Invasive Species Monitoring | MARINe

Last updated July 15, 2021


Sargassum horneriUndaria on dock

photo credits: T. Boyd (left), C. Culver (right)

Help Monitor Invasive Seaweeds

We at are overseeing the monitoring of seaweeds that have invaded coastal and island reefs of California. We encourage you to help keep track of these invasions by reporting your sightings when you find them. Continue reading to learn more about these invasive species and how you can help us determine where they have spread.

Why Monitor?

Invasive species are plants or animals that have been introduced to a new region where they are not native, typically become widespread and abundant, and result in negative impacts on native species, habitats and/or infrastructure. Invasive species can have dramatic ecological and economic consequences in their new environment, with billions of dollars spent each year to prevent, eradicate and control them. The coastal marine environment is vulnerable to invasion through human activities such as international shipping, boating, aquaculture, and the aquarium trade. Identifying and tracking marine species invasions when they occur is particularly challenging due to the expanse and inaccessibility of coastal and island marine habitats. Citizens who spend time on or in the ocean can play a key role in documenting the appearance and spread of invasive species by participating in monitoring.

Monitoring the distribution of invasive species is vital to their management and prevention of further spread. Detecting non-native species before they become a problem, as well as documenting the range of existing non-native species, are critical for controlling and eradicating them. Also, the more we understand about how invasive species spread, the better we will be able to prevent future invasions. We encourage you to assist with monitoring the spread of two seaweeds – Sargassum horneri and Undaria pinnafida - that have invaded California and Baja California waters.

What You Can Do:

Be Safe: Scuba diving can be a dangerous sport possibly resulting in serious injury or death without proper training and safe diving practices. Always dive within the limits of your training and comfort.

Video: 'How You Can Help Fight Invasive Species'

Undaria holdfastHorneri closeup

Species Currently Monitored by

Sargassum horneri

Invasion and Distribution

Sargassum horneri, referred to as ‘Devil Weed’ by many divers and boaters in southern California, is a brown seaweed native to Japan and Korea that attaches to rocks and other surfaces (e.g., docks and boat hulls). It was first detected in California in 2003 by biologists conducting surveys in the Port Long Beach inner harbor1 and originally identified as Sargassum filicinum, which is now considered a type of S. horneri2. By October 2005, the California population had spread within Long Beach Harbor and S. horneri was found adrift in Todos Santos Bay, Baja California, Mexico. It has since expanded rapidly throughout southern California and Baja California3 and in many places, including the northern and southern Channel Islands, dense populations occupy areas where native kelp beds once flourished. To date, this is the only known invasion of S. horneri (filicinum type); it has not been found anywhere else in the world outside of its native range.

How It Spread

The initial detection of S. horneri in a major commercial shipping port suggests it likely came to the US on a commercial vessel. Further spread regionally is attributed to its natural dispersal ability as well as various human activities. Sargassum horneri can spread long distances because as an adult it has buoyant floats that allow it to drift with currents to new sites if detached from the substrate. Just one reproductive individual drifting to a new area may be enough for a new population to become established. It can become detached either naturally (e.g., disturbance from waves or grazers), or by human activity, particularly boating (e.g., becoming wrapped around anchors) and SCUBA diving (e.g., catching on fins and other gear). Sargassum horneri is found at many boating destinations on offshore islands and nearby harbors, suggesting humans may have helped it spread.

Be careful to not inadvertently spread S. horneri by intentionally or accidentally removing and releasing it. Spread the Word, Not the Weed!


The continued expansion and shear abundance and formation of dense mats of S. horneri has raised concerns about its impact on native ecosystems. Potential consequences exist for not only native kelp but also the commercially and recreationally important fished species which depend on kelp for food or habitat, like sea urchins and kelp bass. Definitive studies assessing the impacts of S. horneri are currently underway.

Learn more about Sargassum horneri with this Sargassum horneri information sheet (PDF)

H. horneriL. Marks with S. horneri

photo credit: Jessie Altstatt

Undaria pinnatifida 

Invasion and Distribution

Undaria pinnatifida, often referred to as Asian Kelp or Wakame, is a brown seaweed that is native to Japan where it is cultured for consumption (e.g., miso soup and seaweed salad). It has invaded many areas worldwide. It was first found outside of its native Japanese waters in 1971 in Thau Lagoon along the Mediterranean coast of France. Since that time, it has spread along Atlantic and Mediterranean coasts of Europe, as well as to England, New Zealand, Australia, Argentina, Baja California in Mexico, and California, USA. In California, U. pinnatifida was first found growing in Los Angeles Harbor in 2000 and is now found in numerous California harbors. It has also spread to two locations on the open coast: one small population has persisted off Catalina Island since 20014, and a new population was detected on the north side of Anacapa Island in 2016. It may have spread to other reefs in these areas; surveys have been limited.

