetraspores—which contribute genetic variation to populations—, propagules, and various types of spores have been described in its natural habitat there.
However, in the invaded territory analyzed on the coast of Granada, the pattern changes radically. Throughout the sampling year, researchers found hardly any structures of sexual reproduction : gametes were not detected, and tetraspores appeared only anecdotally.
In contrast, the presence of structures related to asexual reproduction was massive. The team identified abundant asexual propagules and spores , responsible for the clonal multiplication of the species , across virtually the entire surface of the thalli and throughout much of the year.
Propagules are small fragments or tiny thalli capable of detaching from the parent plant and, once released, growing into a new specimen . These units were detected at virtually every station, indicating a continuous capacity to generate new clones.
Asexual spores , on the other hand, were concentrated mainly in spring and summer, periods identified as the times of greatest reproductive activity for the invasive species. These are specialized cells that, upon germination, give rise to complete individuals without the need for sexual fertilization.
More than half a million individuals per square meter
Using the information collected on the number of propagules and spores produced by thalli of different sizes, scientists calculated the so-called propagule pressure , an indicator that estimates how many new individuals a Rugulopteryx okamurae population can generate in a given area of invaded grassland.
The results are striking: according to estimates, in a single year the macroalga could produce more than half a million potential individuals per square meter of Posidonia seagrass meadow. That is, in an area similar to that of a large table, a number of individuals comparable to the population of a city like Málaga could originate.
Clearly, there isn't enough seabed for all these individuals to settle and grow on the substrate. This disproportion leads the team to suggest that a significant portion of these new organisms remain in the water column , living and moving without needing to attach to the seabed.
This behavior aligns with the increasingly frequent presence of large floating masses of algae along the Andalusian coast. Fragments and clones that cannot find a suitable location drift with the currents, reaching other coastal areas and facilitating the colonization of new sites, often many kilometers from the original nucleus.
The study also confirms that larger thalli produce a higher number of reproductive units, so that large individuals act as centers for the emission of propagules and spores, constantly feeding the invasion.
Impact on Posidonia, marine fauna and economic sectors
The spread of Rugulopteryx okamurae has a direct impact on Posidonia oceanica seagrass meadows , one of the most important marine habitats in the Mediterranean. These meadows serve as a refuge and breeding ground for numerous species, help stabilize sediments, and play a vital role in water oxygenation and quality.
By colonizing these enclaves, the Asian algae competes with Posidonia for the space and lighthindering its development and altering the ecosystem's structure. Where Posidonia leaves and a mosaic of native algae once predominated, extensive mats dominated by the invasive species are now observed, resulting in changes to the community. de peces, invertebrates and other associated organisms.
The impact is also noticeable in the fishing activityThe banks de peces Seafood is affected by habitat transformation, and the large amounts of algae in nets, traps and other gear pose an additional problem for professionals in the sector, who must invest more time and effort in cleaning their equipment.
On beaches, the massive influx of Rugulopteryx okamurae seaweed forces local councils and other administrations to allocate significant financial resources to its removal and management . This is compounded by concerns about the image of coastal areas for tourists, as the presence of large piles of seaweed can be off-putting to visitors.
Given this situation, various research groups are also exploring possible uses for the removed biomass, from industrial uses to applications in new materials or biotechnological processes, with the idea of revaluing at least part of the problem generated by the invasive species.
Scientific keys to improving management and control
Understanding in detail how Rugulopteryx okamurae reproduces and disperses is not just an academic matter. The information obtained allows for the design of more effective management strategies to combat this marine invasion, tailoring actions to the time of year and the characteristics of the populations.
Researchers point out that the complete eradication of a species with this capacity for expansion is only viable when it is detected very early and action is taken swiftly and sustainably. In the case of the Andalusian coast, where the macroalga is already firmly established in many areas, the approach involves population controls and measures to limit its impact.
Among the options proposed is the selective removal of biomass during periods of peak reproductive activity —mainly spring and summer—, so as to reduce the number of propagules and spores generated and, consequently, the pressure of propagation on new areas.
These interventions, experts point out, should be accompanied by actions that promote the development of native species , particularly posidonia and other algae native to the Mediterranean, allowing them to recover some of the space currently occupied by Rugulopteryx okamurae beds.
Detailed information on reproductive cycles, propagule density and the behavior of floating masses provides coastal administrations and managers with a solid scientific basis for planning removal campaigns and other mitigation measures with greater precision, prioritizing areas and times when intervention can be most effective.
As data accumulates on this macroalga, the challenge lies in translating that knowledge into concrete decisions that will contain its spread and minimize damage to marine ecosystems and the human activities that depend on them. Coordinated work between universities, government agencies, and affected sectors will be key to ensuring that, despite the presence of this invasive species, the Andalusian coastline retains the greatest possible degree of biodiversity and ecological function.