The etiology of sea star wasting (SSW) disease has eluded us since it caused mass mortality in fall 2013 – mid 2014. Attempts to elicit wasting using a variety of stressors (bacterial challenge, heat stress, tissue homogenate challenges, etc) in aquarium specimens from 2015 – 2017 were not successful. Our early investigation of the Sea Star associated Densovirus (SSaDV) and its link to SSW was not supported by subsequent experiments, re-analysis of 2013-2014 samples using redesigned primers, and re-analysis of metagenomic data. Furthermore, subsequent metagenomic surveys of RNA viruses and bacterial communities failed to yield any candidate pathogens. You can read more about these attempts to substantiate our initial 2014 findings here. One interesting observation as part of these efforts was that water temperature swung wildly from warm to cool to warm immediately prior to SSW onset at a few sites. Hence, we designed an experiment to test whether experimental swings in temperature mimicking these observations could induce wasting.
In May 2018, we traveled to the Long Marine Lab, UC Santa Cruz, to work with Pete Raimondi, Betsy Steele, and REU Citlalli Aquino on these experiments. We collected 12 Pisaster ochraceus from Davenport, CA, and placed them into a flow through sea table in the lab, with the idea that they would adjust to the aquarium environment for several weeks prior to the start of experiments (i.e. to standardize their environmental history). We left the lab with the intention of returning in 3 weeks to start these experiments. To our surprise, the specimens developed SSW lesions and dramatically wasted after 2 weeks… without any external stimuli (other than collection). This was the first time we’d seen actual sea star wasting in 3 years! We couldn’t use these specimens in further experiments, but they proved useful in microbiome studies to understand wasting etiology.
Returning to the lab in late June, we re-focused efforts to understand why sea stars were wasting. If there was no external stimuli that led to wasting in the earlier batch of stars, what types of stressors could result in SSW? We performed several additional experiments on freshly-collected specimens from near Santa Cruz. First, we looked at how aquarium flow rates affected SSW. The idea here was that net water replacement rate would affect accumulation of toxins or other hormones, as well as oxygen saturation. Second, we looked at whether desiccation in the air for a period of time affected SSW; here we thought perhaps that warm weather and exposure at king tides (at the time) may result in faster wasting – so we looked at temperature during these experiments as well as at the collection site. Third, we looked at whether tissue homogenates from a single wasting star we found at the collection site (compared to a proteinase-K treated homogenate… which would be somewhat depleted of proteins) could generate SSW. Fourth, we looked at whether abrasion with pedicellaria and paxillae from a star (via a blast with a syringe, compared to a blast of seawater) would affect SSW. Finally, we examined whether transporting individuals alone (i.e. in separate containers) vs together (i.e. multiple specimens in the same container) would affect SSW. Much to our surprise, every single challenge – low flow rates, dessiccation, challenge with homogenates, abrasion, and close proximity – all resulted in faster wasting than their respective controls.
After this round of experiment, we decided to study what happens to the microbiome and viruses in lesion margins during wasting progression in the absence of external stimuli. We collected 6 fresh specimens, and incubated them individually in a sea table. To differentiate microorganisms that grew/replicated in scar/wound tissues, we knicked specimens every day and collected tissue (lesion/scar margins) to see what changed as animals went from completely healthy to SSW-affected. This last study proved the most influential in terms of understanding wasting etiology, the results of which can be found here.
The experiments and surveys performed in Summer 2018 were mostly facilitated by Citlalli Aquino, then an undergraduate researcher at UC Santa Cruz (and current graduate student at SFSU), and with lab space provided by Betsy Steele.
Publications from this Expedition:
Aquino CA, Besemer RM, DeRito CM, Kocian J, Porter IR, Raimondi P, Rede JE, Schiebelhut LM, Sparks JP, Wares JP, Hewson I (2021) “Evidence that microorganisms at the animal-water interface drive sea star wasting disease” Frontiers in Microbiology. DOI: https://www.frontiersin.org/articles/10.3389/fmicb.2020.610009/
Hewson I, Aquino CA, DeRito CM (2020) “Virome variation during sea star wasting disease progression in Pisaster ochraceus (Asteroidea, Echinodermata)” Viruses 12: 1332
Jackson EW, Wilhelm RC, Johnson MR, Lutz HL, Danforth I, Gaydos JK, Hart MW, Hewson I (2020) “Diversity of sea star-associated densoviruses and transcribed endogenized viral elements of densovirus origin” Journal of Virology DOI: 10.1128/JVI.01594-20
Jackson EW, Pepe-Ranney C, Debenport SJ, Buckley DH, Hewson I (2018) “The microbial landscape of sea star and anatomical and interspecies variability of their microbiome” Frontiers in Microbiology. https://doi.org/10.3389/fmicb.2018.01829