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PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES 
Series 4, Volume 65, 28 Sept. 2018, No. 3 
found at new northernmost localities in the current study and Merlo et al. (2018) is more than dou¬ 
ble the number reported by Goddard et al. (2016), and their mean range extension was 270 km, or 
79% longer. While taxonomic splits since 2016 (Lindsay and Valdes 2016; Uribe et al 2017) 
account for the addition of two species ( Limacia mcdonaldi and Hermissenda opalescens ) to the 
recent total, the 2015-16 El Nino clearly had a significant effect on heterobranch distributions in 
the region. 
Of the 37 species treated herein, 22 were not among the species covered by Goddard et al. 
(2016). The combined 2014-17 marine heat wave in the Northeast Pacific Ocean therefore drove 
range shifts in at least 52 species of benthic heterobranchs, approximately one quarter of the 
species known from the Californian and Oregonian biogeographic provinces (Behrens and Her- 
mosillo 2005). Thirty of the 52 were found farther north than ever observed before. Although clear¬ 
ly significant and likely unprecedented, it is difficult to directly compare these numbers to those 
observed during warm-water events of the last half century, owing to (1) historical gaps in sam¬ 
pling coverage, (2) lack of dates for some of the earlier records, (3) recent taxonomic revisions, and 
(4) increased numbers of observers, especially citizen scientists, combined with increased oppor¬ 
tunities for posting online geo-referenced observations accompanied by images. As evidenced by 
the number of known northern range limits that were not surpassed by species we found in our 
studies, many have been advected as larvae and settled as far north before, especially during strong 
El Nino events. These include, for example, Bulla gouldiana, Aplysia californica, Berthellina ilisi- 
ma, Polycera alabe, Dirona picta, the chromodorids Felimare porterae and Felimida macfarlandi, 
and Flabellinopsis iodinea (Goddard et al. 2016; present study and references therein). However, 
the range shifts since late 2013 by species like Okenia rosacea, Carminodoris bramale, Doriopsil- 
la fulva, Anteaeolidiella oliviae, Noumeaella rubrofasciata, and Phidiana hiltoni are unprecedent¬ 
ed, either in geographic extent or population density, with their prior absence from new northern 
ranges backed by historical records. 
The sheer number of heterobranch range shifts associated with the 2014-17 marine heat wave, 
especially those to new northernmost localities, begs questions about the possible role of global 
warming, beyond simple temperature increases in the California Current System and Northeast 
Pacific, which so far have been less than in other ocean regions (Cheng et al. 2017). Dispropor¬ 
tionate Arctic warming, or “Arctic amplification,” has been implicated in slowing the Northern 
Hemisphere polar jet stream, making it wavier, with more persistent high pressure ridges and low 
pressure troughs (Francis and Vavrus 2015; Francis et al. 2017). This increased amplitude and per¬ 
sistence is reflected in the North Pacific Oscillation (NPO), a leading pattern of atmospheric vari¬ 
ability. A persistent atmospheric ridge was observed over the Northeast Pacific during winter 2013- 
2014, which by altering wind stress and reducing the depth of the mixed layer generated the 2014 
warm anomaly in sea-surface temperatures (Bond et al. 2015). Atmospheric teleconnections 
between the North Pacific and tropical Pacific and back again are then thought to have effectively 
forced phase changes in the Northeast Pacific reflected first in the North Pacific Gyre Oscillation 
(NPGO), then the Pacific Decadal Oscillation (PDO), and ultimately the El Nino Southern Oscil¬ 
lation (ENSO) (Di Lorenzo and Mantua 2016; and see Levine and McPhaden 2016). The direction 
of the observed phase changes are known to be associated with, among other effects, reduced 
coastal upwelling and increased poleward and onshore transport of coastal waters in the NE Pacif¬ 
ic, which can advect planktonic propagules and pelagic organisms poleward, well beyond their 
usual ranges (e.g., Lluch-Belda et al. 2005; Ohman et al. 2017). Global warming, by increasing the 
amplitude and persistence of the NPO, therefore appears to be important in driving increased pole- 
ward range shifts. The unprecedented magnitude and duration of the 2014-17 marine heat wave 
likely also contributed to the continued presence at outer coast sites of recently arrived southern 
