An Instantaneous Gill-Clogging Defense
Deep on the muddy floor of the Pacific Ocean lives a primitive, jawless fish with an exceptionally effective defense mechanism. The Pacific hagfish, Eptatretus stoutii, responds to threats by releasing a glandular exudate that combines with seawater to create a massive volume of gelatinous slime. This deployment happens in less than half a second. The slime is the most dilute hydrogel known, consisting of 99.996% seawater. Its other components are tens of thousands of protein threads and mucus.
The slime is produced by approximately 100 glands arranged in rows along the hagfish's flanks. These glands contain two specialized cell types: gland thread cells (GTCs) and gland mucous cells (GMCs). Each GTC produces a single, coiled protein thread, or skein, that is about 15 cm long but only 1 to 3 micrometers in diameter. GMCs hold thousands of tiny vesicles filled with mucins. When the hagfish is attacked, it secretes these cells, which rupture upon contact with seawater. A protein-based glue holding the thread skeins together dissolves, allowing the threads to rapidly unravel and expand to 10,000 times their initial volume. The mucins swell and mix with the threads, creating a network that entraps a huge amount of water. A single hagfish can produce up to 24 liters of slime from a small amount of exudate. This slime is a fine sieve, effectively clogging the gills of would-be predators and forcing them to release the hagfish to avoid suffocation.