Sea stars (you may know them as starfish) are prominent
features of the marine environment all over the world. They are especially
prominent in Pacific Northwest waters, where our species are among the largest,
most diverse and best studied. In fact, the Pacific Northwest marine
environment shelters the highest diversity of this group (class Asteroidea,
phylum Echinodermata) anywhere in the world.
Perhaps this prominence made even more startling the
discovery in 2013 that sea stars were dying off in great numbers in our waters.
Sunflower stars (Pycnopodia helianthoides)
were the first to be discovered. This largest of all sea stars is a top-level
predator in our intertidal zone, eating just about everything smaller than it,
so its disappearance could have profound ecological effects.
Just as important and much more abundant, the ochre sea star
(Pisaster ochraceus) has long been
known to control the distribution of mussels in the intertidal zone. By preying
upon them in the lower intertidal and thus opening up substrates that would
otherwise be completely covered by mussels, the sea stars provide habitat for
many other species. The ochre sea star has been called a keystone species
because of its importance. This species too has been dying off, in many cases
completely disappearing from areas where it was once abundant.
These echinoderms are dying from sea star wasting disease,
which causes the animal to deteriorate rapidly. Lesions form on the outer
surface, and the arms begin to writhe around and eventually pull loose,
spilling the internal organs. Death follows very soon, the animal turning to
mush. This is a horrible thing to see, and you’ll have to go online elsewhere
to see photos of it, as this is a family blog.
Other sea stars known to be affected include the sun stars (Solaster spp.). Some common species seem
to be less affected or perhaps not at all, for example the blood star (Henricia leviuscula), leather star (Dermasterias imbricata) and bat star (Patiria miniata). Why would this be?
Like everything else about this puzzle, no one knows.
The animals are being monitored in many areas now, both from
shore and under water, and as spring comes and the intertidal begins to be
exposed more during the daytime, it will be even easier to determine the fates
of sea star populations all along the coast. Divers and remote underwater
cameras have reported mass mortality in numerous places, with sea stars going
from common to virtually absent in a shockingly short time, as if an epidemic
swept through them en masse. Their dermal ossicles (the only hard part of a sea
star) litter the bottom in some areas where they were once common.
Dead and dying animals have been brought into several
laboratories, where efforts are being made to determine what pathogen(s) might
be causing this disease. Earlier die-offs of sea stars were blamed on higher
than usual ocean temperatures, but that’s not the case now.
So far, no bacterium or virus has been incriminated in these
searches, but the search goes on. Stay tuned for more about this ongoing and
very disconcerting drama.
Dennis Paulson