For many years, there have been continuous population explosions of various types of echinoderms (sea urchins and sea stars) in the Strait of Georgia and Howe Sound. At the Aquarium, wet lab staff have known for several years that sunflower sea stars, Pycnopodia helianthoides, have a tendency to disintegrate in a fashion that appears to progress rapidly, spreading to other starfish, even across species.
In Howe Sound, there have been such high densities of sunflower stars that the Aquarium has posted a video of a sea star landslide, showing how these crowded sunflower sea stars can literally fall over each other when in a flight response.
In the last few weeks, these sunflower sea stars appear to have simply fallen ill; thousands have disintegrated and died in shallow water where their numbers are most dense. The die-off is concentrated on the east side of Howe Sound. The southern shore of Bowen Island, which had such high densities that we published a blog post on their destruction of kelp beds, seems to have gone through a complete die-off, with no more dead bodies in evidence. The southwestern corner of Howe Sound and areas across the Strait of Georgia appear unaffected in our dive surveys in the last ten days.
The blood star and the leather star do not appear to be affected by this process, whether it is an infectious disease or something different. Leather stars actually feed on the dying sunflower stars (see left photo).
For a quarter of a century, the ochre sea star, Pisaster ochraceus, has suffered from a parasitic castration that would occasionally cause a male ochre star to disintegrate. In the present situation with sunflower stars, it appears that all individuals are susceptible where they are crowded in close proximity.
It might be well to consider the Malthusian principal, which states that human populations will be kept in check by war, famine and pestilence. These sunflower stars have few enemies and they can eat almost anything, so pestilence is a likely suspect for this mysterious and sudden control of their overpopulation. From an ecological perspective, it is likely a healthy thing for the entire ecosystem.
Written by Jeff Marliave, vice president of marine science, Vancouver Aquarium
With the support of Sitka Foundation, the Vancouver Aquarium is embarking on a two-year project to train divers to identify marine life in Howe Sound, as part of our commitment to the research and conservation of this area. The information they glean on Howe Sound’s sea life will be presented in this series of blogs, and will be used to educate students taking part in the Aquarium’s school programs and AquaVan visits to inspire the next generation to keep learning more about marine biodiversity in British Columbia.