Effects of body size and shape on locomotion in the bat star (Patiria miniata)

Among taxa ranging from cnidarians to vertebrates, absolute speed of locomotion generally increases with increasing body size. Despite the unique mode of locomotion in echinoderms, crawling speed also appears to increase with increasing body size, at least in some species of asteroids and echinoids. We used an escape-response assay to assess how maximum crawling speed varied with body size in the bat star Patiria miniata. We also tested the effect of arm number on maximum crawling speed by comparing speeds of five- and six-armed individuals. Contrary to prior reports for a single sea urchin and sea star species, both absolute crawling speed and crawling speed relative to body size actually declined with increasing body mass, increasing arm length, and increasing oral surface area, in both five- and six-armed individuals. Arm number did not appear to have a significant effect on crawling speed. The reasons for this negative relationship between crawling speed and body size in P. miniata remain unclear, but we suspect that the disproportionate increase in body mass relative to total tube-foot cross-sectional area may make locomotion proportionally more difficult in larger-bodied sea stars.