Ochre Sea Star (Pisaster ochraceus): Starfish Facts

What does an ochre sea star look like?

The ochre sea star has a sturdy central disc and five thick, tapering arms. Its upper surface is rough to the touch, covered with small white knobs and spines, no more than about 2 mm high, that form a net-like or pentagon pattern on the disc and broken lines along the arms. Large adults look heavy and stiff, more like a carved stone than a soft animal.

Despite the name, many ochre sea stars are purple. Others are orange, yellowish ochre, reddish or brown, and the mix of colors changes from one stretch of coast to another. Why some stars are purple and others orange is still not fully understood.

On the underside, a groove runs down the middle of each arm, lined with rows of tube feet: small, water-filled tubes tipped with suckers. Hundreds of them work together to grip rock, crawl and pull on prey. A small, pale, rounded plate on the upper surface between two arms is the madreporite, a sieve plate that lets seawater into the animal’s hydraulic water vascular system.

Like every sea star, the ochre sea star has no brain. A ring of nerves around the mouth connects to a nerve running down each arm, and one arm can take the lead while the others follow. At the tip of each arm sits a tiny cluster of light-sensitive cells that works as a simple eyespot. If you want to know why it counts as an echinoderm rather than a fish, see Is a starfish a fish?.

Two relatives can cause confusion. The giant sea star (Pisaster giganteus) has white spines ringed with blue, and the short-spined sea star (Pisaster brevispinus) is pink with small white spines and often digs for clams in sand. The mottled star (Evasterias troschelii) has a smaller disc and longer, slimmer arms.

How big do ochre sea stars get?

Adults usually measure about 25–35 cm (10–14 in) across, and large individuals can reach about 45 cm (18 in), with each arm roughly 10–25 cm (4–10 in) long. They are thick and heavy for their size. Ochre sea stars reach maturity at about five years old and 70–90 g (2.5–3.2 oz), and big adults on rich mussel beds are many times heavier.

Growth depends heavily on food. Stars living on wave-beaten shores packed with mussels grow faster and larger than those on poorer, sheltered shores.

Habitat and range

The ochre sea star lives along the Pacific coast of North America from Prince William Sound in Alaska south to Point Sal in Santa Barbara County, California. A southern subspecies, Pisaster ochraceus segnis, continues from there into Baja California, Mexico. The species is most abundant in the cooler waters of the north.

Its classic home is the wave-washed rocky shore, especially mussel beds in the middle and lower intertidal zone, the band of shore that is covered at high tide and exposed at low tide. Ochre sea stars also live on the pilings of piers and docks and in bays, and they range below the tide line to about 90 m (300 ft). Young stars hide in crevices and under rocks.

Life between the tides is harsh. At low tide an ochre sea star may sit in the sun and wind for hours. It can survive losing about 30 percent of its body weight in fluids, and experiments have shown that after a hot low tide it soaks up extra seawater, which helps keep its body cooler during the next one. Even so, its feeding and growth slow sharply when its body temperature rises above about 23 °C (73 °F), and temperatures near 30 °C (86 °F) can kill it.

What do ochre sea stars eat?

The ochre sea star is a carnivore, and its favorite prey is the California mussel (Mytilus californianus). It also eats bay mussels, acorn barnacles, snails, limpets and chitons, and occasionally sea urchins and crabs.

To eat a mussel, the star wraps its arms around the shell and pulls the two halves apart with its tube feet. The mussel cannot hold its shell shut forever, and the sea star only needs a gap a fraction of a millimeter wide. It then pushes part of its stomach out through its mouth and into the shell, where digestive enzymes turn the mussel’s soft body into a soupy meal. One estimate suggests a single star can eat around 80 California mussels in a year. The whole process is described in How do starfish eat?.

Ochre sea stars hunt mainly while the tide is in. When the water drops, they usually stop moving and cling tightly to the rock until it returns.

