''Pisaster ochraceus'', generally known as the purple sea star, ochre sea star, or ochre starfish, is a common

seastar Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish ...

found among the waters of the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

. Identified as a

keystone species A keystone species is a species which has a disproportionately large effect on its natural environment relative to its abundance, a concept introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in maintaini ...

, ''P. ochraceus'' is considered an important indicator for the health of the

intertidal zone The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species o ...

Description

This sea star has five stout rays that range in length from 10 to 25 centimeters (4 to 10 in). The rays are arranged around an ill-defined central disk. While most individuals are purple, they can be orange, orange-ochre, yellow, reddish, or brown. The aboral surface contains many small spines (ossicles) that are arranged in a netlike or pentagonal pattern on the central disk. The ossicles are no higher than 2 mm.Kozloff, E. N. (1996). Marine Invertebrates of the Pacific Northwest. Seattle: University of Washington Press.McFadden, M. (2002). Pisaster ochraceus. Retrieved May 10, 2010, from https://web.archive.org/web/20090412083835/http://www.wallawalla.edu/academics/departments/biology/rosario/inverts/Echinodermata/Class%20Asteroidea/Pisaster_ochraceus.html In ''Pisaster'' the tube feet have suckers on their distal ends which allow them to attach to the rocky substrate and live in heavily wave-swept areas. ''P. ochraceus'' has a simple nervous system and does not have a brain. A

nerve ring A circumesophageal or circumpharyngeal nerve ring is an arrangement of nerve ganglia around the esophagus/ pharynx of an animal. It is a common feature of nematodes, molluscs, and many other invertebrate animals, though it is absent in all vertebra ...

connects and relays impulses between the star's radial nerves. Two species that can be mistaken for ''P. ochraceus'' are '' P. giganteus'', which has blue rings around white or purple spines, and '' P. brevispinus'', which is pink with small white spines. These two species have different aboral spines and coloration which allows one to distinguish between the species. '' Evasterias troscheli'' may be confused with ''P. ochraceus'' at times as well. It can be distinguished by its smaller disk size and longer, tapering rays which are often thickest a short distance out from their base rather than at the base as in ''P. ochraceus''.

Reproduction and life history

Reproduction

Members of ''Pisaster'' are

dioecious Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproductio ...

but there is no

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

and sexes can be separated only by the presence of eggs or sperm in the gonads. They reproduce by broadcast spawning, which occurs in the Puget Sound around May to July. There is no parental investment beyond spawning. Fertilization occurs in the water column and ''Pisaster ochraceous'' develops through several larval stages. The reproductive system consists of a pair of gonads branching into each ray off a circular genital strand which is along the oral inner surface of the central disc. The gonads look like a feathery collection of tubules. In females there are orange gonads and in males they are whitish.Nybakken, J. (1996). Diversity of the invertebrates. Hayward: California State University During maturation of the gametes, the gonads increase in size and can account for up to 40 percent of the sea star's weight. The gonopores are too small to be seen, and can only be found when the sea stars are spawning.Ramirez,Y. (2002). Pisaster ochraceus, Animal Diversity Web. Retrieved May 10, 2010, from http://animaldiversity.ummz.umich.edu/site/accounts/information/Pisaster_ochraceus.html.

Lifespan

Many sea stars live to a minimal age of four years. ''P. ochraceus'' can live as long as twenty years."Pisaster ochraceus (Brandt, 1835)". Encyclopedia of Life. Retrieved May 10, 2010, from "http://www.eol.org/pages/598469". This species of seastar is often considered a

in many intertidal regions. ''P. ochraceus'' is a predator of the California mussel, '' Mytilus californianus'' and reduces its abundance. This allows for other macroinvertebrates to persist. In an experimental removal of ''P. ochraceus'', it was shown that ''Mytilus californianus'' becomes almost completely dominant of the intertidal community. When ''P. ochraceus'' is present there is a diverse intertidal community.Holsinger, K. (2005). Keystone species. Retrieved May 10, 2010, from

Feeding

At the larval stage, ''Pisaster ochraceus'' are filter feeders and their diet consists of

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

. As an adult, ''P. ochraceus'' feeds on mussels such as ''Mytilus californianus'' and ''

Mytilus trossulus ''Mytilus trossulus'', the bay mussel or foolish mussel, is a medium-sized edible marine bivalve mollusc in the family Mytilidae. ''Mytilus trossulus'' is one of the three principal, closely related taxa in the ''Mytilus edulis'' complex of blue ...

