Meeting Abstract
We investigated the morphology and mechanics of what appears to be a gliding joint in Atlantic longfin squid (Doryteuthis pealeii). The joint, which is formed by the nuchal cartilage and the chitinous pen, has several novel features. First, it isn’t clear what holds the nuchal cartilage and pen together. The joint is not encapsulated in connective tissue but is, instead, open to the seawater in the mantle cavity. Indeed, the joint can be “snapped” apart and then reconnected in anesthetized squid without impairing function (i.e., the joint will still slide over the normal range once the squid recovers from anesthesia). Second, the nuchal cartilage and pen do not touch directly because a thin layer (1-2 mm thick in adults) of tissue covers the pen in the region of the joint. Standard histological methods showed the layer to be a muscular hydrostatic organ with muscle fiber trajectories running longitudinal and transverse to the long axis of the pen. The role of the muscular hydrostatic layer is unclear because relative to controls, the force required to snap apart the joint does not decline in animals anesthetized in an isosmotic MgCl2 solution that prevents muscle contraction. Furthermore, if the muscular hydrostatic layer is slit longitudinally with a scalpel but remains in situ in an anesthetized animal, the force required to snap apart the joint does not decline. If the tissue layer is removed, then force declines significantly. From our preliminary results, we hypothesize (1) that the joint is held together by hydrogen bonding interactions between the pen, the muscular hydrostatic layer, the nuchal cartilage, and the thin film of seawater that covers all three structures, and (2) that the role of the muscular hydrostatic layer is to conform to the shape of the nuchal cartilage and facilitate hydrogen bonding. Funded by NSF grant IOS-0950827.
