Skate Eggs

Winter storms that produce heavy surf and giant waves bring more to the shore than expert surfers. A big skate (Raja binoculata) egg case, which could be mistaken for seaweed, was collected in early December on San Francisco's Ocean Beach.

After a female big skate mates, usually three or four fertilized eggs start developing within her body. The eggs fill with nutritious yolk and move down her oviduct where a soft capsule, or egg case, forms around them. She lays this case, which hardens upon contact with water, on the sandy seafloor.

Several months later, fully developed, palm-sized babies emerge from the case. They may live for 20 to 30 years and grow up to six feet in length.

The tendrils on an egg case easily catch on seaweeds or rocks and help prevent it from being carried away by ocean currents. At the base of each tendril is a small opening where seawater flows in and brings oxygen to developing embryos. Big skates range from Baja California to the Bering Sea and usually occupy relatively shallow water, less than 400 feet deep. They are common inside San Francisco Bay where they feed on small fishes and crustaceans like crabs and shrimps.

 

The big skate's species name, binoculata, refers to the two dark "eyespots" on each wing of its back.
Photo: California Academy of Sciences, Special Collections