It was low tide and forecast to be relatively calm, but when we arrived at the beach, white-crested waves were surging along the shore. Abandoning our plans to swim, we decided to beachcomb instead. The high tide had deposited a thick band of decomposing seaweed, shells and other debris on the strandline. As I pocketed an iridescent pink-lined scallop shell, my attention was drawn to a piece of driftwood with peeling turquoise paint. Crouching down to pick it up, I realised it was surrounded by mermaid’s purses – the cast-off eggcases of cartilaginous fish – that had been perfectly camouflaged against the dark ribbons of kelp.
These protective capsules are made from a combination of collagen and keratin, giving them a similar texture to human fingernails. Keratin is a fibrous protein, tough enough to deter predators and insoluble in water, waterproofing the eggcase when it is first laid. This is vital to protect the embryonic pup within, which won’t develop gills until it’s several weeks old. Later, small holes open in the capsule’s four horn-like projections, allowing the embryo to continuously pump in fresh, oxygenated seawater by beating its tail.
Dried by the wind, the eggcases were brittle and shrunken, like aged leather. There are subtle differences in size, colour, capsule shape and the length and shape of the horns between species. To make an accurate identification, I had to rehydrate them, the capsules growing soft and supple as they swelled to their original size. The Shark Trust runs a citizen science project called The Great Eggcase Hunt, designed to record finds and build a picture of species’ presence and diversity in British waters.
Referring to the guide on its app, I determined that we had found the eggcases of three species – mostly spotted rays (Raja montagui) and thornback rays (Raja clavata), but also, perhaps most significantly, two belonging to undulate rays (Raja undulata), which are listed as endangered by the International Union for Conservation of Nature and are a priority species under the UK post-2010 biodiversity framework. All three species are commonly referred to as rays, but they are actually skates. Though closely related, oviparous skates lay eggs, while true rays are viviparous, giving birth to live young.