A soft origin for a forceful bite: motor patterns of the feeding musculature in Atlantic hagfish, Myxine glutinosa

Despite lacking jaws and substantial rigid support for feeding muscles, hagfishes can forcefully grasp and ingest chunks of flesh from their prey. When feeding, bilaterally folding dental plates protrude from the mouth, then forcefully retract. This cyclic protraction and retraction occurs in the anterior region of the hagfish feeding apparatus (HFA) and involves both a cartilaginous skeleton and a complex array of muscles that act as a hydrostat. We recorded motor patterns from the largest muscles in the HFA in six specimens of Myxine glutinosa: the deep protractor muscle (DPM), clavatus muscle (CM), perpendicularis muscle (PM), and tubulatus muscle (TM). Individuals normally used four gape cycles to ingest food and four gape cycles to intraorally transport food. We measured burst duration from each muscle and the onsets of kinematic events and the onsets of CM, PM, and TM bursts relative to the onset of the DPM. The DPM fired during protraction, while the CM, PM and TM fired during retraction. Our study corroborates our anatomical predictions about DPM and CM function. Activation of the circumferentially and vertically oriented fibers of the TM and PM stiffens the origin of the CM, allowing it to forcefully retract the dental plates. The progressive decrease in retractor muscle activity during gape cycles following ingestion suggests a reliance on passive properties of the musculoskeletal system for retraction.