By: Richelle Tanner


The giant kelp forests of Catalina provide not only a subaquatic wonderland for divers, but also a habitat for countless marine organisms. However, their existence is jeopardized by a number of invasive species, namely Sargassum horneri (hereafter referred to as S. horneri). This species has no common name in English, but is referred to as akamoku in Asia, where it is native (Saccardi). S. horneri and S. filicinum, once thought to be two different strains but recently proven to be both classified as the horneri species by Uwai et al. in 2009, is an invasive seaweed found along the coasts of Southern California, the Channel Islands, and Baja Mexico. It behaves similarly to other invasives of S. CA such as S. muticum, Undari pinnatifida, and Caulerpa taxifolia. S. horneri is found in the intertidal zone and up to depths of 19 m, and exhibits a brown fern-like quality with many air bladders, often growing up to 20 feet (Smith). Its forests can grow to be very dense, blocking out sunlight and stealing nutrients and substrate from native species like the giant kelp. It is very apt at reproducing, as it is capable of self-fertilization (both the male and the female are in one plant) and matures early in its lifespan; this makes for rapid growth and out-competition of native species for resources. S. horneri is an annual species, completing its lifespan in less than a year – the dead individuals, often housing creatures like barnacles, diatoms, invertebrates, and even baby giant kelp plants, drift away carrying reproduction-capable structures of their own to colonize new areas. This colonizing mechanism makes the S. horneri difficult to contain; the amount of beached alga from this species is second only to the giant kelp (Miller). Removal of the S. horneri from S. CA and Baja Mexico is necessary to the livelihood of the natural ecosystem, and the current challenge is how we will accomplish this task.

S. horneri origins:

This seaweed originated in the Northwestern Pacific off the coast of Korea, Japan, and China, extending to Hong Kong (Uwai). It is a key species in the ecosystem there, as it provides a habitat for marine invertebrates and fish, holds the role of primary producer, and is a traditional food source for people of the region. It is an effective biofilter for the nutrients released off of the mainland, and is able to take up a significant percentage of the nutrients discharged. S. horneri forms large, dense, floating biomasses in the intertidal and subtidal zones, as well as in offshore surface waters. This is one of the main attributes of S. horneri that makes it well suited for the NW Pacific ecosystem. Its lifecycle is unique, in that it is relatively short, yet the lifecycles of the organism as a whole overlap. Ironically, populations are on the decline (due to rapid development and urbanization on the densely populated land) in select regions off the Japanese coast, leading scientists to search for ways to bolster the populations in what will soon be, or are already, barren benthic wastelands (Uwai). In the Eastern Pacific, there are no such problems – we can’t seem to find a way to rid ourselves of the weedy organism.

S. horneri in Southern California and Baja Mexico:

This seaweed was first discovered on the coast of California in Long Beach in October 2003. Most of the individuals collected were mature and capable of reproduction, yet even the juveniles were reproductive, due to their ability to spawn as early as four months into their life (Miller). This was quite concerning, as an organism with the capability to reproduce at almost any stage in its lifespan is able to multiply significantly and potentially outcompete native species with a less rapid reproductive rate. S. horneri has since spread along the Southern California coast, extending to Baja Mexico and the Channel Islands (more on these later). By 2005, S. horneri had become commonplace along Terminal Island and both the Ports of Long Beach and Los Angeles. By 2009, it could be found all along the coast of Baja Mexico (Riosmena-Rodríguez). Many scientists compare its spread to a similar S. muticum, which was found along the coast in the 1970s. S. muticum is another invasive Sargassum that is native to the NW Pacific, establishing itself prominently in the coast waters of California in the last 50 years.

But how did these seaweeds migrate and flourish on the other side of an ocean basin? Both Sargassum strains are thought to have hitched a ride on the exterior of shipping containers as well as in the ballast water held within them. This migration of S. horneri is not merely observed, it has been confirmed by genome sequencing. The S. horneri specimens collected in the United States were found to have had the cytochrome oxidase subunit III gene (cox3), which was matched with species found in the Seto Inland Sea in Japan in 2007. This particular cox3 sequence was 469 pairs in length, and matched perfectly with samples from three sites in the above-mentioned Japan site (Uwai). Further studies along the coast, namely in Mexico, have provided further evidence in gene sequencing to prove that the invasive weed that plagues our coastline is indeed the same strain found in Japan. These gene strains can be found in the gene database, GenBank, for public use (Riosmena-Rodríguez).

