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First published online November 2, 2007
Journal of Experimental Biology 210, 3897-3909 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.006940
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Morphology and kinematics of feeding in hagfish: possible functional advantages of jaws

Andrew J. Clark* and Adam P. Summers

Evolutionary and Comparative Physiology, Department of Ecology and Evolutionary Biology, 321 Steinhaus Hall, University of California, Irvine, CA 92697-2525, USA


Figure 1
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  		Fig. 1. Skeletal and dental components of the hagfish feeding apparatus. (A) Dorsal view of an unfolded dental plate (oral mucosa removed) from Eptatretus stoutii, including methods for measuring dental plate width and dental plate length. Thick broken vertical line represents the axis about which bilateral folding occurs. Anterior and posterior tooth rows are indicated by blue and red outlined ovals, respectively. 2–3 medial teeth in each row are fused and fusion patterns of these medial teeth vary with species. For example, a M. glutinosa specimen would have two fused medial teeth in the anterior row (Ferholm, 1998). (B) Ventral view of an E. stoutii with the gray box representing the position of the basal plate. The inset shows a schematic of the basal plate in ventral view with the resting position of the dental plate and methods for measuring basal plate length and width. Below is a photograph of a basal plate in dorsal view. ABP, anterior basal plate; MBP, middle basal plate; PBP, posterior basal plate; LB, lateral bar; MB, medial bar. (C) Lateral view of the cranial skeleton of M. glutinosa, including the notochord (NC) and spinal cord (SC). Material highlighted in blue represents the cartilages and teeth of the feeding apparatus. DP, dental plate. Image has been modified from (Cole, 1905Go).

 

Figure 2
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  		Fig. 2. Morphology of the hagfish feeding apparatus (HFA) in the retracted position and methods for measuring HFA length in (A) lateral view, (B) ventral view and (C) dorsal view. ABP, anterior basal plate; CM, clavatus muscle; DPM, deep protractor muscle; DP, dental plate; MBP, middle basal plate; PBP, posterior basal plate; PC, perpendicularis cartilage; PM, perpendicularis muscle; VK, ventral keel; SPM, superficial protractor muscle; TM, tubulatus muscle. Dental plate cartilages (not shown in C) are covered by oral mucosa.

 

Figure 3
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  		Fig. 3. Schematics showing measurement techniques for determining physiological cross-sectional area and theoretical maximum force production in the clavatus muscle (CM) and deep protractor muscle (DPM). (A) Fiber angle measurements did not apply to the DPM, as DPM fibers coursed parallel with the line of action (broken blue line). (B) Lower inset shows that the two heads of the CM separate once it is removed from the feeding apparatus. Upper inset demonstrates how pennation angles were measured. Fiber angles {theta} are identical near the muscle's midline (line of action) and begin to vary slightly as they course posteriorly, therefore fiber angles were measured near the muscle's midline. Muscle length L was used as a proxy for determining fiber length because mean fiber lengths in the DPM and CM were approximately equal to respective muscle lengths. Darkened areas on the feeding apparatuses indicate positions of the CM and DPM in dorsal and ventral views, respectively. Individual CM and DPM (from insets) are not drawn to scale.

 

Figure 4
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  		Fig. 4. Measurement schemes for feeding kinematic timing and angular variables in hagfish. (A) Gape cycle time is the elapsed time for maximal dental plate protraction and retraction. Time to maximum gape is the elapsed time to attain maximal dental plate protraction from a retracted position. Dental plate retraction time is the elapsed time to retract the dental plate from a maximally protracted position. Head depression time is the elapsed time from the onset to the completion of head depression. Head elevation time is the elapsed time from the onset to completion of head elevation from a maximally depressed state. (B) Kinematic angular variables defined. Lateral view of a hagfish with a maximally depressed head and a maximally protracted dental plate. Head depression angle (top) is the angle between the anterior tip of the snout before the onset of head depression (see light gray image), the pivoting point of the head, and the anterior tip of snout once the head is maximally depressed. Maximum protraction angle is the angle between the anterior tip of the snout, mouth opening and anterior tip of the maximally protracted dental plate.

 

Figure 5
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  		Fig. 5. Box and whisker plots of length-scaled measurements of skeletal dimensions, physiological cross-sectional areas (PCSA), and theoretical maximum muscle force production (Po) in Eptatretus stoutii and Myxine glutinosa. Measurements include length-scaled (A) hagfish feeding apparatus length (HFAL/TL), (B) basal plate width (BPW/TL), (C) basal plate length (BPL/TL), (D) dental plate width (DPW/TL), (E) dental plate length (DPL/TL), (F) deep protractor muscle PCSA (PCSADPM/TL), (G) clavatus muscle PCSA (PCSACM/TL), (H) deep protractor muscle Po (PoDPM/TL) and (I) clavatus muscle Po (PoCM/TL) in E. stoutii and M. glutinosa. Each graph includes the 95% confidence interval (box), maximum and minimum values (upper and lower bars, respectively), and a mean value (thick horizontal bar). *Significant difference (P<0.05).

 

Figure 6
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  		Fig. 6. Box and whisker plots of kinematic variables from Eptatretus stoutii and Myxine glutinosa, illustrated in the cartoon above. (Ai) Gape cycle time (GCT); (Bi) time to maximum gape (TMG); (Ci) dental plate retraction time (DPRT); (Di) head depression time (HDT); (Ei) head elevation time (HET); (Fi,Fii) maximum protraction angle (MPA); and (Gi,Gii) head depression angle (HDA). Each graph includes the 95% confidence interval (box), maximum and minimum values (upper and lower bars, respectively), and a mean value (thick bar). (Aii–Eii) Measurements relative to TL.

 

Figure 7
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  		Fig. 7. Cranial kinematic profiles from Myxine glutinosa (red) and Eptatretus stoutii (blue). Block diagram of relative timing of mean dental plate and cranial kinematic events from two E. stoutii (11 food transport events) and four M. glutinosa (25 food transport events). GCT, gape cycle time; TMG; time to maximum gape; DPRT, dental plate retraction time; HDT, head depression time; HET, head elevation time.

 

Figure 8
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  		Fig. 8. Gape cycle times (GCT) (in ms) plotted against body lengths (mm) of various craniates. In these plots, body length is represented by total length, standard length, snout–vent length, carapace length and disc width. Horizontal gray lines indicate residual means, broken lines are 95% confidence intervals of the best-fit, least-squares regression (black line). (A) Log-transformed plot of GCT in aquatic feeding craniates. (B) Log-transformed plot of terrestrial feeding tetrapod GCT.

 

Figure 9
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  		Fig. 9. Physical models of dental plate and jaw kinematics in hagfish and gnathostomes, respectively. (A) Pulley system model for dental plate retraction (left) and protraction (right). (B) Third-order lever system model for jaw adduction (left) and jaw abduction (right) in a large cat (Panthera). Dotted red arrows demonstrate how a jaw can be geared for closing velocity or mechanical advantage by changing relative input lever arm lengths (output lever arm remains unchanged in this example). BP, basal plate; CMJ, craniomandibular joint; DP, dental plate; f, fulcrum; Fi, input force; Fo, output force; JAM, jaw adductor muscle; JDM, jaw depressor muscle; Li, length of input lever arm; Lo, length of output lever arm; PMG, protractor muscle group; RMG, retractor muscle group.

 

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© The Company of Biologists Ltd 2007