On the swimming function of crinoid cirri

Abstract

Crinoid cirri are generally considered primarily as structures that anchor the animal to the substrate; however, a swimming function for cirri has also been suggested. Most notably, it has been claimed that an extant comatulid, Dorometra nana , was observed swimming with its cirri. Because no muscles have ever been found in the cirri of crinoids, cirrus movement must involve another mechanism, and recent reports of contractile properties of crinoid ligament suggest that it might be the connective tissue that is responsible. Given the reported mechanical properties of cirral ligament, and the morphology and weight in water of D. nana , we tested the claim of cirrus swimming using a biomechanical model. Our results indicate that the thrust generated by cirri can account only for a very small fraction of what would be needed to overcome the weight of the animal in water, suggesting that D. nana cannot swim with its cirri. A similar approach applied to the Jurassic isocrinid, Pentacrinus briareus (= P. dichotomus ), with unusually numerous, long and flattened cirri, also fails to support previously hypothesized cirrus swimming in this taxon.