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The baggy skins of hagfishes confer whole-body flexibility that enables these animals to tie themselves into knots without injury. The skin's looseness is produced by a subcutaneous blood sinus that decouples the skin and body core and permits the core to contort dramatically without loading the skin in tension or shear. Hagfish skin represents a biological composite material comparable in strength and stiffness to the conventionally taut skins of other fishes. However, our understanding of hagfish skin is restricted to only one of 78 species: The Pacific hagfish Eptatretus stoutii. To determine if other hagfish share similar characteristics with E. stoutii, we measured material properties and compared histological data sets from the skins of four hagfish species: E. springeri, E. stoutii, Myxine glutinosa, and M. hubbsi. We also compared these material properties data with skins from the American eel, Anguilla rostrata. We subjected skin samples from all species to uniaxial tensile tests in order to measure strength, stiffness, extensibility, and toughness of skins stretched along longitudinal and circumferential axes. We also used a series of equibiaxial tensile tests on skin samples from E. stoutii, M. glutinosa, and A. rostrata to measure stiffness of skins simultaneously strained along both axes. Significant results of uniaxial and biaxial tests show that the skins from Eptatretus are anisotropic, being stiffer in the longitudinal axis, and more extensible than the isotropic skins of Myxine. Skins of A. rostrata were stiffer in the circumferential axis and they were stronger, tougher, and stiffer than all hagfish skins examined. The skins of Eptatretus are histologically distinct from Myxine skins and possess arrays of fibers that stain like muscle. These interspecific differences across hagfish skins show a phylogenetic pattern with knotting kinematics and flexibility; both genera belong to distinct but major subfamilies within the Myxinidae, and Eptatretus is known for creating and manipulating a greater diversity of knotting styles than Myxine., (Copyright © 2020 Elsevier GmbH. All rights reserved.)

Hagfishes (Myxinidae) often integrate whole-body knotting movements with jawless biting motions when reducing large marine carcasses to ingestible items. Adaptations for these behaviors include complex arrangements of axial muscles and flexible, elongate bodies without vertebrae. Between the axial muscles and the hagfish skin is a large, blood-filled subcutaneous sinus devoid of the intricate, myoseptal tendon networks characteristic of the taut skins of other fishes. We propose that the loose-fitting skin of the hagfish facilitates the formation and manipulation of body knots, even if it is of little functional significance to steady swimming. Hagfish skin is a relatively thick, anisotropic, multilayered composite material comprising a superficial, thin, and slimy epidermis, a middle dermal layer densely packed with fibrous tissues, and a deep subdermal layer comprised of adipose tissue. Hagfish skin is stiffer when pulled longitudinally than circumferentially. Stress-strain data from uniaxial tensile tests show that hagfish skins are comparable in tensile strength and stiffness to the taut skins of elongate fishes that do not engage in knotting behaviors (e.g., sea lamprey and penpoint gunnel). Sheath-core-constructed ropes, which serve as more accurate models for hagfish bodies, demonstrate that loose skin (extra sheathing) enhances flexibility of the body (rope). Along with a loose-fitting skin, the morphologies of hagfish skin parallel those of moray eels, which are also known for generating and manipulating figure-eight-style body knots when struggling with prey., (© 2016 Marine Biological Laboratory.)

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., (Copyright © 2010 Elsevier GmbH. All rights reserved.)

As in gnathostomes, the hagfish feeding apparatus includes skeletal, dental and muscular components. In the present study, we examined feeding morphology and kinematics in two hagfish species, Eptatretus stoutii and Myxine glutinosa, representing the two major hagfish lineages. E. stoutii have larger dental plates, larger basal plates, and stronger clavatus muscles (the major dental plate retractor) than M. glutinosa. Despite morphological differences, kinematic profiles are similar in E. stoutii and M. glutinosa. When protracted, the dental plate unfolds and exposes keratinous teeth, which are then embedded in the prey. Once food is grasped, the dental plate is retracted into the mouth. During retraction, the clavatus muscle can generate up to 16 N of force, which exceeds the bite force of some gnathostomes of similar size. In addition to producing high forces with the feeding muscles, hagfish can evert their dental plates to 180 degrees , exceeding the gape angles attained by virtually all gnathostomes, suggesting vertebrate jaws are not the prerequisites for muscle force generation and wide gapes. We propose that dental plate protraction and retraction can be modeled as a fixed pulley that lacks the speed amplification occurring in gnathostome jaws. Hagfish gape cycle times are approximately 1 s, and are longer than those of gnathostomes, suggesting that a functional advantage of jaws is the speed that allows gnathostomes to exploit elusive prey.