On the swimming function of crinoid cirri
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On the swimming function of crinoid cirri Tomasz K. Baumiller • G. Alex Janevski
Received: 11 May 2010 / Accepted: 19 October 2010 / Published online: 2 December 2010 Ó Akademie der Naturwissenschaften Schweiz (SCNAT) 2010
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. Keywords
Echinoderms Biomechanics Paleobiology
Introduction The Treatise on Invertebrate Paleontology (1978) defines cirri as generally undivided, jointed appendages of the
T. K. Baumiller (&) G. A. Janevski Museum of Paleontology and Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, USA e-mail: [email protected]
crinoid stem or the centrodorsal (Breimer 1978). These appendages characterize most extant crinoids, including isocrinids and comatulids, but are missing from many Paleozoic groups as well as some post-Paleozoic taxa such as the millericrinids and cyrtocrinids. It is generally accepted that one function of cirri is to anchor the animal to the substrate (Clark 1915). Anchoring by cirri is done actively, i.e., each cirrus bends slowly aborally allowing it to grasp the substrate. Release from the substrate is accomplished by unbending orally. Exactly how crinoid cirri can generate movement has been a major puzzle. Whereas the presence of a transverse ridge on the cirral facets in certain crinoids, such as isocrinids and comatulids, clearly serves as a fulcrum around which oral–aboral movement can occur, no muscles have been found in the cirri of extant crinoids (Fig. 1). The cirrals are connected to each other by ligaments that insert into the skeleton on the oral and aboral sides, on opposite sides of the transverse ridge. In addition, a central canal pierces each cirrus and contains extensions of the coelom and the nervous system. Finally, thin layers of dermis and epidermis form the outermost layers of each cirrus. The absence of muscles in cirri necessitates that some other mechanism was responsible for their
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