Context-dependent mechanical reconfiguration is necessary for multifunctional behavior in an intrinsically confined hydrostat.

Muscular hydrostats, muscular structures lacking rigid skeletons, are ubiquitous in animals, but how they perform complex actions remains poorly understood. Understanding their function requires uncovering the mechanisms that couple and constrain their deformations. We investigated how mechanical reconfiguration from interacting shape-changing elements in the feeding system (buccal mass) of Aplysia facilitates different feeding behaviors. Combining analysis of MRI movies of Aplysia feeding with a new biomechanical model, we demonstrate how two context-dependent mechanical reconfigurations produce large protractions in different ways during two key behaviors. During rejection, grasper elongation stretches a protractor muscle, allowing stronger protractions. During biting, the grasper remains compact, requiring additional support from more anterior muscles. The mechanisms integrate shape-changing, bending and conforming muscles, and shifting contacts. We propose two mechanical subclasses of muscular hydrostats, "intrinsically confined" and "unconfined," that may be morphologically similar but employ different control strategies depending on whether mechanical confines are reliably present.