Portugal’s first national update since 2015 identifies 231 non-indigenous marine species across the mainland, Azores and Madeira, while environmental DNA is opening a new frontier in the early detection of biological invasions
The ocean has always carried travelers. Long before ships drew routes across maps, currents transported larvae, seeds and microscopic life between distant shores, while storms and drifting material occasionally delivered unexpected organisms to new environments. But the modern ocean moves differently. Ships now cross entire seas in days, carrying water from one ecosystem to another in their ballast tanks and transporting living communities almost invisibly upon their hulls. Ports connect continents with an intensity unimaginable only generations ago, and the biological geography of the sea is changing alongside the economic geography created by global trade. For islands, whose ecosystems have developed through the peculiar combination of isolation and oceanic connection, these movements acquire particular significance. The Azores now have 81 confirmed non-indigenous marine species, according to a major new national scientific inventory that has updated Portugal’s knowledge of marine biological introductions for the first time since 2015. Across mainland Portugal, the Azores and Madeira, researchers have confirmed 231 non-native marine species, including five species never previously recorded in Portuguese waters. The new inventory, published in the scientific journal Marine Pollution Bulletin, represents the work of 31 researchers from 11 Portuguese universities, three national public entities and several research institutes, led by scientists associated with MARE — the Marine and Environmental Sciences Centre — and ARNET, the Aquatic Research Network. Yet the significance of the study lies not simply in producing a larger number. It provides a new map of how species are reaching Portuguese waters, where they appear to originate, what human activities are helping them travel and, perhaps most importantly, how science may begin detecting new arrivals before the human eye can even see them.
Eighty-One Species in an Archipelago Shaped by Isolation
Of the 231 confirmed non-indigenous species, 159 have been recorded in mainland Portuguese waters, 81 in the Azores and 62 in Madeira. These figures should not simply be added together because individual species may occur in more than one Portuguese region. Rather, they reveal the geographical distribution of confirmed non-native marine organisms across three distinct oceanic environments. The five species newly recorded for Portugal were discovered along the southwestern mainland coast near the Port of Sines, through a long-term environmental monitoring program that has followed the area since 1997. The fact that previously unrecorded organisms continue to be discovered after decades of scientific observation illustrates one of the central challenges of marine biological invasions: the ocean is vast, organisms can remain unnoticed for considerable periods, and the biological consequences of international mobility often develop beneath the surface long before they become obvious.
For the Azores, the presence of 81 non-indigenous species acquires additional meaning because oceanic islands are laboratories of isolation. Their natural communities developed at great distances from continental landmasses, shaped by the relatively small number of species capable of crossing enormous stretches of ocean through natural processes. Modern transportation alters that ancient filter. A hull covered with marine organisms or ballast water collected thousands of kilometers away can accomplish in days what nature might once have made extraordinarily difficult. This does not mean that every newly arriving species will become invasive or produce ecological damage; non-indigenous and invasive are not automatically synonymous. But identifying what has arrived, where it originated and how it may have been transported is essential to determining which species require particular attention and which pathways should be monitored most closely.
Two Atlantic Geographies of Arrival
One of the most revealing findings of the new inventory is that non-indigenous species do not arrive in the same geographical pattern throughout Portugal. Mainland waters show a strong connection with the temperate North Pacific: 39 percent of non-native species recorded on the mainland originate in the temperate North Pacific Ocean, while 24 percent originate in the North Atlantic. In the islands, however, the pattern changes. Madeira has a more pronounced Atlantic signature, with 27 percent of its non-indigenous species originating in the North Atlantic and 24 percent in the Tropical Atlantic. The Azores occupy an intriguing position between these patterns: 37 percent of their non-indigenous species originate in the North Atlantic and 19 percent in the North Pacific, giving the archipelago a profile that, despite its mid-Atlantic geography, is in some respects closer to that observed on mainland Portugal.
These percentages tell a story about an ocean increasingly interconnected by human movement. Biological geography is no longer determined solely by currents, temperatures and natural dispersal. Commercial routes have become ecological corridors. A ship does not transport only containers, fuel, passengers or merchandise; it can also carry an invisible biological cargo. Across Portugal, the study identifies maritime transportation—particularly ballast water and biofouling on ships’ hulls—as the principal pathway through which non-indigenous marine species are introduced. Aquaculture also plays a particularly relevant role on the mainland. The importance of maritime traffic is especially apparent around Sines, described in the study as a strategic entry point because the deep-water port lies at the intersection of maritime routes connecting Asia, the Mediterranean, the United States and Africa.
Ships Carry More Than Their Cargo
Ballast water illustrates the extraordinary unintended consequences of global mobility. Ships use large quantities of seawater to maintain stability, collecting it in one location and potentially releasing it somewhere far away. Within that water can be microscopic organisms, larvae and other forms of marine life capable of surviving the voyage. Biofouling operates differently but produces a similar geographical leap: organisms attach themselves to submerged surfaces of vessels and effectively travel with the ship. What appears from above to be simply another vessel entering port may therefore represent, beneath the waterline, the arrival of organisms from an entirely different ecosystem.