S. horneri on Catalina Island:

This “devil weed” was collected on Catalina Island north of Big Fisherman’s Cove near Two Harbors and USC’s Wrigley Institute in April 2006. An estimated 30 plants were discovered, at varying stages of reproductive maturity, within the four to twelve meter sublittoral zone. The largest of the plants were one to two meters in length, and were encrusted with bryozoa – this indicated that they had been there for a significant amount of time (Miller).

The introduction of S. horneri to the Channel Islands, beginning with Catalina Island, is debated. There are two distinct possibilities for the origin: tankers ships passing by the islands on the way to the ports or via pleasure boats from the US mainland. The transport via pleasure boats seems more plausible, as their actual docking at Two Harbors and the Wrigley Institute increase the chances of depositing polyps and possibly juvenile organisms. For the tankers ships to have deposited the organisms, ballast water from the ship or organisms latched onto the hull of the ships would have to be driven by currents to land along the shores of the Channel Islands. Both explanations are entirely possible and could have equally contributed to the growing infestation of Catalina Island with S. horneri. The species on Catalina Island have been confirmed to contain the same cox3 sequence found in species in Long Beach and Japan. Not only is S. horneri being spread anthropogenically, it is known for its ability to disperse for reproduction purposes. The air-filled “bladders” aid the scattering of fertile fragments of the organism to neighboring areas. This makes removal at an early stage in their lifecycle a high priority.

Current removal efforts:

There are very few widespread removal efforts of the S. horneri, and it is not kept in check by predation by sea urchins or other invertebrates and fish. In this habitat that they have been transplanted in, they have no natural predators, as they harbor toxins called polyphenols. The spread of S. horneri patches has become so extensive and thick that it is beginning to outcompete organisms, even the giant kelp, for sunlight and nutrients. It will eventually come to the point where native organisms are forced to feed on the S. horneri, or face starvation. Currently organisms are starting to adapt to the seaweed’s presence, with giant kelp fish nesting in it and other organisms feeding on the encrusting invertebrates living on it (Bushing). Therefore it is imperative that widespread removal is initiated, or else the workings of the natural ecosystem will be further disrupted.

Currently there is little being done to remove the seaweed from Catalina waters. By law, only ten pounds of marine substrate can be taken from the waters by a diver within the time span of a day, as long as they have a fishing license (the area around the USC Wrigley Institute is a Marine Protected Area – nothing can be taken out of the water). This means that a great number of divers would be required to remove even a small swatch of S. horneri – considering the small population of long-term residents of Catalina Island and the even smaller percentage of them that are certified divers, this plan will never be completely successful. Dr. Bushing has shown through his own informal monitoring projects that, if removed completely from an area, the seaweed will not grow back. He set up three areas that he removed the S. horneri completely from, and in two of the three, it did not grow back. Dr. Bill Bushing created a video to request a permit from the CA Department of Fish and Game to remove more of this invasive species in 2011, but it has not been granted yet. The proposal for the permit has not been written yet, due in part to the fact that the CA Dept. of Fish and Game is expecting Dr. Bushing himself to fund the project. He believes this is not right, as it is partially the fault of this department for not listening to scientists’ concerns 10 years prior when the S. horneri first came to Southern California (Bushing). He is currently seeking private trustees to fund the project.

Another removal effort being conducted is where divers uproot the invasive plants and release them to be carried away by the current. This may or may not be constructive, considering that this is one of their natural tactics for reproduction and spreading. Even though it may be an immediate solution to the infestation in specific areas on Catalina Island, this may actually promote a wider reach of the species in the Southern California and Baja Mexico area. Unfortunately the reality is that the S. horneri is so widespread already, removal and release will do little to expand the reach of the species; it will only strengthen existing populations (Bushing).

