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Sea star wasting disease has differential effects on brooding sea stars

Dr. C. Sarah Cohen, San Francisco State University
Dr. Renate Eberl, Santa Rosa Junior College
Laura Melroy, recent Cohen Lab Master’s student

May 17, 2016

Sea stars are important members of intertidal and subtidal communities along the eastern Pacific coast, yet sea star numbers have been decimated in the past few years by sea star wasting disease (SSWD). A virus has been linked to the disease that progresses from white lesions to loss of arms and ultimately death. Multiple environmental stressors such as changes in temperature, salinity, or pH might increase susceptibility to SSWD. Surveys of changes in sea star abundance are important to understand effects of the disease on marine communities. Survey efforts focus largely on Pisaster ochraceous, a keystone predator of rocky intertidal areas that suffered drastic initial declines but has recently shown some localized signs of recovery. This species, like most sea stars, is a broadcast-spawner that produces large amounts of planktonic larvae that can disperse with currents and are theoretically able to re-colonize large areas decimated by SSWD.

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Adult female Leptasterias reveals brood in place over the mouth. It will remain there for the duration of early embryonic development until offspring hatch out as tiny sea stars and walk out from underneath the mother. Photo Credit: Ashley Smith

Leptasterias spp. co-occur with P. ochraceous, are sometimes considered competitors, and initially appeared unaffected by SSWD. Leptasterias brood their young, which means the female protects her fertilized eggs in a cavity formed by her arms until they develop into juveniles that crawl away from their mother. During this entire time the female remains in one place and does not feed.

Brooding sea stars with smaller numbers of offspring that have low dispersal potential provide a contrasting opportunity to study local effects of SSWD. Abundance and genetic barcoding data from brooding sea stars such as the small six-armed Leptasterias offer insights into population connectivity and disease dynamics on a finer geographic scale. Brooding species like Leptasterias tend to have minimal gene flow between populations, which leads to the potential of both reproductive isolation among relatively close sites and greater fluctuation in population density over time. The likelihood of local extinctions of Leptasterias spp. is therefore extremely high, however separation between populations through genetic isolation could also lead to local adaptation and evolution of disease resistant populations. Barcoding of Leptasterias before the onset of SSWD revealed several cryptic species complexes with clades of overlapping distributions along the eastern Pacific.

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Cohen Lab members conducting timed counts of Leptasterias. Photo Credit: Bing Huey

With COAST funding, we conducted field sampling to investigate changes in the distribution and genetic composition of Leptasterias populations related to the occurrence of SSWD. We re-sampled core sites that had been sampled prior to the outbreak of SSWD and added new sites in Central California and Mendocino County. Fieldwork was performed with San Francisco State University graduate and undergraduate students, Santa Rosa Community College students and citizen volunteers who were trained in sea star identification and field methods and participated in the development of sampling protocols. Sea star abundance was determined via timed counts where a number of trained observers search predefined areas over a set time period, a method that has been shown suitable for varied and complex habitats when low numbers of stars are present. Our group surveyed 22 sites distributed over approximately 500 km of coastline from Monterey to Mendocino County. All sea stars were examined for evidence of SSWD in the field.

SSWD coincided with a significant and sometimes rapid decline in Leptasterias abundance. Data analyzed to date indicate that Leptasterias declined in abundance over time and the onset of the disease appears to be later than in larger species. We documented significant declines in abundance from 10’s of Leptasteriasper meter squared to < 1 Leptasterias per meter squared (e.g., an average of 1.1 Leptasteriasper observer hour for individual timed counts at sites in Central California). Leptasterias numbers are also low in Monterey and Mendocino county with an average of < 2 Leptasterias per observer hour (range 0 – 8.6 Leptasterias per observer hour). We collected samples of stars with signs of SSWD for further qPCR analysis of disease. We are extending our sampling north into Humboldt and Del Norte Counties where Leptasterias populations remained abundant in fall 2015 and early winter 2016, as well as into Oregon and Washington. We are working with researchers from Humboldt State University for field sampling in northern California and others including Oregon State University, Western Washington University, the University of Washington, and Olympic National Park.

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Adult Leptasterias in the central California rocky intertidal, summer 2015. Photo Credit: Caleb Shaw

At sites where we were still able to find Leptasterias, we collected genetic samples using a minimally invasive sampling protocol. We are in the process of molecular barcoding the samples in order to test for changes in clade distributions. Previous work in the Cohen Lab uncovered multiple cryptic clades of Leptasterias in central California and preliminary data related to SSWD indicates a shift in the distribution of clades. For example, a previously undescribed clade that was present around the Bay Area before the onset of SSWD is no longer found at these sites. This project may allow us to identify clades that are severely impacted by the disease compared to more resistant clades. Differential response to SSWD may be linked to differences in habitat preference and behavior.

We presented preliminary findings at the Society of Integrative and Comparative Biology meeting in Portland in January 2016 and at the Sea Star Wasting Summit organized by the Seattle Aquarium in January 2016. We have obtained both intertidal and subtidal Leptasterias tissue samples from researchers in different geographic locations, and we have received funding from NSF and other entities to further increase the geographic extent of our Leptasterias research.

Dr. C. Sarah Cohen is a Professor at the Romberg Tiburon Center for Environmental Studies and in the Department of Biology at San Francisco State University. Dr. Renate Eberl is an Adjunct Professor at Santa Rosa Junior College and a Research Associate at the Romberg Tiburon Center for Environmental Studies. Laura Melroy is a recent Cohen Lab graduate. COAST provided funding for this project: Rapid Response Funding Program Award# COAST-RR-2015-005, August 2015.