Photo: Will Husby
Shores
Purple Stars
(Pisaster ochraceus)
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Purple stars are probably the most common sea star in Kwilákm. They may be purple, orange, brown, reddish, or even yellow. Look for small white spines that form patterns across their backs. Most purple stars have five stout, tapering arms, but they may have four or seven. The best places to find them are in crevices in rocky shores and between and under cobble rocks at low tide.
Sea stars are a keystone species in some communities, where they balance prey population structure and species diversity. In Kwilákm they help control mussel population, which can expand quickly and exclude other species.
Purple stars are predators that hunt mussels, barnacles, snails, limpets, and chitons. Mussels seem to be their prey of choice.
Once a purple star finds its prey, it grips its shell with its tube feet and pries it open. The star then pushes its stomach outward through the crack. Its digestive juices dissolve the prey’s tissues and the extruded stomach absorbs the liquified prey. Complete digestion can take 2 to 3 days.
Purple stars look as if they cannot move or do so with glacial slowness. In fact, the underside of the purple star is covered by hundreds of tiny tube feet that it uses to move and to capture prey.
The tube feet are an intriguing feature of sea stars and their kin. They are powered by an ingenious water-pressure mechanism called the water vascular system.
Each tube foot consists of an ampula located inside the sea star’s body and attached to a single tube foot. When the muscles surrounding an ampula contract, more water is forced into the tube foot, which lengthens. Bands of small muscles in the outer wall of the tube foot contract to point the lengthening tube foot in different directions. When the ampula muscles relax, they expand, drawing water out of the tube foot so that the tube foot shortens.
Each tube foot is tipped with a disc that secretes a sticky substance used to attach to rocks or prey. When the sea star wishes to release its hold, the disc secretes a second chemical that breaks the bond of its glue.
Any one tube foot on a sea star can act independantly in responding to smell, touch, or light, but coupled together, many tube feet can synchronize their motion to produce a bouncing motion—their version of running. Researchers are still working out exactly how a sea star accomplishes this synchronization, given it has no brain and a completely decentralized nervous system.
Purple stars are known to live up to 20 years in the wild. During this time they must evade marauding gulls, their only known predators, who hunt them down for food at low tide.
Because gulls cannot chew or dismember sea stars, they are limited to eating only small specimens of a radius of six cm or less.
Purple sea stars can breed at the age of five, and they spawn during the summer. The sexes are separate, even though indistinguishable externally. A large female can produce 40 million tiny eggs, which are fertilized by sperm released by males. The tiny larvae float around in the plankton for several months before they settle to the sea floor and become adult sea stars.
Sea Star Wasting Disease
From 2013 to 2015 many Bowen Islanders noticed a sudden decrease in the number of sea stars, including purple stars along the intertidal shores of Kwilákm. Many of the stars were damaged, with missing arms and ugly wounds.
Many scientists believe that sea star wasting disease is caused by the “Sea star associated densovirus.” A sea star wasting disease epidemic swept the Pacific west coast in 2013 to 2015. A large proportion of the purple stars found along the coast died. The incidence of wasting was higher in tide pools than on exposed rock surfaces. The major die- off was followed by an unusual increase (up to 300x) in recruitment by young seastars. As a result of this recovery, today the population of purple sea stars on Bowen shores is similar to that before the onset of the wasting disease.