Cross-posted from Kayak-Yak.

Once upon a time in Brentwood Bay, while drifting over rocks studded with orange and purple starfish, and past huddles of starfish in crevasses at the waterline, it occurred to me to wonder why they were these colours, that purple, in particular. The starfish in question were the ochre star, Pisaster ochraceus, and the answer, after intermittent and desultory trawling through the web and the scientific literature, turned out to be (a) carotenoids and (b) maybe what they eat.

The Royal British Columbia Museum Handbook Sea Stars of British Columbia, Southeast Alaska, and Puget Sound, told me a lot about the anatomy, hunting and mating behaviour, but does not account for the colours: P ochraceus is the most common intertidal sea star, with territory from Prince William Sound, Alaska, to Cedros Island, Baja California (lucky it!), and from the intertidal zone to nearly 100 m undersea. It likes rocky shores, waves and currents. I’ve seen plenty in the Broken Islands, the Gulf Islands, and around Saanich Penninsula. P ochraceus eats mussels, barnacles, limpets, and snails. It is the paradigm of a “keystone species” in that its presence and predation significantly affect the numbers and distribution of other species, especially the California mussel, Mytilus californianus; in the absence of P ochraceus, M californianus takes over the beach. Pisaster spawn in May to July, releasing millions of eggs, which turn into larvae, first floating free in the plankton and then (those that survive) attaching themselves and turning into juvenile sea stars. Juveniles grow to adult size and maturity over about 5 years. Larval P ochraceus have a chemical defense that induces filter-feeders to spit them out (got to look that up). The only known predators of adult sea stars are seagulls and sea otters.

Harley et al, 2006 (full text available) looking at the colour variation, note in their introduction that “at least two caroteinoid pigments mytiloxantin and astaxanthin, sequestered in the aboral surface, produce these colors in Pisaster and other asteroids.” Aboral is the upper side side of the sea star, and starfish belong to the Class Asteroidea, under the Phylum Echinodermata. Caroteinoids as a chemical class are named after their best known member, the yellow pigment in carrots, and have in common a long carbon backbone with many concatenated double bonds which generally absorb light at the blue end of the spectrum, hence the orange colour. Mytiloxanthin was named after M californianus, part of P ochraceous’ preferred diet, from which it was first isolated, so it was assumed to be dietary in origin. Astaxanthin arises through “several distinct metabolic pathways”, and is orange. I’m still not sure from my reading what the pigment behind the purple is, though reading descriptions of 1940s-style chromatography makes me oddly nostalgic for undergraduate chemistry.

However, knowing the pigments doesn’t explain why individual starfish should be orange, ochre, brown, or purple, or why starfish on an exposed, wave-beaten rocky coast like the west coast of Vancouver Island should be predominately orange (6-28%) and brown (68-90%), while those in the sheltered waters of the South St Georgia strait should be almost entirely that brilliant purple so familiar on our paddles (95% in the samples collected by Harley). The answer is apparently not genetic: DNA studies don’t suggest that the populations sampled (from Alaska to California, with lots of attention to Puget Sound) are isolated from each other, and conversely do suggest that there is flow of genetic material between them. It’s not apparently to do with wave action, inasmuch as scientists have been able to reproduce in the lab the difference between turbulent water and calm. It may be dietary, in that the distribution of colours correlated with the pattern of prey: in the more exposed waters (where purple starfish are in the minority), P ochraceus preferentially eat M californianus, the big California mussel, whereas M calfornianus is uncommon to absent in interior waters (where purple starfish are in the majority), and the Pisaster there tend to prey on barnacles and bay mussels. So, eats purple mussels -> orange; doesn’t eat purple mussels -> purple. Hmm. And that still doesn’t explain why purple and orange starfish could be found within yards of each other. Another paper by Raymondi et al, 2007 (only abstract) found that the frequency of orange in a population was constant with latitude, but tends to increase with the size of the individuals in that population. So all is not quite explained.

References

• Harley CDG, Pankey MS, Wares JP, Grosberg RK, Wonham MJ. Color Polymorphism and Genetic Structure in the Sea Star Pisaster ochraceus. Biol Bull. 2006 Dec 1;211(3):248-262. And here’s marine biologist Christopher Mah (full name from his Twitter feed), on the Echinoblog, with a crisp and colourful synopsis, complete with photos and diagrams; if I hadn’t written a chunk of this entry while back before I found his entry, I’d just have said, go there!
• Lambert P. Sea Stars of British Columbia, Southeast Alaska, and Puget Sound. 2nd ed. UBC Press; 2000.
• Raimondi PT, Sagarin RD, Ambrose RF, Bell C, George M, Lee SF, et al. Consistent Frequency of Color Morphs in the Sea Star Pisaster ochraceus (Echinodermata:Asteriidae) across Open-Coast Habitats in the Northeastern Pacific. Pacific Science. 2007 4;61(2):201-210.