The Feeding Ecology of the Invasive Chinese Mitten Crab , Eriocheir sinensis : Implications for California ' s Freshwater Communities

The San Francisco Bay area is a highly invaded watershed, where 60% of the species present are non-native. The Chinese mitten crab, Eriocheir sinensis, is an invasive species of particular concern in California because of the ecological damage that it has caused in ecosystems throughout Europe. Despite this concern, little is known about the ecology of this freshwater, catadromous crab. To assess the impact that E. sinensis is having on California's freshwater communities, I characterized the feeding ecology of Chinese mitten crabs with a combination of foregut content analyses of crabs from south San Francisco Bay tributaries and feeding preference experiments. Foregut analyses show that E. sinensis feeds mostly on algae (28.7%) and detritus (49.3%), while occasionally feeding on aquatic invertebrates (8.4%). Feeding preference studies indicate that E. sinensis prefers resident benthic invertebrates (59.2% of food eaten), to green algae (21.7%), leaf detritus, (17.2%) and freshwater shrimp, Palaemonetes paludosus (6%). Preliminary benthic samples taken at the same sites show low benthic invertebrate abundance and diversity, which potentially explains discrepancies between gut contents and feeding preferences. The combined analyses indicate that E. sinensis has the potential to cause a shift in the food web structure due to its large population size and preference for aquatic invertebrates, in habitats where benthic invertebrates are rare and not diverse. Introduction Introduced species can a serious threat to the structure and function of natural communities and ecosystems. Introduced species cause these changes by outcompeting resident species, accelerating native species extinction rates, or by facilitating the establishment of other non-native species (Hill et al. 1993, Light et al. 1995, Adler et al. 1998). Habitat disturbances and increased development often facilitate the introduction and long-term establishment of non-native species (Moyle and Williams 1990), making habitats within or adjacent to urban areas prone to species introductions. The San Francisco Bay area represents one of the most highly invaded urban regions, where 60% of the aquatic species present are non-indigenous (Cohen and Carlton 1998). Many of the successful non-native aquatic species are invertebrates, and crustaceans such as the signal crayfish (Pacifasticus leniusculus), the red-swamp crayfish (Procambarus clarkii) the European green crab (Carcinus maenus) and the Chinese mitten crab (Eriocheir sinensis) are particularly common as invaders, (Cohen and Carlton, 1997, Cohen and Carlton 1998, Grosholz et al. 2000). Despite the widespread invasion of crustaceans throughout Californian aquatic ecosystems, the community and ecosystem-level impacts of these invasions in freshwater have only recently been studied. In freshwater, crayfish have the potential to dominate the benthos both in population number and trophic influence, exerting strong direct and indirect effects on benthic communities (Light et al. 1995, Charlebois and Lamberti 1996, Nyström et al. 1999, Stelzer and Lamberti 1999). This dominance often occurs regardless of the presence of other ecologically similar crayfish (Hill et al. 1993). The Chinese mitten crab is native to the Province of Fukien, China and the Korean Peninsula. It was first documented in the San Francisco Bay Area in 1992. According to Cohen and Carlton (1997), the Chinese mitten crab was either accidentally introduced to California via ship ballast water or it was intentionally introduced. The Chinese mitten crab is catadromous, migrating from freshwater habitats where it resides during its juvenile years to saltwater habitats to reproduce. The range of this species has spread to cover over several hundred square miles and population numbers are estimated to be in the millions throughout the freshwater and estuarine ecosystems of the San Francisco, San Pablo and Suisun Bays, the Sacramento Delta and the Central Valley since its introduction eight years ago (Vedhuizen and Stanish 1999). The Chinese mitten crab has a history of successful invasion. It was introduced to Germany in 1912 and since then has become established in much of the rest of Europe and the Baltic States (Jadzdzewski and Konopacka, 1993). In many European countries, E. sinensis has adversely affected the ecology of aquatic habitats and the economy associated with these areas (Panning 1939, Hoestlandt 1948, Clark 1998). These adverse effects range from erosion and collapse of banks and levees (from crab burrows), to interference with commercial and sport fisheries (net damage, predation on and competition with harvested species) and alteration of community structure and function. In the San Francisco Bay area, hastened bank erosion in the South Bay since invasion is already evident (Halat 1997, Rudnick et al. 1999). Previous studies and observations of the trophic impact of E. sinensis on freshwater communities from Europe and China have indicated that, like crayfish, mitten crabs are opportunistic omnivores (Thiel 1938, Panning 1939, Hoestlandt 1948, Dan 1984). Despite this knowledge, we have little data to characterize the feeding ecology and potential trophic impacts of a large mitten crab population in the urban San Francisco Bay area (Halat 1997, Rudnick et al. 1999). Mitten crab populations have increased dramatically over the last four years in the south San Francisco Bay area; in some streams, densities have increased from less than 5 burrows/m2 to over 40 burrows/m2, where each burrow is evidence of one crab inhabitant (Rudnick et al. 1999). Such a large mitten crab population size, is capable of causing major changes in local community structure and function, regardless of their feeding ecology. Future decisions made regarding the control of the mitten crab or protection of native communities will need to be aware of the feeding ecology of E. sinensis in California. To address this need for knowledge, I assessed the natural diet of the mitten crab in three streams in Santa Clara County, California, through gut content analyses. The gut content analyses provide information on the current diet of the mitten crab in Santa Clara County. Based on a previous natural diet study of the Chinese mitten crab in the same habitat, I expected to find that E. sinensis feeds primarily on detritus and algae, very rarely feeding on invertebrate prey (Halat 1997, Rudnick et al. 1999). However, gut content analyses alone do not allow prediction of potential diets in other habitat areas.