Large-scale impacts of sea star wasting disease (SSWD) on intertidal sea stars and implications for recovery

Abstract

Disease outbreaks can have substantial impacts on wild populations, but the often patchy or anecdotal evidence of these impacts impedes our ability to understand outbreak dynamics. Recently however, a severe disease outbreak occurred in a group of very well-studied organisms–sea stars along the west coast of North America. We analyzed nearly two decades of data from a coordinated monitoring effort at 88 sites ranging from southern British Columbia to San Diego, California along with 2 sites near Sitka, Alaska to better understand the effects of sea star wasting disease (SSWD) on the keystone intertidal predator, Pisaster ochraceus. Quantitative surveys revealed unprecedented declines of P. ochraceus in 2014 and 2015 across nearly the entire geographic range of the species. The intensity of the impact of SSWD was not uniform across the affected area, with proportionally greater population declines in the southern regions relative to the north. The degree of population decline was unrelated to pre-outbreak P. ochraceus density, although these factors have been linked in other well-documented disease events. While elevated seawater temperatures were not broadly linked to the initial emergence of SSWD, anomalously high seawater temperatures in 2014 and 2015 might have exacerbated the disease’s impact. Both before and after the onset of the SSWD outbreak, we documented higher recruitment of P. ochraceus in the north than in the south, and while some juveniles are surviving (as evidenced by transition of recruitment pulses to larger size classes), post-SSWD survivorship is lower than during pre-SSWD periods. In hindsight, our data suggest that the SSWD event defied prediction based on two factors found to be important in other marine disease events, sea water temperature and population density, and illustrate the importance of surveillance of natural populations as one element of an integrated approach to marine disease ecology. Low levels of SSWD-symptomatic sea stars are still present throughout the impacted range, thus the outlook for population recovery is uncertain.

An initial comparative genomic autopsy of wasting disease in sea stars

“…One of the most intriguing aspects of the 2013 outbreak of sea star wasting disease has been its wide zoonotic impact and yet apparently few consistent responses among species in subsequent studies (e.g., see Hewson et al (2018) and Miner et al (2018)). Our synthesis of prior datasets (Chandler & Wares, 2017; Fuess et al, 2015; Gudenkauf & Hewson, 2015; Schiebelhut et al, 2018) enabled by the common reference of the P. ochraceus genome however begins to suggest some potential commonalities (Figure 5), although we do not consider the role of complex interactions among stressors, which may manifest in unintuitive ways that do not necessarily result in shared differential expression of the same genes.…”

“…The 2013 outbreak of SSWD in the northeastern Pacific Ocean was remarkable for several reasons, including that both ecological and population genetic sampling were underway preceding the outbreak (Harley, Pankey, Wares, Grosberg, & Wonham, 2006; Miner et al, 2018; Schiebelhut, Puritz, & Dawson, 2018). This enabled documentation of the outbreak's greatest geographic extent (Eisenlord et al, 2016), highest mortality rates recorded for a noncommercial marine species e.g., 67% (Eisenlord et al, 2016), 90% (Menge et al, 2016), 99% (Miner et al, 2018), and diversity of sea star species affected (Eisenlord et al, 2016; Montecino‐Latorre et al, 2016). Over 20 species of subtidal and intertidal asteroids were impacted (Hewson et al, 2014), of which some species, such as Pisaster ochraceus and Pycnopodia helianthoides , are important predators in their communities (Duggins, 1983; Gravem & Morgan, 2017; Paine, 1974).…”

“…Given the large geographic extent (Miner et al, 2018) and taxonomic scale (Eisenlord et al, 2016; Montecino‐Latorre et al, 2016) of SSWD, events leading up to the outbreak were heterogeneous, multifaceted, and often unrecognized at the time (Miner et al, 2018). Moreover, the disease status of specimens in the field progresses rapidly from apparently healthy through morbid to dead, leaving few opportunities for diagnoses prior to, or in the presence of, visually identifiable symptoms (Miner et al, 2018), so clarifying evidence has been elusive. Proposed causes are diverse, including infection, injury, and environmental shifts (Harvell et al, 2019; Hewson et al, 2018).…”

“…Despite coincidence of the outbreak of wasting (i.e., SSWD/AIWS) with formation of a northeastern Pacific “warm blob” (Bond, Cronin, Freeland, & Mantua, 2015), the role of temperature in disease onset and intensification is ambiguous (Hewson et al, 2018; Miner et al, 2018), suggesting that wasting might be caused by a combination of environmental (Hewson et al, 2018; Miner et al, 2018) and biological factors (Menge et al, 2016). Exploration of differential gene expression in P. ochraceus under thermal stress suggested heritable variation in transcriptional response to elevated temperature (Chandler & Wares, 2017).…”

To improve ecological understanding, collect infection data

“…For example, on the North American Pacific coast, millions of sea stars across 20 species experienced mass mortality attributed to sea star wasting disease (Hewson et al 2014). Extensive monitoring of marine invertebrates along the West Coast enabled researchers to detect and evaluate the effects of sea star wasting disease on hosts and their communities (Menge et al 2016, Montecino-Latorre et al 2016, Schultz et al 2016, Burt et al 2018, Miner et al 2018, Moritsch and Raimondi 2018. Extensive monitoring of marine invertebrates along the West Coast enabled researchers to detect and evaluate the effects of sea star wasting disease on hosts and their communities (Menge et al 2016, Montecino-Latorre et al 2016, Schultz et al 2016, Burt et al 2018, Miner et al 2018, Moritsch and Raimondi 2018.…”