There is another animal that is always trying to get inside the screwcap’s shells – but not to eat them. When it sees a predator, the sticker worm folds itself over and attaches each half on either side of the seam of a screwcap using pads of variable stickiness. Microscopic grooves in these pads take advantage of every deviation from perfect smoothness. Flexing its muscular body, it twists the shell open and then quickly slips inside before it can shut again. While hiding from predators, the screwcap and the sticker worm will not hurt each other. When the shell opens again, the sticker worm will leave.
Sticker worms are incapable of digesting meat and only eat weeds, which lack some of the compounds that hold meat together. However, there is one genus of sticker worm that will eat meat anyways. It opens screwcaps not to hide, but to bite off chunks of their gills. It holds this mostly undigested meat in its digestive tract in order to deliver it to creatures called beps in exchange for medical supplies and services.