A Southern California Icon Surfs North: Local Ecotype of California Grunion, Leuresthes tenuis (Atherinopsidae), Revealed by Multiple Approaches during Temporary Habitat Expansion into San Francisco Bay

California Grunion Leuresthes tenuis, a beach spawning marine teleost, was described from a specimen purchased at a San Francisco market in 1859, but not subsequently seen there for 140 years. From 2001–2007, L. tenuis spawned on beaches in San Francisco Bay (SFB, 37°45′N, 122°15′W) at multiple locations, but disappeared after 2008. In 2005, L. tenuis started spawning for the first time on a beach in Tomales Bay (TB, 38°14′N, 122°58′W), 64 km north of SFB, but this population disappeared after 2009. Adult size, clutch volumes, and egg diameters of L. tenuis in SFB were consistently smaller than L. tenuis from southern California, though the population was not genetically distinct. Population size structure suggests few L. tenuis survived more than one year in these northern bays, rather than the two or three years expected in the typical southern California habitat. Climate change models predict conditions supporting poleward expansion of ranges of marine organisms, but colonization of northern habitats by this beach spawning species resulted in significant phenotypic changes including smaller size, shorter life span, and reduced reproductive output. The multiple environmental challenges and rapid extirpation of two disjunct colonization events indicate this species will require repeated events for habitat expansion to succeed.