Monterey Bay National Marine Sanctuary






Sancutary Program Accomplishments

Intertidal Systems

Rocky Subtidal Systems

Open Ocean & Deep Water Systems

The Physical Environment

Wetlands and Watersheds

Endangered & Threatened Species

Marine Mammals

Bird Populations

Harvested Species

Exotic Species

Human Interactions

Further Reading



A critical issue for the Monterey Bay National Marine Sanctuary is the unabated, human-mediated, worldwide transport of marine species. A variety of transport mechanisms exist, resulting in the invasion of native communities by exotic species. The most important of these vectors is ballast water—untreated seawater taken aboard ships for weight, balance, and stability—which can harbor hundreds of species. While Monterey Bay is not a significant port for commercial ship traffic, its proximity to San Francisco Bay makes it vulnerable to ballast water invasions.

A prime example of an invasion that started in San Francisco Bay and has now spread to the Sanctuary is the European green crab, Carcinus maenas (see accompanying article). Interestingly, genetic evidence gathered by Moss Landing Marine Laboratories indicates that Carcinus in San Francisco Bay actually derive from introduced populations in New England; green crab populations are playing leap frog across the world!

While ballast water is the primary cause, other human activities have spread marine organisms for decades. Of note, the movement of oysters from the Atlantic coast and Japan resulted in dozens of established exotic species on the Pacific coast of the United States. While exotic oysters themselves have rarely become established, the organisms that live under the ridges and crevices of their shells (sponges, bryozoans, worms) are now permanent members of our biota. Aquaculturists, seafood restaurants, and bait shops also share respon-sibility for the inadvertent release of exotic species into our local waters. Lastly, U.S. customs agents in San Francisco report a surprising numbers of travelers carrying exotic species into the country illegally.

Biological invasions in the Sanctuary remain largely unstudied. The seemingly straightforward task of counting exotic species is actually a daunting challenge for three reasons. First, the incredible species diversity of the Pacific coast marine fauna makes it difficult simply to identify which species in a sample are native and already known to live in Sanctuary waters. Second, by no means are all the native species in the Sanctuary known to science, and it requires careful study to determine whether a newly-discovered species is native or a new invader. Third, it takes continuous monitoring and a practiced eye to recognize a suspect exotic organism and the cooperation of taxonomic experts from around the world to identify the creature.

We can, however, make some predictions about which exotic species might be present and where they will be found. We know that protected bays and estuaries are much more invaded than the open rocky or sandy coasts. In the Sanctuary, Elkhorn Slough is the probable hot spot for invasions. Indeed, many invaders have already been detected there.

For example, Japanese mudsnails (Batillaria atramentaria) and Atlantic gem clams (Gemma gemma) are among the most numerous mollusks in the slough, and its banks are eroding from the actions of the burrowing isopod Sphaeroma quoyonum. Half Moon Bay is another especially vulnerable site, as are the harbors at Moss Landing, Monterey, and Santa Cruz. Our best indication of the extent of the invasion threat comes from extensive studies in San Francisco Bay by Drs. James Carlton of Williams College and Andrew Cohen of the San Francisco Estuary Institute. With the aid of an army of taxonomic experts, they listed more than 200 exotic species of plants, protozoans, invertebrates, and vertebrates in San Francisco Bay, and we can expect a substantial proportion of this list to reside within the Sanctuary now. Although the rocky shore is less invaded, the Japanese kelp Sargassum muticum appears now to be established in the rocky intertidal zone in the Sanctuary.

How will biological invasions change the natural communities in the Sanctuary? From case studies, we know that exotic species in marine systems can be important predators and competitors, that introduced parasites can spread rapidly in native populations, and that primary consumers can radically alter food webs. However, it remains a challenge to ecologists to develop a theory of biological invasions that will predict which species will invade and what their impact will be.

--Jonathan Geller
Moss Landing Marine Laboratories

Green Crabs in Elkhorn Slough

The European green crab, Carcinus maenas, is on the Nature Conservancy's "Ten Least Wanted" list. This crab, though often not green, is easily identified by five spines on either side of its carapace. They live three to five years in brackish to full sea water. Adult males can grow a carapace width up to 9 cm. As Dr. Geller indicates in the adjoining article, exotic species can be important predators and alter a natural community. In Bodega Harbor, the green crab has been associated with tenfold decreases in both native shore crabs (Hemigrapsus oregonensis) and clams (Nutricola tantilla); preliminary experiments show green crabs readily consume juvenile Dungeness crabs, thus raising concerns about impacts on this fishery.

The European green crab has been associated with significant decreases in the populations of native shore crabs and clams in Bodega Harbor. (©Jonathan Geller)

The green crab invaded Elkhorn Slough in 1994, after its West Coast introduction to San Francisco Bay in 1989. Initial studies in Elkhorn Slough focused on documenting the presence of green crabs, then in 1998 a collaborative program between the Sanctuary, Elkhorn Slough Reserve, and University of New Hampshire Sea Grant began assessing their distribution and abundance as well as quantitative impacts to mudflat communities. This year, an additional study was initiated to assess indirect effects of green crabs on shorebirds. If green crabs are impacting shorebird prey species in the mudflats, this may influence how shorebirds use a critical stop in their heroic migration along the Pacific Flyway.

Preliminary data indicate that the green crab invasion into Elkhorn Slough is unique. The green crabs are maintaining relatively low densities relative to the native shore crab, and they are found mostly on mudflats mid-way up the slough, with low abundances at the harbor entrance and upper reaches—both areas where currents can run faster. Green crabs use open coast rocky shore habitats as well as mudflats on the East Coast of North America and in Europe, and the population growth and impacts to native species seem less in Elkhorn Slough than in San Francisco Bay or Bodega Harbor.

An interesting hypothesis is that sea otters, currently abundant in Elkhorn Slough and only historically abundant in more northern bays, are limiting green crab populations. One paradigm in the study of exotic species is that healthy ecosystems, with a full complement of native species, are more difficult to invade than modified systems. In this case, sea otters may be eating the introduced species, as evidenced by green crab parts in recent scat analyses. While the current West Coast range for green crabs is Morro Bay, California to Barkley Sound, British Columbia, studies at Elkhorn Slough may change how scientists and resource managers predict the impacts of introduced marine species.

--Andrew DeVogelaere
Monterey Bay National Marine Sanctuary
--Edwin Grosholz
University of California Davis

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Last modified on: March 31, 2000