The transmissibility and severity of black spot and wasting disease in sea urchins from laboratory experiments and field studies

Abstract Sea urchins can be important in structuring multiple ecosystems, but the causes of population fluctuations including the role of infectious disease are often not well understood. We performed laboratory experiments and field studies with purple sea urchins (Strongylocentrotus purpuratus) to investigate the transmissibility and severity of two diseases, wasting and black spot. We found that wasting disease and wasting mortality were equally likely in tanks with and without a wasting urchin. In contrast, the presence of a sea urchin with black spot disease led to faster growth of abrasions and significantly higher (41%) mortality than without a diseased sea urchin (0%). Finally, in the field, the prevalence of black spot disease decreased significantly with distance from a focal urchin with black spot disease but not in plots centered on a healthy reference urchin; black spot prevalence also decreased with sea urchin density. These results suggest that wasting disease likely results from non-transmissible physiological stress, while black spot disease is caused by a pathogen that can be transmitted without contact, is transmitted locally, and can cause substantial mortality. Despite these differences, both diseases can cause mortality and population declines in sea urchin populations with cascading effects on kelp forests.