A team of researchers from the departments of Botany and Plant Physiology and from Geology and Ecology at the University of Málaga, in collaboration with the Department of Botany at the University of Granada, has demonstrated that the Asian macroalga Rugulopteryx okamurae expands along the Andalusian coasts by forming clones asexually, from small fragments of itself and through specialized and vegetative cells. The work analyzes for the first time the reproductive strategies of this invasive species in a colonized territory, specifically in the seagrass meadows of Posidonia oceanica on the Granada coast. The reproductive structures detected develop new complete individuals, capable of dispersing through the water and reaching other nearby areas.
The study, published in the 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", estimates that the macroalga could generate more than half a million potential specimens per square meter over a year, a figure comparable to the population of a city like Málaga. Scientists point to spring and summer as the periods of highest reproductive activity, when the algae produce more structures capable of originating new organisms.
These results help explain the rapid proliferation of the species and provide information to improve management and control strategies for this marine invader. "Although eradication is only successful with early detection and rapid and sustained action over time, it could be tested with population controls, applying selective removal strategies at the most appropriate times, to give native species a chance to thrive," explains researcher Jesús Rosas-Guerrero from the University of Málaga to the Fundación Descubre, co-author of the article.
Originating from the western Pacific, Rugulopteryx okamurae was first detected in Ceuta in 2015 by researcher María Altamirano from the University of Málaga, the principal author of this work. Since then, the macroalga has colonized large areas of the Andalusian coast, displacing native species and generating unprecedented ecological and economic impacts, among others, on the fishing sector and from the accumulation of biomass on the beaches that require costly removal and gathering labor.
To better understand the mechanisms explaining this invasion, the team conducted a year-long follow-up of a population of the algae colonizing a Posidonia oceanica meadow on the Granada coast, specifically in the Cala de Cambriles, where one of these ecosystems is protected by European regulations for harboring great biodiversity. A team of divers carefully collected samples every two months and, once in the laboratory, the scientists analyzed the thalli —the plant structure of the algae— classifying them according to their size. Subsequently, they checked how many presented reproductive structures and in what quantity, to estimate the potential for propagation among individuals of different sizes and at different times of the year.
Previous studies have described that, in its native area, the species reproduces through clonal mechanisms —propagules, small thalli capable of breaking off and growing to form a new specimen, or spores, cells that generate an individual when germinated—, but also sexually, through gametes and tetraspores, which provide genetic variability to populations. However, the results change when studying behavior in an invaded territory. "During the year analyzed, reproductive sexual structures were absent in the case of gametes, and anecdotal in the case of tetraspores, while asexual propagules and spores appeared in almost all of the algae's surface and during most of the year," Rosas-Guerrero specifies.
The propagules were detected practically in all seasons, while the production of spores concentrated mainly in spring and summer, with the larger individuals generating more units capable of forming new organisms. Based on this data, the team calculated the so-called propagule pressure, to estimate the invasive species' capacity for expansion in a given area. "We have discovered that the algae can produce more than half a million new individuals per square meter invaded by Posidonia, but the reality is that there is no seabed that can host such a quantity," asserts Rosas-Guerrero.
This finding leads the team to consider that some of those specimens remain in the water column, living and traveling in it to new areas, without the need to settle in a territory. This process would explain both the rapid expansion of Rugulopteryx okamurae and the presence of large floating masses along the coast.
Scientists point out that understanding these reproductive mechanisms is fundamental for designing more effective management strategies, especially in planning actions for control and removal of biomass, with the aim of minimizing ecological and socio-economic impact. "Our results can support managers, in their decision-making, to act against the worst invasive species in the Andalusian marine environment," concludes the researcher.