The black-pouched egg cases are washed up in stormy weather

They want to identify why several species are declining in numbers.

The Shark Trust, based in Plymouth, says the best time to look for egg cases is after stormy weather when seaweed and debris are thrown onto beaches by the sea.

The cases are black pouches - those from sharks have curly tendrils at both ends, while those from their close cousins, skates and rays, have a pointed horn at each corner.

The cases also vary in size, but many are as big as a child's hand.

The trust wants people to record the details of egg cases, including where they were found and at what time.

It says people can post the empty pouches to its headquarters in Coxside.

It is also keen to involve children who can fill in forms about their discoveries online.