ABSTRACT
Stalked crinoids are well known from the fossil record. Physiological data on recent species, however, have almost not been obtained. The arm of crinoids contains oral muscles without antagonistic muscles on the aboral side. It was suggested that the aboral ligament would work like an elastic spring, but neither ultrastructural nor biomechanical studies have been done. We investigated the arm of Metacrinus rotundus to find out whether the aboral ligament could act as the antagonist. Electron-microscopically, the aboral ligament consists of collagen, microfibrils, fibroblasts and neurosecretory-like cells and is similar to echinoderm catch connective tissue. The microfibrils exhibit a banding pattern after fixation in the presence of polyethyleneimine (PEI). The banding periodicity is variable like in vertebrate microfibrils that are thought to be elastic. In load-unload experiments on the frozen and thawed arm ligament of Metacrinus rotundus we found that the ligament is an elastic material with an average elastic stiffness of 2.28 MPa. The maximal stress ever observed in a muscle is much less. In other words, the ligament is so stiff that a muscle would not be able to stretch it, and the arm would not to move. We therefore suggest that the aboral ligament is a catch connective tissue, which can change its stiffness. During stress-relaxation experiments the ligaments relaxed in 10 minutes by values between 2.4 % and 29.1 % of the initial stress. A mechanical model that best fit the data consisted of a spring in series to two Maxwell elements with different viscocities. In summary, we suggest, that the aboral ligament is stiff and fixes the arm in a certain position, e.g. for filtration of food particles in strong currents. The ligament would soften to allow the oral muscles to bend the arm. When the muscles relax, the ligament recoils elastically and bends the arm to the other direction. Thus muscle and elastic ligament work antagonistically.
