Hagfish Houdinis: biomechanics and behavior of squeezing through small openings

On the fit of skins with a particular focus on the biomechanics of loose skins of hagfishes

There is a considerable diversity in how skins fit. Here, we review the function of both tight and loose skins and note that the latter are poorly understood. Analysis of loose skin examples suggest five functional categories: (I) freedom of movement, (II) surface area enhancement, (III) increased structural extensibility, (IV) lubrication, and (V) maladaptive examples arising through sexual or artificial selection. We investigate the skins of hagfishes as a model for understanding loose skin function by examining its structure using histology, standardized puncture resistance testing using the ASTM F1306 protocol, and the effect of internal pressure using a simple inflated balloon model. Skins of hagfishes are composed of multiple layers of cross-helically wound connective tissue fibers of a 45° angle to the longitudinal axis, resulting in a skin that functions as fabric cut “on the bias”. Hagfish skins are relatively yielding; however, skin looseness adds a “structural extensibility” that may allow hagfishes to compensate for low puncture resistance. Physical balloon models, with stiff cores that limit length changes, show that only low pressures allow short loop radii without local buckling. Hagfishes represent ideal organisms for studying loose skin function because their skins seem to fit in all functionally adaptive categories.

Flaccid skin protects hagfishes from shark bites

Hagfishes defend themselves from fish predators by releasing large volumes of gill-clogging slime when they are attacked. Slime release is not anticipatory, but is only released after an attack has been initiated, raising the question of how hagfishes survive the initial attack, especially from biting predators such as sharks. We tested two hypotheses that could explain how hagfishes avoid damage from shark bites: puncture-resistant skin, and a loose and flaccid body design that makes it difficult for teeth to penetrate body musculature and viscera. Based on data from skin puncture tests from 22 fish species, we found that hagfish skin is not remarkably puncture resistant. Simulated shark bites on hagfish and their closest living relatives, lamprey, as well as whole animal inflation tests, revealed that the loose attachment of hagfish skin to the rest of the body and the substantial 'slack volume' in the subcutaneous sinus protect hagfish musculature and viscera from penetrating teeth. While recent work has found evidence that the capacious subcutaneous sinus in hagfishes is important for behaviours such as knot-tying and burrowing, our work demonstrates that it also plays a role in predator defence.