First records of Anemonia manjano (Carlgren 1900) in the Hawaiian Islands and the aquarium hobby as an increasingly important pathway for marine invasive species.

The ocean is rarely a static place; it is a dynamic system where species constantly move, adapt, and sometimes settle in new homes. When a species arrives in a place where it has never lived before, it is called an introduction. If that new arrival spreads quickly and harms the local environment, it becomes an invasive species. For decades, scientists have known that ships carrying ballast water or cargo have been the primary way marine life travels across the globe, accidentally hitching rides on hulls or in water tanks. However, a quieter, more personal pathway is gaining attention: the aquarium trade. Millions of people keep marine animals in tanks at home, and when these hobbyists release their pets or discard unwanted tank decorations into the ocean, they can inadvertently transport entire ecosystems. The question of how these small, intentional acts might reshape vast natural reefs is a growing concern for marine biologists, who are now watching to see if the colorful creatures in our living rooms can become the troublemakers in our oceans.

In the Hawaiian Islands, a team of researchers has documented the arrival of a new, potentially dangerous visitor: a sea anemone known as Anemonia manjano. This creature, which looks like a small, soft flower with pink-tipped tentacles, was first spotted in Hawaiian waters in June 2020. It was found in Kāne'ohe Bay on the island of O'ahu, growing among a patch of non-native stony corals that had been planted there earlier that year. The anemone was not alone; it was accompanied by a second, separate population discovered in February 2023 along the shoreline of Pearl Harbor, specifically near Ford Island. These two sightings, separated by time and distance, suggest that the anemone arrived through two different events rather than spreading from a single point. The species is native to the western Indian Ocean, where it is known to compete with and sometimes kill coral, but it had never been recorded in Hawaii before.

The researchers, working with the Division of Aquatic Resources and various university and military partners, quickly realized they were dealing with an established problem. In Kāne'ohe Bay, the anemone was found on a variety of surfaces, including live coral, dead coral rubble, and even the cement blocks of the reef. By early 2026, the area covered by the anemone in this bay had grown from a small patch to over 1,100 square meters. In Pearl Harbor, the situation was even more expansive. What began as a handful of individuals on mangrove roots and floating wood in 2023 had, by early 2026, spread to cover nearly three acres of the shoreline and shallow waters. The anemone was found at depths ranging from the surface down to about 25 feet, attaching itself to mangrove roots, driftwood, and the hard surfaces of the harbor floor.

What makes this discovery particularly urgent is the behavior of the anemone itself. In its native range and in aquariums, this species is known to be aggressive. It can kill other corals by using stinging cells in its tentacles to smother them. In the aquarium hobby, it is often considered a pest because it grows rapidly and is difficult to remove, sometimes hiding deep within rock structures where it is hard to see. In Hawaii, the researchers observed the same pattern: the anemone was growing over native corals and other sea life. The populations in both locations included individuals of all sizes, from tiny, nearly microscopic babies to large, adult specimens. This mix of ages indicates that the anemones are not just surviving; they are reproducing and establishing permanent communities.

The path these creatures took to reach Hawaii appears to be linked directly to human activity. In Kāne'ohe Bay, the anemones were found exclusively with non-native corals that were likely brought in for a reef restoration project or released from a private aquarium. The close association suggests the anemone arrived as a "hitchhiker," hiding inside the coral or on the rocks used for the project. In Pearl Harbor, the situation was slightly different. The anemones were found in a brackish, mangrove-lined area without the presence of the non-native corals seen in the bay. This suggests a different entry point, possibly from an aquarium release into the harbor or from debris floating in the water. The fact that two separate populations appeared in two very different environments points to the aquarium trade as a significant and growing vector for marine invasions.

Once the presence of the anemone was confirmed, the researchers moved to identify it with certainty. They collected samples from both locations and sent them to the Bishop Museum for examination. Using genetic testing, they confirmed that the specimens were indeed Anemonia manjano. This genetic confirmation was crucial, as the species can be easily confused with others, and its scientific name has been spelled in different ways over the years. The researchers also noted that the anemone is capable of moving on its own. In aquariums, these creatures can detach from their spot and "walk" to a new location, moving as much as a foot in four hours. In the wild, they can also spread by breaking off pieces of their own bodies, which then grow into new individuals, or by attaching to floating debris and ships.

The response to this invasion has been a race against time. In Kāne'ohe Bay, teams have been working to remove the anemones, sometimes covering them with epoxy to smother them. Despite these efforts, the population has continued to expand. In Pearl Harbor, access restrictions and permitting requirements have made it harder to conduct full underwater surveys, so much of the monitoring has been done from the shoreline. Even with these challenges, the data shows a clear trend: the anemone is spreading rapidly. In less than a year, the known area of the infestation in Pearl Harbor grew six times larger. The researchers describe the situation as an invasive infestation that requires immediate management.

The story of the Anemonia manjano in Hawaii is more than just a local problem; it is a warning about how marine life moves in the modern world. For a long time, the focus of invasive species prevention has been on large ships and their ballast water. However, this case highlights the power of the aquarium hobby. With millions of hobbyists worldwide and a growing number of people importing marine animals, the risk of accidental releases is increasing. The anemone's ability to thrive in different habitats, from open reefs to sheltered lagoons, and its capacity to reproduce quickly, make it a formidable invader. The researchers emphasize that without prompt action to control the spread and prevent future introductions, the ecological impact could be severe.

The work done by the team in Hawaii serves as a call to action for a broader change in how we manage marine introductions. It suggests that preventing invasions requires more than just monitoring ships; it requires addressing the aquarium trade directly. This means raising public awareness about the dangers of releasing aquarium animals into the wild and creating better systems for people to return unwanted organisms to facilities rather than the ocean. The rapid expansion of the anemone populations in just a few years shows that once these species are established, they are very difficult to stop. The researchers conclude that while the full extent of how these introductions will change the ocean is still unknown, the evidence is clear: the aquarium trade is a major pathway for marine invasive species, and it needs to be treated with the same seriousness as any other vector of biological change.