A Chinese mitten crab, thought to be a threat to some native wildlife in the Bay Area, was found in Matadero Creek last week.
Local biologists say that the crabs could have a negative effect on some wildlife by eating the food of native animals. It also could cause erosion because it burrows into the sides of creeks.
The crab was found Oct. 20 by Michelle Sanders, 10, whose family lives along the creek. She told her mother, who immediately realized it was something unusual.
"We've lived by the creek for 15 years, and I've never seen a crab here before," Denise Sanders said.
Sanders had trouble finding anyone who recognized the strange crab. Eventually, she called the Marine Science Institute in Redwood City. They identified the crab by its unusual gray and white color and the dense covering of hair on the outside of its claws.
Known as the Chinese mitten crab, or Erlocheir sinensis, this species, is destructive to local ecosystems, said Karen Grimmer, educational director of the Marine Science Institute in Redwood City.
"They burrow into levees and the mud on river beds, and weaken the banks," she said. They also eat local vegetation, which can cause problems for farmers whose crops lie near creeks and streams that run into the bay.
In their native China, the crabs are known to thrive up to several hundred miles inland, often living in rice paddies and eating the roots of the plants, according to a report from the institute.
The mitten crab is unusual because it lives in fresh water and spawns in salt water, Grimmer said. This is exactly the opposite of American crabs.
She said mitten crabs spend their 3- to 5-year lives in streams and rivers that feed into the ocean. In the late fall and winter, the older crabs move downstream to mate. Once the eggs have hatched in the late spring, the young move up the rivers and the adults crawl to the river banks to die.
The mitten crab apparently made its way into the Bay Area in the late '70s and early '80s, Grimmer said. The crabs originally were brought over to be sold in San Francisco as food, but since the Mitten Crab carries a lung fluke that caused symptoms similar to tuberculosis, the state and federal governments banned it from importation and sale in the late 1980s.
The crabs were first found in Bay Area waters by shrimp harvesters in San Pablo Bay in 1993. Several more were found in San Francisco Bay in 1994.
Grimmer said that it is unclear whether the crabs were introduced to the Bay intentionally or accidentally. "We believe it was accidental," she said, but there is no way to be sure.
After Sanders contacted the Marine Science Institute, she thought that the crab would have to be destroyed. However, they told her that because there are so many crabs in the area now, destroying the one she had found wasn't really vital. Sanders was somewhat relieved since, she said, "we've grown somewhat attached to it."
--Bjorn Sunde "We've lived by the creek for 15 years, and I've never seen a crab here before." @id:--Denise Sanders said.
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