Hagfishes thwart attacks by fish predators by producing liters of defensive slime. The slime is produced when slime gland exudate is released into the predator's mouth, where it deploys in a fraction of a second and clogs the gills. Slime exudate is composed mainly of secretory products from two cell types, gland mucous cells and gland thread cells, which produce the mucous and fibrous components of the slime, respectively. Here, we review what is known about the composition of the slime, morphology of the slime gland, and physiology of the cells that produce the slime. We also discuss several of the mechanisms involved in the deployment of both mucous and thread cells during the transition from thick glandular exudate to ultradilute material. We review biomechanical aspects of the slime, along with recent efforts to produce biomimetic slime thread analogs, and end with a discussion of how hagfish slime may have evolved.

Conspecific density is widely recognized as an important ecological factor across the animal kingdom; however, the physiological impacts are less thoroughly described. In fact, population density is rarely mentioned as a factor in physiological studies on captive animals and, when it is infrequently addressed, the animals used are reared and housed at densities far above those in nature, making the translation of results from the laboratory to natural systems difficult. We survey the literature to highlight this important ecophysiological gap and bring attention to the possibility that conspecific density prior to experimentation may be a critical factor influencing results. Across three taxa: mammals, birds, and fish, we present evidence from ecology that density influences glucocorticoid levels, immune function, and body condition with the intention of stimulating discussion and increasing consideration of population density in physiology studies. We conclude with several directives to improve the applicability of insights gained in the laboratory to organisms in the natural environment.

Hagfishes thwart predators by releasing large volumes of gill-clogging slime, which consists of mucus and silklike fibers. The mucous fraction originates within gland mucous cells, which release numerous vesicles that swell and rupture when ejected into seawater. Several studies have examined the function of hagfish slime mucous vesicles in vitro, but a comprehensive model of their biophysics is lacking. Here, we tested the hypothesis that vesicles contain polyanionic glycoproteins stabilized by divalent cations and deploy in seawater via exchange of divalent for monovalent cations. We also tested the hypothesis that vesicle swelling and stabilization are governed by "Hofmeister effects". We found no evidence for either hypothesis. Our results show that hagfish mucous granules are only stabilized by multivalent anions, and pH titration experiments underscore these results. Our results lead us to the conclusion that the hagfish slime mucous gel is in fact polycationic in nature.

ZOO*4300 (Marine Biology and Oceanography) is a senior-level field course offered by the Department of Integrative Biology at the University of Guelph. This two-week course is held at the Huntsman Marine Science Centre in St. Andrew’s New Brunswick, Canada. Students enrolled in the course study various aspects of the ecology, behaviour, physiology, biochemistry and genetics of marine organisms using a variety of oceanographic techniques. The course also includes group exercises to study various intertidal and sub-tidal environments as well as boat cruises to collect plankton, benthic invertebrates, marine fish, and to observe marine mammals. The course provides excellent opportunities for students to familiarize themselves with state-of-the-art techniques involved in various branches of marine biology and oceanography and conduct an individual research project. This feature highlights three individual research projects by University of Guelph students. More information about the field course in marine biology and oceanography is accessible at the following link: http://www.uoguelph.ca/ib/undergrad/fieldcourses_marine.shtml.