Description:
Pisaster ochraceus is a common local sea star (Phylum Echinodermata). Their star-shaped body consists of five short, stout arms (or rays) that arise from a central disc. Although commonly there are five arms the number of arms can vary from 4 to 7. The average body size of P. ochraceus is approximately 25 cm in diameter, but they can grow as large as 40 cm in diameter. A thin layer of soft skin is usually purple, but they can also be orange-red or yellow-brown. Individuals can change colour over time, which may be linked to what the sea star eats.
Underneath the soft skin is an internal skeleton composed of calcareous plates that project spines or bumps giving the sea star's body surface a warty appearance; hence the name echinoderm - "spiny skin". The somewhat flexible skeleton allows the arms to bend and twist.
On the underside there are grooves that extend from the central disc along each arm. These grooves contain tube feet (or podia) that the sea star uses to move. The tube feet are extended by hydraulic pressure created by the sea star's water vascular system. The tips of these tube feet are flattened, forming suckers that the sea star uses to grasp the substratum, to move across irregular surfaces, and to right itself if it gets turned upside down. The tube feet also grasp and handle prey, moving the food to the sea star's mouth located on the bottom surface of the oral disc.
The tip of each arm bears a red eyespot that allows the sea stars to detect changes in light intensity. The skin of P. ochraceus also contain sensory cells that allow the sea stars to detect touch and chemicals. These sensory cells are particularly abundant on the suckers of the tube feet. The body surface of P. ochraceus also has very small, claw-like appendages, called pedicellaria, that are used for protection, especially against small animals or larvae that might try to settle on the sea star.
Underneath the soft skin is an internal skeleton composed of calcareous plates that project spines or bumps giving the sea star's body surface a warty appearance; hence the name echinoderm - "spiny skin". The somewhat flexible skeleton allows the arms to bend and twist.
On the underside there are grooves that extend from the central disc along each arm. These grooves contain tube feet (or podia) that the sea star uses to move. The tube feet are extended by hydraulic pressure created by the sea star's water vascular system. The tips of these tube feet are flattened, forming suckers that the sea star uses to grasp the substratum, to move across irregular surfaces, and to right itself if it gets turned upside down. The tube feet also grasp and handle prey, moving the food to the sea star's mouth located on the bottom surface of the oral disc.
The tip of each arm bears a red eyespot that allows the sea stars to detect changes in light intensity. The skin of P. ochraceus also contain sensory cells that allow the sea stars to detect touch and chemicals. These sensory cells are particularly abundant on the suckers of the tube feet. The body surface of P. ochraceus also has very small, claw-like appendages, called pedicellaria, that are used for protection, especially against small animals or larvae that might try to settle on the sea star.
Ecology:
Pisaster ochraceus lives along exposed and protected shorelines from Alaska to California. They inhabit the mid intertidal zone to water depths of 90 m. Larger individuals, which can withstand prolonged exposure to the air, tend to be found higher in the intertidal zone.
P. ochraceus prefer to feed on blue mussels (Mytilus spp.); but, they will also feed on barnacles, limpets, clams, chitons, snails and crabs. Mobile prey species can move to escape sea stars, but sessile animals such as mussels and barnacles do not have this option. When feeding on mussels, the sea star uses its arms and tube feet to pull open the two halves of the shell. The sea star can then evert its stomach through its mouth and digest its prey externally!!
P. ochraceus has few natural predators (e.g. gulls and sea otters). But, if attacked, P. ochraceus can withdraw their tube feet into the groove in which they are located. If they are damaged, P. ochraceus can regenerate their arms, but part of their central disc must remain intact.
Over the past few years, P. ochraceus (and other species of sea star) throughout their entire range have been dying in very large numbers due to a wasting disease (90-95% mortality in some areas). Sea stars with this disease decay until their body fragments and all that remains is a white, mushy blob. Similar die offs have been observed periodically since the 1970's but never with the same magnitude or wide geographic range. Research suggests that the most likely cause of this disease is a virus. This die-off of P. ochraceus is a major concern for biologists because P. ochraceus helps to control the population size of blue mussels (Mytilus spp.), which allows other invertebrates and algae to inhabit rocky intertidal areas. In the absence of predation by P. ochraceus, blue mussels can potentially take over a large rocky intertidal area.
P. ochraceus prefer to feed on blue mussels (Mytilus spp.); but, they will also feed on barnacles, limpets, clams, chitons, snails and crabs. Mobile prey species can move to escape sea stars, but sessile animals such as mussels and barnacles do not have this option. When feeding on mussels, the sea star uses its arms and tube feet to pull open the two halves of the shell. The sea star can then evert its stomach through its mouth and digest its prey externally!!
P. ochraceus has few natural predators (e.g. gulls and sea otters). But, if attacked, P. ochraceus can withdraw their tube feet into the groove in which they are located. If they are damaged, P. ochraceus can regenerate their arms, but part of their central disc must remain intact.
Over the past few years, P. ochraceus (and other species of sea star) throughout their entire range have been dying in very large numbers due to a wasting disease (90-95% mortality in some areas). Sea stars with this disease decay until their body fragments and all that remains is a white, mushy blob. Similar die offs have been observed periodically since the 1970's but never with the same magnitude or wide geographic range. Research suggests that the most likely cause of this disease is a virus. This die-off of P. ochraceus is a major concern for biologists because P. ochraceus helps to control the population size of blue mussels (Mytilus spp.), which allows other invertebrates and algae to inhabit rocky intertidal areas. In the absence of predation by P. ochraceus, blue mussels can potentially take over a large rocky intertidal area.
Life History:
Pisaster ochraceus may live for 20 years. They don't reach sexual maturity until they are five years old. P. ochraceus has separate sexes but there are no external featuers that can be used to distinguish males from females. In the Pacific Northwest, P. ochraceus reproduce from March to June. Fertilization is external - the eggs and sperm are released into the ocean. A female may release as many as 40 million eggs!! Fertilized eggs develop into planktonic larvae that float in the water column for 2-6 months before settling to the ocean floor and developing into juvenile sea stars.
To learn more:
http://linnet.geog.ubc.ca/efauna/Atlas/Atlas.aspx?sciname=Pisaster%20ochraceus
Harley, D. CG., Pankey, M.S., Wares, J.P., Grosberg, R.K. and Wonham, M.H. 2006. Colour polymorphism and genetic structure in the sea star Pisaster ochraceus. Biological Bulletin, 22(3)" 248-262.