Optimization principles applied to the mechanism of neurocranium levation and mouth bottom depression in bony fishes (Halecostomi)

Abstract

The mechanism of raising the neurocranium and lowering the bottom of the mouth in stationary feeding bony fishes has been modelled by a four-bar linkage consisting of the hyoid, urohyal-sternohyoid complex, pectoral girdle and neurocranium-suspensorium complex. This mechanism can be brought into a locked position and then be prestressed by the epaxial and hypaxial muscles. A small deviation from this locked position, initiated by a trigger muscle, causes a quick-release movement of the four-bar chain. This enables very fast dorsorotation of the head and depression of the mouth bottom (5–50 msec). Optimal dimensions of the four-bar linkage can be derived with regard to the attuning of head levation and mouth bottom movement. The amplitude of neurocranial levation can be maximized (e.g. in Pleuronectes ) or minimized (e.g. in Amphisile ). An extremely large neurocranium levation (as in e.g. Chauliodus ) limits high kinematic transmission (an indicator for high velocity or acceleration of the neurocranium). Different trigger mechanisms are discussed for some pipefishes ( Gasterosteiformes ), flatfishes ( Pleuronectiformes ) and some voracious deep sea fishes ( Stomiatoidei ).