How it Spread

Undaria pinnatifida arrived in other countries through several different ways. It initially arrived in France with the importation of Japanese oysters. Then, in 1983 it was intentionally introduced along the coast of Brittany, France, for cultivation as food. The original source of U. pinnatifida in Californian waters is unknown. However, its genetics indicate that it may be related to a population in Japan that could have been transported via commercial shipping5. This idea is supported by its first detection in a major port (Los Angeles Harbor) and its ability to grow on the hulls of boats/ships. Further, its occurrence almost exclusively in harbor environments in California suggest that it was also transported along the California coast through boating activities.

Unlike Sargassum horneri, fragments of U. pinnatifida are not capable of dispersing long distances as they do not contain any float-like structures. Nonetheless, adult plants are capable of releasing microscopic spores that will eventually become new plants. If encountered, divers should not disturb or remove adult plants as it could trigger the release of spores and help the population persist and spread.


Undaria pinnatifida is considered a highly invasive species and is listed as one of the world’s 100 worst invasive alien species by the International Union for the Conservation of Nature (IUCN). In some places where it has invaded it has caused a decrease in the number and abundance of native species and interfered with shellfish farming.

Learn more about Undaria pinnatifida with this Undaria pinnatifida information sheet (PDF)

Undaria 2 stagesUndaria on boat keel

photo credit: Jessie Altstatt

Identification Guides and Resources

Identification Guides (PDF)

Identification and Reporting Flyer

Common marine invasive species of the U.S. West Coast

Sargassum identification guide

Sargassum horneri vs. Sargassum muticum identification guide

Undaria pinnatifida identification guide 1

Undaria pinnatifida identification guide 2

Other Resources

World Register of Introduced Marine Species

Invasive Species - Smithsonian Ocean Portal

Articles, Publications, and News Broadcasts

Bacteria Growth-restraining Nano Composite Developed from Invasive Algae (May 26, 2021)

Unseasonal attack by brown macroalgae from China causes damage to sea farms in S. Korea (Feb 19, 2021)

Jeju Island Inundated with Invasive Algae (Jan 20, 2021)

An Orange County marine biologist wants to weed the ocean to help kelp grow (Sept 8, 2020)

Sargassum horneri, known as "Devil Weed," invades Monterey Bay National Marine Sanctuary (June 25, 2020)

Two Types Of Non-Native, Invasive Seaweed Prompt Concern For Marine Protected Areas Off South Coast (May 2, 2017)

Park launches campaign to stop invasive seaweed (Apr 28, 2017)

Super Sucker targets highly invasive Sargassum in Southern California (Mar 11, 2016)

Nancy Foster Scholar Lindsay Marks Takes On Invasive Species (Feb 16, 2016)

Devil Weed invades O.C. Waters (Jan 1, 2015)

Questions? Please contact Lindsay Marks (

Funding provided by:

                               CA Sea Grant


1. Miller KA, Engle JM, Uwai S, Kawai H (2007) First report of the Asian seaweed Sargassum filicinum Harvey (Fucales) in California, USA. Biological Invasions 9: 609-613, 10.1007/s10530-006-9060-2

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

3. Marks LM, Salinas-Ruiz P, Reed DC, Holbrook SJ, Culver CS, Engle JM, Kushner DJ, Caselle JE, Freiwald J, Williams JP, Smith JR, Aguilar-Rosas LE, Kaplanis NJ (2015) Range expansion of a non-native, invasive, macroalga Sargassum horneri (Turner) C. Agardh, 1820 in the eastern Pacific. BioInvasions Records 4: 243-248,

4. Miller KA and JM Engle. 2009. The natural history of Undaria pinnatifida and Sargassum filicinum at the California Channel Islands: Non-native seaweeds with different invasion styles. In: Proceedings of the 7th California Islands Symposium, eds CC Damiani and DK Garcelon. pp. 131-140.

5. Uwai, S., Nelson W, Neill D, Wang WD, Aguilar-Rosas LE, Boo SM, Kitayama T, Kawai H. 2006. Genetic diversity in Undaria pinnatifida (Laminariales, Phaeophyceae) deduced from mitochondria genes—origins and succession of introduced populations. Phycologia 45: 687-695,