For an archipelago such as the Azores, situated within the great circulation routes of the North Atlantic, understanding these pathways is particularly relevant. The islands have always existed at a crossroads between continents, but their strategic geography has consequences beyond aviation, shipping, communications and geopolitics. The same position that gives the Azores importance in the movement of people and goods can also expose its marine environments to biological movement. The ocean that connects the islands to the world can become the road by which unfamiliar organisms reach them.
When DNA Finds an Arrival Before We Can See It
Perhaps the most fascinating dimension of the research lies in the development of a pioneering watch list containing 22 species detected exclusively through molecular techniques, including environmental DNA, but whose physical presence has not yet been confirmed through collected specimens. Environmental DNA—or eDNA—has transformed ecological monitoring by allowing researchers to search for genetic traces organisms leave behind in their surroundings. Instead of relying entirely upon seeing or capturing an organism, scientists can analyze environmental samples for molecular evidence suggesting that it may be present.
The result is something approaching an early-warning system for the ocean. A species may leave a genetic signature before researchers visually identify it in the field. This can potentially give scientists and environmental authorities valuable additional time to investigate an arrival, determine whether the signal can be confirmed and assess whether intervention is necessary. The distinction between molecular detection and physical confirmation remains important, which is why the 22 species are maintained on a surveillance list rather than simply being added to the 231 confirmed species. But the approach changes the possibilities of marine monitoring. Science is increasingly learning to listen for organisms genetically before it can see them biologically.
A Genetic Library Still Missing Portuguese Pages
The same molecular technology, however, has revealed a significant weakness in Portugal’s capacity for precise detection. According to the study, 71 percent of the species included in the inventory are represented in international genetic databases, but only 12.5 percent have genetic sequences derived specifically from specimens collected in Portuguese waters. In practical terms, much of the genetic reference material used to identify organisms in Portugal comes from populations collected elsewhere in the world. That may reduce the reliability of rapid detection tools such as environmental DNA and can make it more difficult to distinguish between species that are genetically very similar.
The problem resembles attempting to identify someone using a library of photographs in which the correct family is represented but the closest relatives are missing. The comparison may still work, but precision improves when reference material comes from the actual populations and environments being studied. Strengthening the collection and genetic sequencing of specimens from Portuguese waters is therefore identified by the researchers as a priority. The more complete Portugal’s own genetic reference library becomes, the faster and more accurately scientists can interpret the molecular signals appearing in environmental samples.
As Romeu Ribeiro, a MARE researcher at the Faculty of Sciences of the University of Lisbon and first author of the study, explained, marine invasions are “a dynamic and transboundary phenomenon” that require science capable of operating at the same scale. The importance of the project, he emphasized, lies in moving well beyond a numerical update: the research identifies patterns of introduction, gaps in genetic knowledge and priorities for future monitoring.
A National Scientific Network Watching a Shared Sea
The scope of the collaboration itself reflects the nature of the challenge. The work was coordinated by Paula Chainho of MARE at the Faculty of Sciences of the University of Lisbon (MARE-ULisboa) and involved the Directorate-General for Natural Resources, Safety and Maritime Services, the Portuguese Institute for the Sea and Atmosphere and the Institute for Nature Conservation and Forests, alongside researchers from 11 Portuguese universities and other research institutions. Funding includes support from the Recovery and Resilience Plan through the NEXUS Agenda and from Portugal’s Foundation for Science and Technology.
That breadth of cooperation matters because marine species recognize none of the administrative boundaries through which humans organize the sea. An organism transported into one Portuguese port can eventually appear elsewhere; a species detected around an island may belong to a much larger Atlantic movement. Monitoring therefore requires collaboration between universities, laboratories, environmental authorities, ports and regions, as well as scientific knowledge capable of moving between traditional taxonomy and increasingly sophisticated molecular technologies.
The Ocean Is Keeping a New Record of Globalization
The new inventory ultimately tells two stories at once. One concerns biodiversity: 231 confirmed non-indigenous marine species in Portuguese waters, 81 of them recorded in the Azores, together with another 22 species whose molecular traces have placed them on a scientific watch list pending physical confirmation. The other concerns the world human beings have constructed above the water. Ships move faster and more frequently, ports connect once-distant ecosystems, aquaculture transfers organisms, and the boundaries that once helped shape distinctive marine communities have become increasingly permeable.
For the Azores, this is another reminder that isolation has never meant disconnection. The archipelago is simultaneously remote and profoundly central, separated from continents by hundreds of kilometers of ocean yet positioned within one of the great maritime spaces of the planet. Its biodiversity developed through that tension between distance and arrival. Today, however, the speed and scale of arrival have changed.
The task of science is therefore not to imagine an ocean frozen permanently in one biological moment. Seas have always changed. Species have always moved. The challenge is to distinguish natural change from human-accelerated introduction, to identify newcomers before some become ecological problems, to understand the pathways that brought them and to build monitoring systems sophisticated enough to detect what the naked eye cannot.
For centuries, islanders looked toward the horizon to see what was approaching: a storm, a sail, a ship, a returning relative, perhaps news from another continent. Today, some of the most consequential arrivals cannot be seen from shore at all. They may be attached beneath a hull, suspended invisibly in ballast water or present only as fragments of genetic material captured in a laboratory sample. The horizon is still bringing strangers to the Azores. The difference is that now, to see some of them coming, we must learn to read the water itself.
Based on a story in Diário dos Açores, Paulo Viveiros-direcor. Photos from DA.