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Discovering Kwilakm
  • Discovering Kwilákm
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  • Changing Climate
    • Hotter Ocean Temperatures
    • Changing Ocean Chemistry
    • Rising Sea Levels and Intensifying Winter Storms
    • When Seashore Temperatures Spike – Killer Heat Dome 2021
  • Terminal Creek
    • Where does Terminal Creek’s Water Come From?
    • Signal Crayfish
    • Terminal Creek Fish Hatchery
  • The Lagoon
    • The Tidal Inlet that became the Lagoon
    • Aquatic Plants
    • Chum Salmon
    • The Beaver
    • Canada Geese
    • Three-Spined Stickleback
  • Shores
    • Nearshore Forests
    • Beaches
    • The Terminal Creek Sand Flats
    • The Curious Clay Beds of Kwilákm
    • Blue Mussels
    • Clams
    • Purple Stars
    • Oysters in Kwilákm
  • Shallows
    • Eelgrass
    • Young Chum Salmon
    • Winter Bay Birds
    • Year-Round Bay Birds
  • Deeper Waters
    • Plankton
    • Northern Anchovy
    • Harbour Seal
    • Octopus
Conservancy logoBowen Island Conservancy
    • About
    • Get Involved
  • Discovering Kwilákm
    • About
    • Get Involved
  • Changing Climate
    • Hotter Ocean Temperatures
    • Changing Ocean Chemistry
    • Rising Sea Levels and Intensifying Winter Storms
    • When Seashore Temperatures Spike – Killer Heat Dome 2021
  • Terminal Creek
    • Where does Terminal Creek’s Water Come From?
    • Signal Crayfish
    • Terminal Creek Fish Hatchery
  • The Lagoon
    • The Tidal Inlet that became the Lagoon
    • Aquatic Plants
    • Chum Salmon
    • The Beaver
    • Canada Geese
    • Three-Spined Stickleback
  • Shores
    • Nearshore Forests
    • Beaches
    • The Terminal Creek Sand Flats
    • The Curious Clay Beds of Kwilákm
    • Blue Mussels
    • Clams
    • Purple Stars
    • Oysters in Kwilákm
  • Shallows
    • Eelgrass
    • Young Chum Salmon
    • Winter Bay Birds
    • Year-Round Bay Birds
  • Deeper Waters
    • Plankton
    • Northern Anchovy
    • Harbour Seal
    • Octopus

Photo: Will Husby

Shores

Purple Stars

(Pisaster ochraceus)

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Discover Kwilakm » Story » Shores » Purple Stars
Ocre Stars
Photo: Will Husby

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.

Orca stars on a rock
Two purple stars crammed into a crevice to hide from predators such as gulls and to reduce water loss until the tide returns. Photo: Will Husby

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.

Ochre stars feeding
Purple stars feeding: note how their central bodies are hunched upward as they extrude their stomachs into their prey below. Photo:Will Husby

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.

Starfish tube feet
An upside down star displaying its many tube feet. Photo:Will Husby

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.

Figure: Will Husby

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.

Figure: Will Husby

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.

Seagull with a sea stars in its mouth
A gull harvesting a young purple star near Mothers Beach. Photo:Will Husby

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.

Dying purple sea star
A dying purple star: Photo: Oregon State University

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.

More About Shores

  • Shores
  • Nearshore Forests
  • Beaches
  • The Terminal Creek Sand Flats
  • The Curious Clay Beds of Kwilákm
  • Deep Bay Brickyards
  • Blue Mussels
  • Clams
  • Purple Stars
  • Oysters in Kwilákm
  • Does Kwilákm Have Two Species of Oyster?
  • Oyster are Pretty Awesome Creatures
  • Oysters and People
  • Oyster Harvesting: Health and Safety


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