Sunday, August 10, 2014

Ochre Sea Star

An ochre sea star found on the Olympic Coast in Washington.
Photo by: Matt Brincka
The ochre sea star (Pistaster ochraceus), also known as the purple sea star or the ochre starfish, is a popular site among tide pools and waters of the Washington coast and can live up to twenty years. Where ochre stars are present, it is often considered the keystone species of their environment. Ochre stars are a highly effective predator who mostly feed on muscles, but are known to eat snail, limpets, barnacles, and chitons. It has been shown that a few ochre stars have a profound impact on mussel populations. The loss of just a few purple sea stars can send mussel populations skyward, which inherently reduces the health of the ecosystem. The presence of Pisaster Wasting Syndrome on the West Coast of America highlights fears of massive starfish die-offs, effectively changing the ecosystem.

Like most starfish, P. ochraceus has five arms (rays) that give it the appearance of a star. On the ventral (bottom) side of the starfish it has small tubular suckers that allow them to adhere to rocky surfaces; a perfect tidal adaptation for being constantly pounded by the waves of the changing tides. While the majority of individuals are purple, you can also find ochre stars in oranges, red, browns, and yellows. The dorsal (top) side contains many small spines, or ossicles, that give it a rough to the touch.

Ochre stars in the crevasse between two rocks.
Perfect natural protection from the pounding waves.
Photo by: Matt Brincka
Like other sea star, the ochre star can expel its stomach from its body to help digest their meals that can't fit into its mouth... which is literally almost everything that it eats. Its stomach is so versatile that it can push it into snail shells and the narrow slits of bivalves (a shellfish that has a shell with two hinged parts... like a mussel, clam, oyster, etc) that are 0.1 mm wide. For slits that it can't push its stomach into, they will use their suckers to pry the poor, yet yummy, bivalve open.

Mussels. An ochre star's feast.
Photo by: Matt Brincka

The ochre star is actually a dioecious species, meaning there are both male and female individuals. Though there are both male and females, there is no external sexual dimorphism... meaning they look exactly alike. Reproduction actually occurs through broadcast spawning, which means males and females will expel sperm and eggs into the water column where fertilization happens. Corals are often used as a prime example of broadcast spawners.

Orange and purple ochre sea stars.
Photo by: Matt Brincka

The ochre star has very few natural predators besides the sea otter and gulls. Scientists believe one of the main sources of any types of species number disturbance stems from human collectors or casual tide pool observers. If you are wandering the coast and exploring tide pools, please refrain from stepping on or removing any organism from its natural ecosystem.

Interesting Tid-Bits

  • The gonads (sexual organs) when fully matured can take up roughly 40% of the sea star's weight.
  • Sea stars can regrown limbs! If an arm is broken off by a predator, the sea star will gladly still live on with four limbs until it can regrow a new one.

Scientific Classification
Kingdom: Animalia
Phylum: Echinodermata
Class: Asteroidea
Order: Forcipulatida
Family: Asteriidae
Genus: Pisaster
Species: P. ochraceus

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