The original keystone species

The ochre sea star is famous in science because of one long experiment. In the early 1960s the American ecologist Robert T. Paine began removing every ochre sea star from a stretch of rocky shore at Makah Bay, on Washington’s Olympic Peninsula, while leaving a nearby stretch untouched.

Without the sea stars, California mussels spread across the rock and crowded out almost everything else. The cleared shore started with 15 species of animals and seaweeds attached to the rock; after three years only 8 remained, and over the following years mussels came to cover most of the space. Paine described the experiment in 1966, and in 1969 he coined the term “keystone species” for a predator like this one, whose effect on its community is far larger than its numbers suggest, like the keystone that holds up an arch.

Later research showed the ochre sea star is a powerful keystone on wave-exposed shores but much less important in sheltered places. The idea spread to many other animals, including the sea otter of kelp forests, whose story is told in Why are sea otters a keystone species?.

Behavior: slow but persistent

Like other sea stars, the ochre sea star walks on its tube feet, usually covering only a few tens of centimeters a minute. It cannot chase prey, so it relies on patience and on prey that cannot run away. When a star is turned upside down, it twists the tips of its arms, grips the rock with them and slowly rights itself.

Its upper surface carries tiny pincer-like structures called pedicellariae, which snap at small animals and larvae that try to settle on its skin and help keep it clean. When disturbed, an ochre sea star can pull in its tube feet and its tiny skin gills, close the grooves under its arms and spread its spines over them for protection.

Like many sea stars, it can regrow a lost arm, although a new star can only form if part of the central disc is still attached. Can starfish regrow their arms? explains how this works.

Reproduction and life cycle

Ochre sea stars have separate sexes, but males and females look exactly alike from the outside. Each arm holds a pair of feathery gonads, which swell before the breeding season until they can make up as much as 40 percent of the animal’s weight.

Breeding happens by broadcast spawning in spring and early summer; in Puget Sound it takes place from about May to July. Males and females release sperm and eggs into the water, where fertilization happens, and the parents give no further care.

The fertilized eggs develop into tiny, see-through larvae that drift with the plankton and feed on microscopic algae, using bands of beating hairs called cilia. After several larval stages, including one called the brachiolaria, the larva settles on rock, attaches with sticky arms and transforms into a miniature five-armed sea star. The young star then needs about five years to reach adulthood.

Lifespan, predators and threats

Ochre sea stars are long-lived. Field studies suggest many reach 20 years or more, and one estimate puts the maximum at about 34 years.

Adults have few predators because they are tough, spiny and taste unpleasant. Sea otters and gulls, such as the western gull, are known to eat them. The main threats are heat stress during warm low tides, marine heat waves, disease and people who collect stars from tide pools or trample the shore.

The biggest blow came in 2013, when sea star wasting disease swept the Pacific coast of North America. Ochre sea stars were among the species with the highest death rates, and at many sites most adults died within months. Declines were proportionally greater in the southern part of the range.

Conservation status

The ochre sea star has not been evaluated by the IUCN Red List and has no special protection under US federal law. It is still common along much of the coast.

Since the outbreak, many shores have seen large numbers of young ochre sea stars settle and grow, especially in the north of the range. The total weight of sea stars on many shores, and the pressure they put on mussel beds, has stayed below the levels seen before 2013, so scientists keep monitoring the species closely. In many parks and marine protected areas along the coast, removing sea stars from the shore is not allowed.

Relationship with people

For millions of visitors, the ochre sea star is the animal that makes a tide pool memorable. It is easy to see at low tide on the coasts of Washington, Oregon, California and British Columbia, often in clusters of purple and orange on the same rock.

It is also one of the best-studied invertebrates on the coast. Long-term monitoring programs, such as the Multi-Agency Rocky Intertidal Network (MARINe), have counted ochre sea stars at many sites along the coast for years, and their records allowed scientists to measure exactly how hard sea star wasting disease hit.

When you meet one, look closely but do not pull it off the rock. Watching how it holds on, how its arms slowly move and how it fits into the mussel bed shows why such a quiet animal has had such a loud voice in ecology.