RAPID: A subtle epidemic: unique mortality of Mytilus californianus on the Oregon coast

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On temperate rocky shores, mussels are a “foundation” species, serving as prey for multiple predators, able to filter particles out of huge volumes of water, and harboring hundreds of other species, thereby serving a central role in community function. Hence, threats to their abundance and persistence are of significant ecological and societal concern. Past research has shown that mussels are highly resilient and well-adapted to acute, short-lived stresses such as disturbance from winter storms or short-term warming. In summer 2023, a novel pattern of mortality was discovered during routine field research: high numbers of dead mussels, many with tissue remaining in the shell were observed in a scattered pattern. That is, rather than a mass mortality (i.e., all mussels in a large area are killed), single dead mussels surrounded by live and apparently healthy mussels occurred, with multiple dead individuals per square meter. This project investigates the cause of the mortality event, which preliminary data suggests may be a harmful algal bloom. The project also builds capacity by supporting research training for several undergraduate students and informs the public and resource managers about an emerging threat to the stability of rocky intertidal ecosystems.The hypothesis under investigation is that the 2023 “scattered” mortality event at 8 study sites along the Oregon and northern California coasts affecting the rocky intertidal dominant, Mytilus californianus, may be an unusual consequence of a harmful algal bloom. During this event, densities of dead mussels have ranged from 0.7 to 10.6/m2. This novel pattern contrasts with typical mass mortalities of organisms where all individuals within a specific site or area die due to (e.g.) thermal stress or severe storms. The scattered mortality pattern persisted through late September 2023. Preliminary histological evidence suggests the ability of mussels to digest food is being suppressed, most likely by a water-borne toxin. To investigate the cause of the mortality event, the team is (1) collecting monthly (a) water samples for identification of dinoflagellate species and (b) mussel tissue samples for histological investigation of digestive and kidney tissues and high performance liquid chromatography and mass spectrometry analyses for the presence of algal-derived toxins; (2) continuing mussel bed surveys to determine the seasonality of the mortality event; and (3) quantifying mussel bed persistence through winter storms at sites with higher or lower rates of mortality. This research advances understanding of community responses to intense environmental stresses and how these might influence the future of rocky intertidal ecosystems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.