Meeting Abstract
Hagfish use coordinated head and body knotting movements to dismember large carcasses into ingestible items. When feeding, keratinous teeth protrude from the mouth and contact the food surface while a body knot, formed at the tail, slides towards the head and pushes against the food surface surrounding the area in contact with the dentition. In these situations, the body knot creates a stable platform and an ad hoc lever for tooth movements. We propose that knotting in hagfishes might be facilitated by the absence of vertebrae, a complex arrangement of axial musculature, and loose skin. Between the axial muscles and the hagfish skin is a large blood-filled subdermal sinus devoid of the intricate myoseptal tendon networks characteristic to the taut-fitting skins of other fishes. This morphology raises the assumption that hagfish skin is ineffective at transmitting forces generated by axial muscles to the surrounding water, rendering hagfish skin a poor external tendon. Results from quasi-static uniaxial tensile tests to failure on fresh skin samples from specimens of Pacific hagfish, sea lamprey, and penpoint gunnel show that hagfish skin is as strong and stiff as the taught skins of more evolutionarily derived fishes, and is 60% stiffer and 20% stronger in the axial orientation relative to the hoop orientation. These anisotropic properties violate Laplace’s law (by which hoop stress should be twice that of axial stress) suggesting that the body of a hagfish is not a thin-walled pressurized cylinder, and thus does not function like an external tendon. Instead, the subdermal sinus encased between loose skin and axial musculature could be functionally important during knotting when large associated axial strains could impose damaging levels of tension in the skin.
