Predicting crawling speed relative to mass in sea stars

Larger animals typically move faster than smaller individuals of the same species. This relationship is highly conserved across many diverse groups of vertebrates with varied modes of locomotion. But the relationship between body size and locomotion speed is much less clear among invertebrate taxa - the Asteroidea (sea stars) in particular. One sea star species crawls faster at larger body sizes, three species show no correlation between body size and crawling speed, and another sea star species actually crawls slower at larger body sizes. To further examine these unusual patterns of locomotion in sea stars, the relationship between body size and crawling speed was quantified in three unstudied species of Northeast Pacific sea star: Pycnopodia helianthoides, Solaster stimpsoni, and Dermasterias imbricata. The two multi-armed species P. helianthoides and S. stimpsoni followed the general trend: larger individuals crawled faster. But the five-armed D. imbricata crawled slower, like the previously reported five-armed star Patiria miniata. Arm number and scaling differences among species are not sufficient to explain why some sea star species crawl faster at larger body sizes. But two shape ratios relating arm length, arm width, and oral disk diameter may enable predictions of the relationship between body size and crawling speed in unstudied sea star species. Crawling speed differences within and among species could have implications for predator-prey dynamics as well as feeding mode.