The strandline is the high water mark on a beach where waves dump materials which can later be found by discerning beach combers.
Winter is a brilliant time to go beach combing because the stormy weather tends to wash up interesting things onto the strandline more frequently and in greater quantity.
Mermaid’s purses are not the only egg cases that can be found in the strandline. Common whelk (Buccinum undatum) egg cases are also a fairly common sight. They are a mass of small spongy balls that could indeed be confused for sponges. In the past, sailors did actually use them as wash balls! The first whelks to hatch are cannibalistic and will eat their still developing siblings to give themselves a burst of energy to help them survive in the open ocean.
Black sea grapes are another egg you may come across in the strandline. These interesting looking bunches are actually cuttlefish eggs stained black by cuttlefish ink. If you find some on the beach, you may even be able to see the baby cuttlefish inside the eggs, as when they are close to hatching the eggs become more transparent. There could be a chance the cuttlefish babies are still alive, so the best thing to do it is to return the eggs to the ocean which will give them the best chance of hatching and growing into adult cuttlefish.
Black sea grapes © Ryan Greaves