Removal efforts/successes of other invasive seaweeds:

A success story in the saga of invasive seaweed removal is with the Caulerpa taxifolia (C. taxifolia), commonly used in home aquariums. Discovered on the California coast in 2000 near San Diego in Agua Hedionda Lagoon, it was already on a “watch list” of invasives due to its 15-year history wreaking havoc in the Mediterranean Sea. Due to the increased awareness about the detrimental effects of its spreading nature, eradication was a priority from the get-go. A group of state, local, non-governmental organizations (NGOs), and private groups formed the Southern California Caulerpa Action Team (SCCAT), dedicating time and resources to the extermination of this species from the western hemisphere, as this was the first occurrence (Monteiro).

It was not necessarily the removal method that differentiated this occurrence, but the timing with which it happened: only 17 days passed between the discovery of the C. taxifolia and the initiation of the removal method. The removal method agreed upon involved isolating the invasive with PVC piping and tarp and pumping in a solution of sodium hypochlorite. Later a simpler method was developed involving similar materials, but solid chlorine-releasing tablets that were much easier to handle for the divers implementing the procedure. A major concern in this technique is in water contamination, as special permits and research authorizations had to be obtained from the CA EPA for the use of sodium hypochlorite in such high concentrations (Monteiro). However, it was determined that the chlorine would dissipate relatively quickly and pose few major risks to native species. This was confirmed in later studies, as the C. taxifolia and its seeds were not found anywhere in the area, while the native eelgrass had survived, even when treated with the chlorine. The aggressive stance taken by the SCCAT is both criticized and admired, as it solved the problem effectively but did not allow for non-eradication methods such as scientific research regarding the spread to contribute to the conversation. The project’s relatively high-anticipated cost ($1.2 million per year) also drew skepticism, but the venture was largely successful in the long term.

Possible actions for removal:

There are a number of possible directions that we can take on this issue. The removal actions being taken currently are not sufficient enough to curb the S. horneri populations, as they are continuing to grow out of control. A lesson to be taken from the eradication of the C. taxifolia is time efficiency. Obviously it is many years past how long the C. taxifolia was left unkempt, but actions taken now will be more effective than actions taken a year from now. Dr. Bushing’s video is well informed and, if approved, the permits could potentially lead to control over the S. horneri populations. However this method requires disposing of the individuals while they are still in their juvenile state. This means that scuba divers would constantly have to monitor populations, as the life cycles of the seaweed overlap and there could potentially be juvenile presence year-round. The methods utilized by SCCAT are honorable in their vigor; however, the use of highly saturated chlorine tablets in a marine protected area (MPA) such as the cove near the USC Wrigley Institute would hardly be condoned.

Instead, I suggest an alternative method, one that takes from Japan’s increasing problem of this particular seaweed disappearing from their waters. Although the causes may be largely anthropogenic, it would still be beneficial to do toxin analyses on the now dead S. horneri. If there is an organic compound that is not harming the other organisms in Japanese waters that almost “targets” this particular seaweed, Catalina Island may have found its solution. This is not to say we should imitate what keeps the S. horneri in check in Japan outside of anomalies, as introducing sea urchins into the environment will also wreak havoc on the native giant kelp.

With the populations helplessly out of control, it is inevitable that eradication is impossible. However, Dr. Bushing raised a compelling point – our greatest motivator is economic benefit. S. horneri can be used for food, biofuel, and fertilizer; recently he received a call from a prospective biofuel manufacturer using the seaweed. If S. horneri can be marketed as a desirable product, its removal will be pushed along by market demand – the supply is, for all intensive purposes, limitless.


Eradication of a species is never an easy task; it is often questioned whether we should facilitate it in the first place. In this case, it is a necessity if we wish to keep the native species functioning as an ecosystem. Not only does the S. horneri crowd out other organisms for nutrients and sunlight, it often impedes safe diving with its thick forests on the ocean floor. Removal of this seaweed from Southern California, Baja Mexico, and the Channel Islands has become quite an undertaking with the rapid spread of the species; an effective and efficient solution is needed so that the colonization of new areas can be prevented. S. horneri, nicknamed the “devil weed” by certain scuba websites, must be eradicated in the near future or else our marine ecosystem will suffer great consequences.

