EXOTIC SPECIES

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

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