''. They also feed on

chiton Chitons () are marine molluscs of varying size in the class Polyplacophora (), formerly known as Amphineura. About 940 extant and 430 fossil species are recognized. They are also sometimes known as gumboots or sea cradles or coat-of-mail s ...

limpet Limpets are a group of aquatic snails that exhibit a conical shell shape (patelliform) and a strong, muscular foot. Limpets are members of the class Gastropoda, but are polyphyletic, meaning the various groups called "limpets" descended indep ...

snail A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gastro ...

barnacle A barnacle is a type of arthropod constituting the subclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in eros ...

echinoids Sea urchins () are spine (zoology), spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard s ...

, and even decapod crustacea. ''P. ochraceus'' uses its tube feet to handle its

prey Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...

. If the prey is too large to be swallowed whole, then it can use its tube feet to open shells. It can evert its stomach through its mouth and engulf its prey, liquify it with digestive enzymes and ingest the processed food. Mussels hold their valves together very securely but ''P. ochraceus'' can insert part of its everted stomach, or some digestive juices, through the narrow gap that exists where the byssal threads emerge from the shell. The mussel needs to open its valves periodically to feed and breathe and the sea star can exert a powerful traction with its tube feet, pulling the two valves further open. Once the stomach is inside the mussel, digestion takes place. It is thought one sea star can consume eighty Californian mussels in a year.

Ecology and distribution

Conservation

''Pisaster ochraceus'' has been described as a

. Experiments by

zoologist Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the Animal, animal kingdom, including the anatomy, structure, embryology, evolution, Biological clas ...

Robert T. Paine in the 1960s demonstrated that a loss of only a few individual ''P. ochraceus'' seastars had a profound impact on mussel bed population, thereby reducing the health of the intertidal environment. With only a few natural predators (sea otters and seagulls) it is suggested that the principal enemies of ''P. ochraceus'' are human collectors and casual tidepool visitors. ''Pisaster ochraceus'' has not been evaluated by the International Union for Conservation of Nature (IUCN).

Geographic range

''P. ochraceus'' can be found from

Prince William Sound Prince William Sound (Sugpiaq: ''Suungaaciq'') is a sound of the Gulf of Alaska on the south coast of the U.S. state of Alaska. It is located on the east side of the Kenai Peninsula. Its largest port is Valdez, at the southern terminus of the Tr ...

in Alaska to Point Sal in Santa Barbara Co., California. The subspecies found within the warmer waters from Santa Barbara County to Baja California is ''P. o. segnis''.Humphreys, V. (2003). The Biogeography of the Purple Ochre Sea Star (''Pisaster ochraceus''). Retrieved May 10, 2010, from

Habitat

This sea star can be found in great numbers on mussel beds and on wave-washed rocky shores. The juveniles are often found in crevices and under rocks. Its depth range is from above the low-tide zone to 90 m. ''P. ochraceous'' is very durable and can tolerate a loss of thirty percent of its body weight in body fluids.

Effects of ocean acidification

A study found that ''P. ochraceus'' will not be affected by

ocean acidification Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...

in the same way as most

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...

marine animals. This normally causes decreased growth due to the increased acidity dissolving calcium carbonate. Researchers found that when ''P. ochraceus'' was exposed to and 770 ppm (beyond rises expected in the next century) they survived. It is thought that this is because the animal's calcium is nodular and so it is able to compensate for the lack of carbonate by growing more fleshy tissue instead.

References

External links

''Pisaster ochraceus''Species by zone heightOchre sea star''Pisaster ochraceus'' (Brandt, 1835)
San Francisco State University

North Island Explorer * {{Taxonbar, from=Q2727642 f Asteriidae Fauna of the Pacific Ocean Starfish described in 1835 Taxa named by Johann Friedrich von Brandt