by Amanda Kahn
One day while exploring an island in Trevor Channel during a low tide, I happened across this sea star.
It’s Pisaster ochraceus! And the logo of the Madreporite…almost in the correct pose too… Credit: A Kahn 2012
Apart from looking like it was dancing a happy dance on a barnacle-encrusted rock, there was something notable about this individual. This species of sea star usually comes in one of three colors: orange, reddish-brown, or purple. However, this one was orange with purple tips on its arms.
Credit: A Kahn 2012
The purple tips on the sea star I found made me wonder what causes colors to change in these sea stars. It’s not fully known yet, but people are thinking about it.
Pisaster ochraceus is commonly found in the intertidal zone around Barkley Sound and is known to be a keystone predator for intertidal habitats. This means the sea stars are largely responsible for the way intertidal communities are arranged, with bands of mussels in the middle intertidal zone and very few mussels and barnacles beneath that zone.
What is known is that color variation is not genetically determined, at least not directly. A study done partly in Bamfield by Chris Harley and co-authors found no meaningful distinctions between orange versus red versus purple sea stars, even though they looked very different. Instead, within an area, such as within the San Juan archipelago or the Strait of Georgia, all sea stars had very similar gene sequences, indicating that all color morphs can and do breed freely and easily with other color morphs. Fitting that idea with the commonly-accepted theory that confines individuals within a “species” (two individuals can’t reproduce to create viable offspring that are also capable of reproduction if they are different species), Harley and co-authors concluded that there don’t seem to be multiple similar-but-not-quite-the-same species, called cryptic species, living amongst each other on the intertidal rocks of Barkley Sound.
Different colors of Pisaster ochraceus coexist along the Bamfield shoreline, but why they are different colors, and what determines what color they grow to be, remains unknown. Credit: Jackson Chu via Flickr
Orange and purple (and sometimes white) color morphs have also been observed in a deep-sea heart urchin, Echinocrepis rostrata. Urchins are part of the same phylum as sea stars, so they are expected to be similar to each other. The color morphs of E. rostrata were also not genetically distinct from each other, indicating that the colors do not seem to be cryptic species of each other either.
So if colors are not related to the genetics of the sea stars, then what’s left? Well, as in the debate of nature vs. nurture, if the sea stars are not born with the genetic predisposition to be purple or orange (the “nature”), then it must be something in their environment that determines their color (the “nurture”). This is where it gets really interesting, with Harley and co-authors finding that the differences in diet might affect the colors of sea stars (though that was not conclusively tested), while size and location did not. The authors in the paper even noted,
“Notably, small fractions of orange stars have purple coloration at the tips and along the undersides of their rays, especially if a ray is actively growing (e.g., following injury; CH, pers. obs.). Even smaller fractions have a tracework of purple coloration on their aboral surface (CH, pers. obs.). Very small Pisaster are not chromatically differentiated, and some orange adults may turn purple when held for long periods under laboratory conditions (J. Pearse, Long Marine Laboratory, University of California Santa Cruz, pers. comm.). These observations imply that individuals express different pigments as they grow or age; however, the effects of age and diet remain confounded for the time being.”
The idea that sea stars may change colors depending on changes in their diet or age was a new concept to me. Why do you think different color morphs happen? Is it related to diet, like Harley and co-authors suppose? Or age-related? Maybe you’ve read a more recent paper about it in a class? Please let me know in the comments section below–I’d love to learn more about this!
Harley, C. D. G., M. S. Pankey, J. P. Wares, R. K. Grosberg, and M. J. Wonham (2006). Color polymorphism and genetic structure in the sea star Pisaster ochraceus. Biological Bulletin 211(3), 248-262.
Vardaro, M. F. (2010). Genetic and anatomic relationships among three morphotypes of the echinoid Echinocrepis rostrata. Invertebrate Biology 129(4), 368-375.