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The notochord of hagfish Myxine glutinosa: visco-elastic properties and mechanical functions during steady swimming
1 Mount Desert Island Biological Laboratory, Salsbury Cove, Maine, 04672,
USA
2 Department of Biology, Vassar College, Poughkeepsie, New York, 12604,
USA
3 Division of Skeletal Biology, Shriners Hospital for Children, Tampa,
Florida, 33612, USA
* Author for correspondence (e-mail: jolong{at}vassar.edu)
Accepted 9 September 2002
To determine the possible locomotor functions of the hagfish notochord, we
measured its flexural stiffness EI (N m-2) and flexural
damping C (kg m3 s-1), under in vitro
conditions that mimicked the body curvature and bending frequency measured
during steady undulatory swimming. To assess the notochord's contribution to
the mechanical behavior of the whole body, we also measured EI and
C of the whole body, the body with skin removed, and the notochord
with the outer fibrous sheath removed. When subjected to dynamic bending at
angular frequencies from 
to 6 rad s-1 and midline
curvatures from 11 to 40 m-1, 1 cm in situ body segments
(N=4), located at an axial position of 37% of the body length, showed
significant changes in EI, C, the Young's modulus or material
stiffness (E, MPa), the net work to bend the body over a cycle
(W, J) and resilience (R, % energy return). When skin,
muscles and the outer fibrous sheath of the notochord were removed
sequentially, each structural reduction yielded significant changes in
mechanical properties: C decreased when the skin was removed,
E increased when the muscles were removed, and EI and
R decreased when the outer fibrous sheath was removed. Although
occupying only a small portion of the cross-sectional area, the notochord
provides the body with 75% of its total EI and 80% of total
C, by virtue of its high E, ranging from 4 to 8 MPa, which
is an order of magnitude greater than that of the whole body. Thus, as the
body's primary source of EI and C, the notochord determines
the passive (i.e. internal, non-muscular) mechanical behavior of the swimming
hagfish. EI and C covary inversely and non-linearly such
that as C increases, EI decreases. However, the bending
moments M (Nm) produced by each property increase proportionally, and
the ratio of stiffness to damping moments, also known as the amplification
ratio at resonance, is nearly invariant (approximately 7) with changes in
driving frequency. If the body operates in life at or near resonance, the
variables EI and C interact over a range of swimming speeds
to produce passive mechanical stability.
Key words: notochord, backbone, axial skeleton, hagfish, swimming, bending, damping, stiffness, flexural stiffness, kinematics, visco-elastic properties, Myxine glutinosa
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