Great White Shark
This is a photo of a bigeye thresher, Alopias superciliosus. It's a circumtropical species and can be found in the waters off southern California. The thresher shark more common to our region is Alopias vulpinus.
The thresher shark occurs relatively far offshore, and is considered a pelagic or open ocean shark. It has a gray upper body and white belly, large eyes, and is nocturnal. The thresher can be quite long -- eleven feet -- though nearly half its length is in its tail. It uses its long tail to round up, stun, and even kill small food fish such as sardines, anchovies, mackerel, and squid.
The thresher is elusive and not much is known about its habits. It is considered harmless.
It's also called blue thresher, green thresher, longtail shark, swiveltail, fox shark, or sea fox.