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PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES 
Series 4, Volume 65, 28 Sept. 2018, No. 3 
Introduction 
The 2015-16 El Nino in the tropical Pacific was one of the strongest on record. However, its 
physical effects on the California Current System, including coastal waters, were weaker than 
expected (Jacox et al. 2016). This resulted in part because as the El Nino formed, the entire north¬ 
east Pacific was already in the midst of a multi-year marine heat wave of unprecedented magnitude 
(Bond et al. 2015; Di Lorenzo and Mantua 2016), causing extensive biological impacts at all troph¬ 
ic levels and poleward range shifts typically observed only during strong El Nino events (reviewed 
by Cavole et al. 2016; and see Jones et al. 2018). The range shifts driven by the 2014-15 warm 
anomaly included 30 species of heterobranch sea slugs (heterobranchs), nine of which were found 
in new northernmost localities (Goddard et al. 2016). During the 2015-16 El Nino, Merlo et al. 
(2018) found Hermissenda opalescens, which was separated by Lindsay and Valdes (2016) from 
its more northerly pseudocryptic sister H. crassicornis, on the west coast of Vancouver Island, 
British Columbia. Merlo et al. (2018) also reported Dirona picta for the first time north of Oregon. 
Here, we document additional range extensions of northeast Pacific heterobranchs associated with 
the 2015-16 El Nino and the warm water anomaly that persisted into 2017 in the northern Califor¬ 
nia Current System (Well et al. 2017) and summarize the effects of the combined warm water 
events (hereafter the 2014-17 marine heat wave) on the distribution of heterobranchs in the region. 
Methods and study sites 
From fall 2015 through 2017, we sampled for benthic heterobranchs at mostly intertidal sites 
from Baja California Sur, Mexico to Oregon (Table 1), including many of the same sites sampled 
annually to quarterly by Goddard et al. (2016) using timed counts. We photographed living speci¬ 
mens in the field or laboratory. A few were collected and deposited in the Invertebrate Zoology 
Collection at the California Academy of Science (CAS) as voucher specimens and are referenced 
below by their CASIZ catalog numbers, records of which can be accessed online via the CAS 
Invertebrate Zoology Collection Database <http://researcharchive.calacademy.org/research/izg/ 
iz_coll_db/index.asp>. We were also sent new records of occurrence by colleagues, particularly in 
Washington and British Columbia, and we also monitored citizen science posts on websites such 
as iNaturalist, OCDiving, and Flickr for sightings of heterobranchs beyond their usual ranges. 
From both of these latter types of sources we include below only records accompanied by an image 
and the date and locality of observation. 
We calculated the range shift for each species by using the Ruler tool on Google Earth to meas¬ 
ure the straight-line distance between the previous northernmost known locality and its new north¬ 
ernmost locality documented below. This measure is conservative and actually measures range 
extensions, because it is based on the distance between recorded northernmost localities, typically 
reached during strong El Nino events, rather than the distance between the new northernmost local¬ 
ity and a species’ usual northern range limit. However, information on the latter has not actually 
been compiled, and is spotty, scattered throughout a wide range of sources, and for many species 
qualitative in nature. 
Nomenclature used below follows the World Register of Marine Species (WoRMS) 
<http://www.marinespecies.org/index.php>, as of June 2018. 
Results 
Including Hermissenda opalescens and Dirona picta, whose ranges were extended by Merlo 
et al. (2018), 23 species of heterobranchs were recorded from new northernmost localities from late 
