Intertidal life, especially in the upper shore regions, may be especially risky because of exposure to a greater variety of terrestrial, aerial, and aquatic predators than would be experienced in subtidal areas.  Other than the shell and operculum, which offer physical protection, and the checker-board patterns of Littorina scutulata and L. plena, which may provide camouflaging protection, winkles have no defenses against predators.  The main predators of littorinids and lacunids are crabs, fishes, birds, and sea stars. 

Although quite a lot is known about inducible defenses in adult marine invertebrates, including shell-thickening in molluscs in response to crabs and longer spines in cladocerans in response to planktonic predators, comparatively little is known about similar processes in larval invertebrates.  A researcher at Friday Harbor Laboratories, Washington approaches the topic by rearing veliger larvae of winkles Littorina scutulata in the presence of caged zoea larvae of crabs Cancer spp. for periods of several weeks.  The larvae develop significantly smaller apertures and rounder shells than cohort veligers reared in the absence of zoeae (see graph on Left). Later, when these “experienced” veligers are exposed to short-term (3.5h) predation by zoeae along with “naïve” veligers, the experienced ones suffer significantly less mortality.  The zoeae capture the veligers as the latter swim by, then rotate their shells while they chip away the aperture edge until the soft tissues are exposed (see photos on Right). Total handling time per veliger in a successful attack is about 3min. Thus, exposure to chemical stimuli from the zoeae induces developmental changes in shell shape and aperture size in the veligers that can contribute to decreased vulnerability on later exposure to the same predators.  However, it is not clear how this works. Does it owe to the more rounded shell-shape, to the smaller-sized aperture, or to a possibly thicker shell at the aperture edge?  This would be an interesting research project.  Vaughn 2007 Ecology 88: 1030.