IMPORTANCE OF MUTABLE COLLAGENOUS TISSUE TO CRINOID BIOMECHANICS, WITH SPECIAL REFERENCE TO LOCOMOTION

It has been recognized since the 1960s that some echinoderm connective tissue can undergo rapid and dramatic changes in its mechanical properties, such as viscosity, stiffness and strength; for this reason, this tissue has been referred to as “Mutable Collagenous Tissue” (MCT). The presence of MCT in crinoids has been known since the 1970s. However, it has recently been reported that in addition to having mutable mechanical properties, some MCT found in the cirri and arms of crinoids, in spite of lacking myocytes, is contractile, i.e., it is capable of active shortening (Birenheide and Motokawa 1996, 1998, Birenheide et al. 2000, Motokawa et al. 2004). Here, I explore the implications of the MCT to crinoid biomechanics.

The functional importance of the mutable properties of ligament cannot be overstated—through irreversible destabilization, it is involved in autotomy, the detachment of body parts, through destiffening it allows the animal to move its body parts with the least amount of force, through stiffening, it allows the animal to withstand high external forces without much deflection and without expending much energy. The functional importance of the recently discovered contractile properties of MCT, however, has yet to be investigated. While it has been experimentally shown that contraction of ligament can produce movement in crinoid arms, it is unclear for what crinoid functions ligament contractility is actually important. For example, in locomotion it has been generally assumed that muscles and the elastic properties of ligament alone suffice.

In order to assess whether ligament contractile properties play an integral role in crinoid crawling, I used published data on maximum stresses and speeds of contraction for crinoid ligament and calculated the maximum speeds of arm bending that these could produce. A comparison of these calculated speeds to observed maximum crawling speeds of comatulids and isocrinids, suggests that ligament contractility alone cannot account for this behavior. Nevertheless, this approach allows an estimate of maximum movement of arms that lack muscles entirely, an estimate that can be useful in constraining the functions of some Paleozoic crinoids.