Corophium acherusicum (Costa, 1857)
This amphipod, which is not native to the San Francisco Bay, has spread from its unknown point of origin to warm temperate waters throughout the world. The specimen in the upper photo, with the greatly enlarged second (lower) antennae, is male. The smaller specimen, in the lower image, is female and is carrying eggs on her abdomen. Because these animals tolerate fresh water, they can be found as far upstream as the Carquinez Strait and the Petaluma River.