Sea stars are some of the largest mobile animals able to live in the harsh flow environment of wave-exposed, rocky intertidal shores. In addition, some species, such as the northeastern Pacific Pisaster ochraceus, are ecologically significant predators in a broad range of environments, from sheltered lagoons to the most wave-exposed shorelines. How they function and survive under such an extreme range of wave exposures remains a puzzle. Here we examine the ability of P. ochraceus to alter body form in response to variation in flow conditions. We found that sea stars in wave-exposed sites had narrower arms and were lighter per unit arm length than those from sheltered sites. Body form was tightly correlated with maximum velocity of breaking waves across four sites and also varied over time. In addition, field transplant experiments showed that these differences in shape were due primarily to phenotypic plasticity. Sea stars transplanted from a sheltered site to a more wave-exposed site became lighter per unit arm length, and developed narrower arms, after 3 months. The tight correlation between water flow and morphology suggests that wave force must be a significant selective factor acting on body shape. On exposed shores, narrower arms probably reduce both lift and drag in breaking waves. On protected shores, fatter arms may provide more thermal inertia to resist overheating, or more body volume for gametes. Such plastic changes in body shape represent a unique method by which sea stars adapt to spatial, seasonal and possibly short-term variation in flow conditions.
K.J.R.H. designed and performed the transplant experiment, collected and measured samples, analysed the data and wrote the original draft of the manuscript. A.R.P. suggested the original study, and provided input on experimental design, statistical analysis, data presentation, and manuscript preparation. T. MacKeracher provided helpful comments on an early version manuscript.
Supplementary material available online at http://jeb.biologists.org/cgi/content/full/216/9/1717/DC1
No competing interests declared.
The study was funded by a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant A7245 to A.R.P., and NSERC CGS-M and Province of Alberta Queen Elizabeth II Graduate scholarships to K.J.R.H.
LIST OF SYMBOLS AND ABBREVIATIONS
- lateral projected area of one half of the sea star (mm2)
- average arm length (mm)
- maximum lateral projected area (mm2)
- analysis of covariance
- planar area (mm2)
- coefficient of drag
- coefficient of lift
- Dixon Island, sheltered
- force of drag (N)
- force of lift (N)
- height of the sea star in the centre of the body (mm)
- Helby Island, exposed
- height at the tip of the sea stars arm (mm)
- sample size
- Seppings Island, exposed
- Seppings Island, sheltered
- water velocity (m s−1)
- density of seawater (kg m−3)
- © 2013. Published by The Company of Biologists Ltd