Slime for defense – Rheological design principles in a marine environment

Abstract

Hagfish are genetically one of the oldest living creatures on earth. Neither charming in appearance or task they brought their defense mechanism to perfection during the last 150 million years. When attacked by predators, hagfish excrete a concentrated mucin – protein fiber solution, which forms within milliseconds a large body of slime. Although the slime consists out of vast amounts of water, the diluted mucin and fibers span a cohesive network eventually clogging mouth or gills of the predators and enabling the hagfish to escape. Here, we present both the rheological properties of the slime in defense and escape situations as well as the biophysical principles of slime formation in the saline marine environment. We show that the different rheological responses of the slime under shear and elongational flow lead to strain hardening under attack but shear thinning during escape, promoting the survival chances of the hagfish. Considering the slime as a complex polyelectrolyte, a fine-tuned interaction with the different ions present in seawater must be given to employ the full defense capability of the slime, i.e. the rapid unraveling and network formation of both mucins and skeins under charge screening conditions. By elucidating the molecular and biophysical design principles and their consequences on the rheological properties of the slime, we are able to provide guidelines for tailoring mechanical properties of other mucin-like polyelectrolyte systems.