Strings of periwinkles, tiny rock-dwelling gastropods, take advantage of low tide to make a crossing.

Lot of periwinkles, crossing over sand from one rock to another
Periwinkles, Littorina, on a sandy crosswalk | September 10, 2017

Here, they glide, en masse, from the tip of rock in the lower left, to new grounds on the rock stretching across the top. Pioneers mark the crossing with mucus. Less adventurous gastropodestrians follow along, single file.

Bunch of Littorina, putting some heavy grazing on a patch of Ulva grazing
Littorina, grazing down a tuft of Ulva

But not all the winkles are on the move. Nearby, some stay put, feasting.