Species that play an important role in shaping an ecosystem are called keystone species. The havoc purple (ochre) sea stars play on mussels, barnacles, limpets and snails earns them that distinction.
So how do they accomplish their dastardly deeds? As the tide rises and covers their prey, they are close behind, quickly covering their victims with hundreds of pulling tube feet. These begin to apply a massive outward pulling pressure upon the shells of their victims. The muscles of a mussel keep the shells tightly closed, at least for a while but soon fatigue sets in. Muscles just can't compete with the constant pull of hydraulics. The mussel relaxes ever so little and when the shells open less than a 25th of an inch, the slippery stomach of the sea star slides out and slithers through the tiny opened crack. Still attached to the sea star, the stomach begins to digest the mussel within its own shells. If the tide drops before the sea star finishes its meal, it will rip its prey from the rocks and move down with the tide as the stomach continues its work.
The stars of this story were photographed on one of the Copeland Islands near Campbell River, British Columbia. We kayaked and explored these tiny islands by Zodiac under bright blue skies.
