Biomechanics and biophysics of anti-predator defence in hagfishes

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Hagfishes are an ancient group of eel-like, benthic, marine animals that are believed to have appeared over half a billion years ago. One possible explanation for the evolutionary staying power of hagfishes has to do with their ability to defend themselves from predators by secreting large volumes of fibrous slime in a fraction of a second. Over the course of my last two Discovery grants, my lab has elucidated several mechanisms underlying the function of the slime. We have shown that, in addition to seawater, the slime consists of two main components, mucus and threads, which are present in very low concentrations. We have also shown that the deployment of exudate from the slime glands involves the unraveling of coiled slime threads, the swelling of mucus vesicles, and a complex interplay among these two components. We have shown that the deployment of the fibrous component of the slime involves the dissolution of a seawater soluble glue and the release of strain energy stored in the coiled thread. We recently elucidated the three dimensional structure of slime threads within the specialized cells that produce them, which has inspired us to investigate the biophysical mechanisms by which the thread is produced within these cells. We have found that there are two main kinds of mucous vesicles, and they differ in their requirement for calcium ions to rupture. We have also shown that aquaporins in the vesicle membrane accelerate their swelling in seawater. The research proposed here will attempt to answer the following questions about how hagfish slime functions and how it is produced in the slime glands: How do gland thread cells produce an exquisitely coiled and remarkably strong slime thread in their cytoplasm that is 150,000 times longer than it is wide? What drives the swelling of the mucus vesicles in seawater and what keeps them condensed within the slime gland? We have shown that hagfish slime is remarkably good at clogging the gills of fish predators, and until recently, we assumed that the slime was effective at preventing attacks from predators. However, recently published deep-sea videos of hagfishes show that the slime is released only after it is bitten by a predator. This observation leads to the intriguing question of how a hagfish survives the initial bite, especially when that bite comes from a formidable predator like a shark. To answer this question, we will test biomechanical hypotheses about how hagfishes avoid injury from these initial bites. Overall, the research proposed here will illuminate the mechanisms by which hagfishes defend themselves from predators, and may lead to novel biomimetic technologies in the areas of puncture resistance and the manufacture of high performance materials. It may also help us understand how these unique animals have survived on this planet for so long.