Ochre sea stars are the most common sea stars found in the waters around Vancouver Island, and we see them in three distinct colours: purple, orange, and red-brown. What causes one ochre star to be purple while its neighbour is orange and the next star over is brown?
Scientists don’t know for sure what causes ochre sea stars’ varied colour schemes, but several theories currently exist:
- Colour may be influenced by diet. An ochre sea star’s diet consists of barnacles, limpets, snails and/or mussels. Mussels, in particular, contain a high concentration of carotenoids — orange and red pigments that give colour to certain plants and animals, like carrots and salmon. Ochre stars that eat mostly mussels are thought to take on an orange hue, while stars that eat a more varied diet of mussels and other prey tend to be varying shades of red-brown. Ochre stars that feed mostly on barnacles — which don’t contain carotenoids — are purple.
- Colour may be determined by genes. Although findings to date suggest that ochre stars of all colours share similar genetic structures — and that their genes intermix regularly — scientists still believe genetics may play a role in determining the stars’ colouration, through relationships more complex than those identified so far.
- Colour may be affected by ocean salinity. Over the length of their lives, it’s possible that some ochre sea stars tolerate salinity better than others, and that their varying colours reflect this difference. More research is necessary here.
- Colour may be a protective mechanism. Ochre sea stars living in areas with high concentrations of sea otters — past or present — may have developed an orange/red colouration to blend in with the marine landscape, as sea otters hunt visually and are red-green colourblind.
- Colour may be influenced by geography. One study found that in Washington’s Olympic Peninsula, Oregon and California, 68-90% of ochre sea stars were brown and 6-28% were orange, while in Georgia Strait (British Columbia) and Puget Sound (Washington), 95% were purple. How might geography cause this striking difference in colour distribution? Could it be due to different diets, ocean salinity, genes, predators, or something else?
Interestingly, one study found that larger-sized ochre sea stars were most often orange, and that when a large orange ochre star lost and regenerated an arm — sea stars can do this to escape predators — the new arm initially grew back tinged with purple. Does this mean that purple is an ochre sea star’s “default” colour?
As scientists continue to study ochre sea stars, perhaps the real reason(s) behind the stars’ different hues will surface. In the meantime, let’s enjoy these colourful stars in all their beautiful mystery.
Other cool facts about ochre sea stars:
- Ochre sea stars are the largest and longest living sea stars in our waters. They can grow over 14 inches in diameter and can live for over 20 years.
- Ochre sea stars are an indicator species for the health of the intertidal zone. The more ochre sea stars you see in an area, the healthier those waters generally are.
- Ochre sea stars are one of the only sea stars that feed year round.
- The underside of an ochre sea star is covered with hundreds of tube feet — tiny tentacles with suckers on the end — which the sea star uses to grip rocks, move (slowly) around, and even pry open the shells of its prey.
- After an ochre star uses its tube feet to pry open a prey’s shell, it extends its stomach out of its body and into the shell to consume the prey inside. A shell opening as narrow as one millimetre will do!
- Ochre sea stars range along the West Coast of North America from Alaska to Baja California.
- Over the last decade, ochre sea stars have been hard hit by a condition called Sea Star Wasting Disease. The disease causes the stars’ bodies to disintegrate (or “melt”), turn white, deflate and deform. The disease is thought to be caused by a virus or bacterial infection, and it kills the stars within weeks.
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