Taxonomy

Rarity

Abundance rates how many are in suitable habitat. Find difficulty rates how hard it is to locate one on a daylight snorkel or freedive through 50 ft. A higher find score means a harder search. The rarity category also reflects the kind of encounter; it is not an average of the 2 scores.

Seasonality

Location

Ochre sea stars live along the eastern Pacific from Prince William Sound in Alaska south to Baja California. They hold in the middle and low intertidal on wave-swept rocky shore, which is the band a person walks through at low tide and a diver swims over at high, and they extend subtidally to about 295 feet. Juveniles hide, in crevices, under rocks and inside the mussel beds their parents feed on, and only move into the open as they grow.

Identifying one is easier than it sounds despite the color range. Most are purple. They also come in orange, orange-ochre, yellow, reddish and shades of brown, and the color does not indicate age, sex or health. The reliable mark is on top: white spines on the central disc arranged in a star-shaped pattern, which separates this species from the other large stars sharing the same rock. Most individuals have 5 arms, and 4 to 7 occur. The average arm radius in California and Oregon is about 3.5 inches, measured from the center of the disc to the arm tip, and a large one runs roughly triple that. They can regrow a lost arm and are thought to live up to 20 years.

Deep Dive

The ochre sea star is where the phrase "keystone species" comes from, and the experiment that produced it is one of the most important pieces of field ecology ever done.

In the 1960s Robert Paine picked a stretch of rocky intertidal at Mukkaw Bay, on the Washington coast, and physically removed every ochre star from it, then kept removing them. The plot had held a mixed community: mussels, barnacles, limpets, chitons, several algae. With the sea stars gone, the California mussel, which is the ochre star's main prey on open coast, spread unchecked and crowded out 7 other species that had been holding space on the same rock. Paine published the result in 1966 in The American Naturalist under the title "Food Web Complexity and Species Diversity." It became the most-cited empirical article that journal has ever published.

The insight was that a single predator, not especially abundant and not at the top of any obvious pyramid, was holding a whole community open by eating the one competitor that would otherwise take everything. Remove it and diversity collapses, not because the other species were eaten but because they were crowded out. That is what a keystone species means, and the ochre star is the original one.

The eating is worth describing, because a sea star has no jaws and mussels close hard. An ochre star climbs onto a mussel and grips both shells with its tube feet, the hundreds of small hydraulic suction cylinders on the underside of each arm. It then pulls, steadily and without letting go, for as long as it takes. A mussel can hold its shell shut against far more force than a sea star can generate in a single pull, but it cannot hold it shut indefinitely, and the star does not tire. When the shells gape even slightly, the star everts its own stomach out through its mouth, pushes it through the gap, and digests the mussel inside its own shell. What is left afterward is a clean empty pair of shells still attached to the rock.

Diet shifts with exposure. On open coast, mussels dominate the menu, along with barnacles, snails, limpets and chitons. In protected inland waters, where mussel beds are thinner, the diet is mostly barnacles and whelks. Reproduction is broadcast spawning, with eggs and sperm released into the water and larvae that swim and feed in the plankton before settling.

The wasting disease is the modern half of the story and it now has a culprit. For more than 10 years after the 2013 outbreak the cause was unknown, which is a long time to watch an epidemic without knowing what is doing the killing. In 2025 a team from the University of British Columbia, the Hakai Institute and the University of Washington published the answer in Nature Ecology and Evolution after a 4-year search. Deep sequencing of the internal body fluid of diseased sea stars, from both laboratory experiments and field outbreaks, was dominated by a single bacterium, a marine species in the same group that causes disease in shellfish. A strain of it cultured from a diseased sunflower sea star caused disease and death when healthy sunflower stars were exposed to it.

The precision matters. The causal experiment was run on sunflower sea stars, not on ochre stars, so what is established is that this bacterium causes the disease in that species. Sunflower stars were the hardest hit of all, and their loss is what released purple urchins and drove the kelp forest collapses of the last decade, which is why that species was the focus. Whether the same bacterium drives the disease in ochre stars is a reasonable expectation and not yet a demonstrated fact.

For the ochre star itself, the recruitment record offers the more hopeful reading. Between the fall of 2014 and the spring of 2015, juvenile settlement at Oregon study sites ran 7 to 300 times higher than the year before, an unprecedented surge that gave the population a real chance at rebuilding. Whether those juveniles survive to adulthood, and whether a rocky shore with far fewer keystone predators looks the same in 20 years as it did in 2012, is being watched rather than known.

Where to Find Them

Researched habitat and range candidates

These locations have suitable habitat within the species range. An assignment is a possible encounter, not a confirmed sighting at every beach. Season, depth, and local conditions still matter. Historical habitat without present-day evidence is excluded.