Opisthobranch- Philine auriformis

Terry Gosliner, Ph.D., Senior Curator of Invertebrate Zoology

Scientists first described the opisthobranch species Philine auriformis in its marine habitat off the New Zealand coast. Opisthobranchs, commonly known as sea slugs, are a sub-class of often brightly colored marine mollusks in which the shell is reduced or lost. This species was probably introduced to the San Francisco Bay from New Zealand as a result of freighter ships discharging ballast water containing larvae and juvenile slugs. It was known to be abundant in the southern part of San Francisco Bay in 1993, but had not been found elsewhere at the time. In 1995, I documented the presence of this species throughout San Francisco Bay and its spread to Bodega Harbor on the Sonoma County Coast. It has now spread from isolated bays to the open coast as far south as San Diego. Philine auriformis has become a dominant marine organism in communities of plants and animals that inhabit the soft bottom of the ocean floor. Since it feeds on bivalves and other mollusks that are primary prey for shorebirds, its introduction is likely to have a profoundly adverse impact on native species. In deeper water, this pest may be eliminating native species of opisthobranchs and other mollusks by preying on them and competing for the same food resources that the natives need to survive.

The introduction of species through ballast discharge can be virtually eliminated by mid-ocean ballast water exchange. Since ballasts are filled with water in bays or other shallow waters, the organisms that inhabit those waters are adapted to shallow waters and would thus not usually survive in the deeper ocean waters. Likewise, any organism that was picked up in the mid-ocean water wouldn't survive in the shallow bays where the water is released. Although it is time consuming for the boats to stop and release ballast water in the open ocean, and therefore has some cost associated with it, it is far less expensive than the ecological damage that has been done by the species which have been introduced to our shores.

Additional Resources:

California Academy of Sciences Invertebrate Zoology Geology Department

Exotic Species of the Monterey Bay National Marine Sanctuary

Nonindigenous Aquatic Species Information Task Force

Nonindigenous Aquatic Species in United States Estuary: A Case Study of the Biological Invasions of the San Francisco Bay Area

Push for Rules on Ballast Exchange Gains Support

The Slug Site

California Academy of Sciences, Golden Gate Park, San Francisco, California