Sea Stars and Sand Dollars

The low tides, especially those below mean lower low water (MLLW and 0.0′ on your tide table), allow us to easily observe Pisaster ochraceus wherever rocky outcrops jut from the sand. On the exposed coast, P. ochraceus live attached to rocks where they are slow movers but get around, maneuvering up and down as the tides rise and fall. I sometimes get the feeling that all things being equal, Pisaster would be most at home when submerged. But all things aren’t equal.

Dense accumulation of purple and orange Pisaster
Sea stars Pisaster ochraceus

On the high side, their frequent prey, mussels and barnacles, tend to live above 0.0′, which means lots of exposure, and on the low side, sometimes they just run out of rocky real estate. In the image above, the sandy dusting at the bottom foreshadows the meeting of rock and beach. These sea stars bunched up at the lower edge of their rock, which meets the sand at about 1.3′ below MLLW. The photo below, another low tide scene, reveals the bigger picture, extending from the mussel bed down to sand-meets-rock.

This high rock is home to Piaster’s favorite prey— mussels, barnacles, and gastropods. The P. ochraceus association with mussels, barnacles, and gastropods is well-known and well-studied. Pisaster ochraceus is an effective predator that can shape its prey’s and other species’ distributional patterns. Species with this kind of profound effect have been called keystone species. If you want to learn about this concept, an excellent place to start is Paine 1966. A good review and experimental test of Paine’s ideas almost 30 years later is given by Menge et al., 1994. If you don’t want to work quite that hard, check out this summary of the keystone species idea.

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Mytilus californianus, frequent Pisaster prey

Pisaster ochraceus go by a variety of common names. When I was preparing this post, the first three references I picked up used purple star, ochre star, and common seastar, respectively. That amount of variation, even in common names, seemed a little unusual, so, seeking resolution, I consulted a fourth guide, Lamb and Hanby’s Marine Life of the Pacific Northwest (2005), where I found this list of common names: purple sea star, ochre sea star, ochre seastar, purple starfish, ochre starfish, Pacific sea star, common seastar, common sea star, common purple star, warty sea star and common starfish. So what do you call P. ochraceus?

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Pisaster ochraceus

The scientific name P. ochraceus suggests an ochre body color, but most sea stars I encounter on the outer shores are purple. The photo above shows two color phases, purple and ochre-ish. Whenever I see two or more color forms, I wonder if there is a genetic versus environmental basis and if there are tradeoffs that allow both forms to exist. This particular polymorphism has been studied, and if you want to learn more about it, check out Harley et al., 2006. In the opening photo of this post, the purple-to-ochre ratio is about 5:1. Next time you are out at low tide, try counting the color phases to determine the ratio for your area.

Sea star on the sand, a little eel grass debris
Pisaster ochraceus and eelgrass

I’ve always assumed that if you find P. ochraceus out on a sandy beach, as I did early in the morning on June 15, 2012 (above), something is wrong. I imagine a gull or human has plucked it off a rock, or some other accident has befallen it, but there could be other explanations. Thus, beachcombers seldom see P. ochraceus unless there are rocks, but they will almost always encounter evidence of sand dollars on the beaches.

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Dendraster excentricus

Above is the test or shell of the eccentric sand dollar Dendraster excentricus. Sand dollars, like sea stars, are echinoderms, a group that also includes brittle stars, sea urchins, and sea cucumbers. Eccentric sand dollars are common on sandy seabeds in the subtidal zone, and their tests frequently wash up on sandy beaches. Live D. excentricus are dark colored and covered with tiny spines. It’s not too common to encounter live sand dollars on the exposed coast; it takes a good low tide. However, I occasionally encounter them in the lowest reaches of the infralittoral fringe, and the best beds I know of are in the bays.

Live sand dollar burrowing into sand
A lone live sand dollar at low tide on an exposed outer coast beach

If you love echinoderms and think perhaps radial symmetry is for you, check out Christopher Mah’s The Echinoblog.

References

Lamb, A. and B. P. Hanby.  2005.  Marine Life of the Pacific Northwest.  Harbour Publishing.

Harley, C. D. G., Pankey, M. S., Wares, J. P., Grosberg, R. K., and M. J. Wonham.  2006.  Color polymorphism and genetic structure in the sea star Pisaster ochraceus.  Biol. Bull. 211: 248-262.

Menge, B. A., E. L. Berlow, C. A. Blanchette, S. A. Navarrete, and S. B. Yamada.  1994.  The keystone species concept: variation in interaction strength in a rocky intertidal habitat. Ecological Monographs 64 (3): 249-286.

Paine, R. T.  1966.  Food web complexity and species diversity.  The American Naturalist, 100 (910): 65-75.


I updated this post on October 22, 2022.

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