Syncaris pacifica is an endangered species of freshwater shrimp in the family Atyidae that occurs only in a limited range within the northern San Francisco Bay Area, California, USA. Specifically, this species occurs only in 17 stream segments within Sonoma, Napa and Marin Counties. This species is often translucent to transparent, with both sexes capable of considerable coloration altering, as a sophisticated form of camouflage. This decapod is commonly known as California freshwater shrimp, and is the only extant decapod shrimp in California that occurs in non-saline waters (its congener Syncaris pasadenae from the basin of the Los Angeles River is extinct).

S. pacifica is one of only four members of the family Atyidae in North America. Genetic studies have been conducted to compare specimens of Syncaris pacifica from various drainages, with the results showing a variety of well-defined genetic variations within these populations. The species has a superficial appearance to its better known marine relatives, and may attain a body length of about 5 centimetres (2.0 in).

...