Figure 1.
Anatomy of the head and illustration of the hyoid locking hypothesis in Hippocampus reidi.
Left lateral view of the CT-reconstruction of the cranium with addition of the stimulated muscles based on histological sections (A), a medial view of the left side of the head with the opercular bone removed (B). Below, a detailed view on the hyoid and suspensorium illustrate the morphological configuration hypothesised to be responsible for locking of hyoid and head rotation where the posteriolateral part of the anterior ceratohyals lies dorsally of one of the medial grooves of the adducted preopercular bones (C), and the unlocked configuration where the ceratohyals are free to rotate ventrally after slight abduction of the preopercula (D). Orange lines in (A) and (B) represent tendinous connections. The arrows indicate the most likely contact region between the ceratohyals and the preopercula. Abbreviations: m-aap, adductor arcus palatini; m-epax, epaxial muscle; m-hypax, hypaxial muscle; m-st, sternohyoideus muscle; o-ch-a, anterior ceratohyal bone; o-ch-p, posterior ceratohyal bone; o-cl, cleithrum; o-den, dentary bone; o-fr, frontal bone; o-hm; hyomandibular bone; o-ih, interhyal bone; o-iop, interopercular bone; o-meth, mesethmoid bone; o-mx, maxillary bone; o-op, opercular bone; o-para, parasphenoid bone; o-pop, preopercular bone; o-prmx, premaxillary bone; o-q, quadrate bone; o-uh, urohyal bone. Scale bar, 5 mm.
Figure 2.
X-ray images showing the placement of the electrodes for both individuals in the electromyography analysis.
The uncoated electrode tips are indicated by white circles. Scale bars, 10 mm.
Figure 3.
Experimental set-up for electromyography (A) and for the stimulation experiment (B).
Figure 4.
Calculation steps in the analysis of the recorded EMGs.
The rectified EMG signal with the duration of the prey capture phase (narrow grey bar) is shown in (A). In (B) noise range selection is illustrated, with determination of the onset and offset times for 25 ms intervals exceeding the signal threshold of the 99% confidence limit of the noise. In (C), the six EMG variables are shown.
Figure 5.
Example of the calculation of prey distance and mouth travel distance.
The formulae and
were used to calculate these two respective variables via digitization of landmarks at the mouth and at the contours of the prey on video images shot with two perpendicular cameras. In the lower panels, “+” indicates the position of the centre of the mouth at one frame before the start of head rotation. See text for further information.
Figure 6.
Activation patterns for the epaxial muscle (m-epax) and the sternohyoideus-hypaxial muscle (m-st-hyp).
The start of the prey-capture movement equals time = 0 s with a grey bar indicating the duration of the head rotation phase. Boxes (mean±s.e.) and whiskers (mean±s.d) denote the onset and offset times. Black profiles represent the within-individual means of the integrated rectified EMGs above or below the noise level. Mean profile amplitudes are separately scaled to improve clarity.
Figure 7.
Relationship between prey distance and mouth travel distance.
Note that no significant correlations were calculated for each of the individuals (individual 1, filled circles; individual 2, open circles).
Figure 8.
Results of the stimulation experiment.
The figure shows the time-dependent kinematic profile of head rotation (mean±s.d.) when the epaxial, hypaxial and adductor arcus palatine muscles are stimulated, and when only the epaxial and hypaxial muscles are stimulated. The grey zone represents the time of stimulation. N = 2 individuals, five repetitions per individual and stimulation treatment.