Assessing muscle function in the hagfish feeding apparatus of Eptatretus stoutii
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Hagfish are marine chordates that, despite lacking jaws, are able to feed on large, tough carcasses with the protraction and forceful retraction of toothplates. Using muscle stimulation experiments, we studied the function of the feeding apparatus muscles in the Pacific hagfish, Eptatretus stoutii, and compared our results with muscle activity patterns recorded from previously studied Atlantic hagfish Myxine glutinosa. Most of our efforts addressed function in the four major feeding apparatus muscles (the deep protractor, sphincter, perpendicular, and retractor muscles), and our stimulations of superficial musculature (superficial protractor muscles and rectus muscles) were assessed qualitatively. Stimulation experiments (tetanic contractions stimulated at 80 V, 60 Hz, 1 ms duration) were performed on in situ and excised feeding apparatuses from five animals positioned supine, while toothplate movements and muscle shape changes (strain) were analyzed from video recordings at 120 Hz. Our results confirm previous observations of M. glutinosa; in E. stoutii, the deep protractor muscles induce toothplate protraction, while activity in the retractor, sphincter, and perpendicular muscles power retraction of the toothplates. In both species, toothplate protraction causes the cylindrical feeding apparatus to decrease in length and increase in diameter, and vice versa during retraction. Upon excision from the body, the feeding apparatus in E. stoutii undergoes greater proportional length changes but smaller changes in width, suggesting that the soft tissue connections to the body wall impose constraints on muscle displacement. Because it comprises a large part of the head region (anterior 20% of total length), constraining the cylindrical shape of the feeding apparatus should work in retaining the cylindrical shape of the head while impeding unwanted head deformations in situations where the head must fit and maneuver in tight spaces (e.g. knotting and burrowing behaviors).