What Turns Water Into Slime in a Fraction of a Second? An Animal With No Jaw

Glands along the body release a small quantity of material consisting of mucus and of protein threads coiled into tight bundles. On contact with seawater the bundles unravel and the mucus expands, and within a fraction of a second a teaspoon of the secretion becomes many litres of a soft fibrous gel. The threads are the crucial component, since they are extremely fine, extremely strong and enormously long relative to their thickness, and they hold the whole mass together rather than letting it disperse. A predator that bites one gets its mouth and gills filled, and video of sharks attempting it shows them releasing immediately and departing, since blocked gills are an immediate threat to life.

Almost nothing about the animal follows the vertebrate pattern:

• •No jaws, feeding instead with a rasping plate of keratin teeth
• •No paired fins, no scales and no bones in the usual sense
• •A skull of cartilage but no vertebral column in most of the body
• •Skin attached to the body at very few points, so the animal moves loosely inside it
• •Four hearts and blood at a pressure far below that of other vertebrates
• •The ability to absorb nutrients directly through the skin

The position of these animals in the tree of life has been argued over for more than a century and matters for understanding vertebrate origins. They and lampreys are the only surviving jawless vertebrates, and the question is whether the two are each other's closest relatives or whether hagfish branched off earlier, before several vertebrate features appeared. Anatomical evidence long favoured the earlier branching, which would make the animal a window onto an early stage. Molecular evidence consistently supports grouping them with lampreys instead, which implies the animal lost features its ancestors had rather than never having them. The molecular result is now generally accepted, which changes the interpretation of nearly every unusual feature listed above.

An animal that fills the surrounding water with gel faces the obvious problem of choking itself, and the solutions are as unusual as the defence. It ties itself into a knot, passing the knot along its body from head to tail, which scrapes the material off and clears the head, and the same movement is used to gain leverage when tearing food from a carcass. Its gills are arranged with a separate exhalant opening, so water taken in can be expelled through a route away from the worst of it. It sneezes, forcing water out through the nostril to clear it. And it appears to tolerate a degree of blocked gill function that other fish would not, which fits an animal that already has unusually low blood pressure.

Materials scientists have taken serious interest in the protein threads, and the reasons are specific. They compare favourably with spider silk in strength for their weight, they are produced by an animal at ambient temperature in seawater rather than requiring industrial conditions, and they consist of proteins whose sequences are known and which can be produced by engineered bacteria. Proposed applications include fibres for textiles that would replace petroleum-derived synthetics, and the material has been drawn into usable threads in laboratories. Naval interest has been reported in the gel itself as a means of fouling propellers and disabling vessels. The practical obstacle throughout is producing the proteins in quantity and getting them to assemble correctly outside the animal.

Glands release mucus and tightly coiled protein threads that unravel in seawater, turning a teaspoon of secretion into litres of gel within milliseconds and blocking a predator's gills. The animal has no jaws, no paired fins, four hearts and skin attached at few points. Molecular evidence groups it with lampreys, implying it lost features rather than never having them, and the threads are under investigation as a replacement for synthetic fibres.