The Northeast Pacific Ocean was anomalously warm in 2014, despite ENSO-neutral conditions in the tropical Pacific. We document northern range shifts associated with this anomaly for 30 species of nudibranchs and other shallow-water, benthic heterobranch gastropods from southern California to southern Oregon. Nine of these (Placida cremoniana, Trapania velox, Doriopsilla fulva, Janolus anulatus, J. barbarensis, Flabellina cooperi, Anteaeolidiella chromosoma, A. oliviae, and Noumeaella rubrofasciata) were recorded from new northernmost localities, while the remainder were found at or near northern range limits which we show were established mainly during El Niño events. All 30 species have planktotrophic larval development, and six were observed spawning at northern localities, increasing the likelihood that their ranges will continue to shift poleward as the strong 2015-16 El Niño develops. Notable among these was Okenia rosacea, usually found south of San Francisco and last observed in Oregon as a single specimen found during the 1997-98 El Niño. In 2015 this bright pink nudibranch reached high densities and was observed spawning throughout northern California and into southern Oregon. Okenia rosacea is therefore poised to exploit abundant prey resources previously out of its reach in northern Oregon and Washington. Our results not only demonstrate a striking biological response to the 2014 warm anomaly in the North Pacific Ocean, but also support early physical indications of a larger regional climate shift, one reinforced by long-term global warming. Combined with historical data, these results highlight how shallow-water nudibranchs, with their planktotrophic larvae, short life cycles, conspicuous coloration, and accessibility are excellent biological indicators of ocean climate in the region.
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