I was watching a Great Egret stalking in low tide when a flicker of movement closer in caught my eye. I thought I saw a small bird perched on a partly submerged log. As I focused in, I had a big surprise. The “small bird” was the dorsal fin of a Leopard Shark that might have been four feet long.
The fish lay quietly in water only a few inches deep. Its head and whole body down almost to the tail rose above the water. Apart from that one flick of its fin, it showed no movement while I stood and watched it. Eventually I moved on. Half an hour later when I returned, the fish had straightened its body and buried its head in the mud. The incoming tide rose to cover its whole body, leaving just a small tip of the dorsal fin exposed. Then that, too, went under. Soon the fish became invisible.
Leopard sharks live in coves and bays along the Pacific Coast from Oregon down to Baja California (Mexico). They are well known in Tomales Bay, just up the coast in western Marin county. It is typical of the species to inhabit very shallow waters and mudflats. I have never seen one before in the North Basin.
There is extensive commentary about this fish in Wikipedia. They are very slow growing, live about 30 years, and don’t travel very far. They don’t bite humans. They eat crabs, shrimp, a wide variety of worms and fish eggs.
They feed by creating a powerful suction; they’ve been known to suck a big clam’s whole body out of its shell. Sport fishers catch them regularly in San Francisco Bay and offshore. It’s illegal to take individuals less than 36 inches long. Despite an extensive catch for food, as a bycatch in bottom trawls, and for the aquarium trade, their population is considered secure and of “least concern.”