Morphological basis and mechanics of arm movement in the stalked crinoid Metacrinus rotundus (Echinodermata, Crinoida)

Abstract

We obtained live specimens of the stalked crinoid Metacrinus rotundus from Suruga Bay, Japan, in 1992 and managed to keep them for several months in aquaria. Video observations of crawling individuals showed that both the aboral bending of the arm and the oral bending can be power strokes. When climbing up a lattice, the crinoids bend their arms aborally to pull the body upward. The oral side of the arm contains muscles, but the aboral side has ligaments without muscles. We found that ligaments on the aboral side are divided into two parts, the aboral ligament and the newly found fossa ligament. Ultrastructurally, the aboral ligament resembles typical echinoderm catch connective tissue. Its microfibrils exhibit a variable banding pattern after special staining and might be an elastic material. The fossa ligament contains only collagen fibrils which are often closely connected. Biomechanically the frozen and rethawed aboral ligament behaves like a spring. It shows between 2.4 and 29.1% stress relaxation, and it is so stiff that the oral muscles would not be able to stretch it. We suggest that the connective tissue of the ligament is a catch connective tissue and softens when the arm moves. During long-term filtration posture the ligament would stiffen so as to maintain its posture for a long time without muscle contraction and thus without using much energy.