Works Cited

Anderson, L. W. J. (2005). California’s reaction to caulerpa taxifolia: a model for invasive species rapid response. Biological Invasions, (7), 1003-1016.

Bushing, B. (2013, March 25). Interview by R. Tanner [Personal Interview]. S. horneri interview.

Choi, H. G., Lee, K. H., Yoo, H. I., Kang, P. J., Kim, Y. S., & Nam, K. W. (2008). Physiological differences in the growth of sargassum horneri between the germling and adult stages. J Appl Phycol, (20), 729-735.

Klinger, T., Miller, K. A., Trowbridge, C., Walters, L., & Whiteside, K. (2007, November). In S.N. Murray (Chair). Improving understanding of invasive seaweeds in california’s coastal waters: Moving beyond caulerpa taxifolia. , CSU Fullerton.

Miller, K. A., Engle, J. M., Uwai, S., & Kawai, H. (2007). First report of the asian seaweed sargassum filicinum harvey (fucales) in california, usa. Biological Invasions, (9), 609-613.

Monteiro, C. A., Engelen, A. H., & Santos, R. (2009). Macro- and mesoherbivores prefer native seaweeds over the invasive brown seaweed sargassum muticum: a potential regulating role on invasions.Marine Biology, (156), 2505-2515.

Monteiro, C., Engelen, A. H., Serrao, E. A., & Santos, R. (2009). Habitat differences in the timing of reproduction of the invasive alga sargassum multicum over tidal and lunar cycles. J. Phycol., (45), 1-7.

Pang, S. J., Liu, F., Shan, T. F., Gao, S. Q., & Zhang, Z. H. (2009). Cultivation of the brown alga sargassum horneri: sexual reproduction and seedling production in tank culture under reduced solar irradiance in ambient temperature. J Appl Phycol, (21), 413-422.

Riosmena-Rodríguez, R., Boo, G. H., López-Vivas, J. M., Hernández- Velasco, A., Sáenz-Arroyo, A., & Boo, S. M. (2012). The invasive seaweed sargassum filicinum (fucales, phaeophyceae) is on the move along the mexican pacific coastline. Botanica Marina, 55(5), 547-551.

Saccardi, S. (n.d.). Faunal communities associated with sargassum horneri.

Sargassum horneri: The “devil weed”. (2012, July 20). Retrieved from

Smith, J. (n.d.). Marine monitoring reveals first intertidal occurrence of invasive seaweed sargassum horneri. Retrieved from

Uwai, S., Kogame, K., Yoshida, G., Kawai, H., & Ajisaka, T. (2009). Geographical genetic structure and phylogeography of the sargassum horneri/wlicinum complex in japan, based on the mitochondrial cox3 haplotype. Marine Biology, (156), 901-911.

Author Bio: Richelle Tanner is a rising junior in the USC Dornsife College and the USC Thornton School of Music pursuing a double degree in Environmental Studies, B.S., and Jazz Studies, B.M.. She intends to pursue a PhD in Marine Science and is from Seattle, WA.

Editor’s note: Scientific Research Diving at USC Dornsife is offered as part of an experiential summer program offered to undergraduate students of the USC Dana and David Dornsife College of Letters, Arts and Sciences through the Environmental Studies Program. This course takes place on location at the USC Wrigley Marine Science Center on Catalina Island and throughout Micronesia. Students investigate important environmental issues such as ecologically sustainable development, fisheries management, protected-area planning and assessment, and human health issues. During the course of the program, the student team will dive and collect data to support conservation and management strategies to protect the fragile coral reefs of Guam and Palau in Micronesia.

Instructors for the course include Jim Haw, Director of the Environmental Studies Program in USC Dornsife, Assistant Professor of Environmental Studies David Ginsburg, Lecturer Kristen Weiss, SCUBA instructor and volunteer in the USC Scientific Diving Program Tom Carr and USC Dive Safety Officer Gerry Smith of the USC Wrigley Institute for Environmental Studies.

Previously in this series:

The 2013 Guam and Palau Expedition Begins

A New Faculty Member on the Team