This shiny vessel from the sea is a mermaid’s purse—a capsule for the eggs of the big skate (Raja binoculata). Sharks also make mermaid’s purses to protect their eggs. Rays, the cartilaginous cousins of skates and sharks, don't use purses but instead bear live young.
The hooked ends of a mermaid’s purse latch to solid surfaces on the seafloor, but the capsules can tear loose from the ocean bottom and wash onto shore. Beach-cast purses often arrive empty, but sometimes they contain eggs, and sometimes embryos wiggle inside, ready to burst free.
Some aquariums will try to hatch baby skates from stranded mermaid’s purses, but success rates are low. Replicating the complex conditions of the ocean to nurture a sea creature through its lifecycle is no small task, and many of the mermaid’s purses that beachgoers find and give to aquariums have been brutalized by waves, torn by rocks, and baked by sun.
When a mermaid’s purse first washes up, the rubbery case glimmers as if covered with gold leaf, and its edges are streaked with shimmering blue and green, like tempered steel. In time, a beached purse stiffens and turns black. Whether it’s a supple golden case tumbling in the surf, or a shriveled leather lump lying in the wrack line, a mermaid’s purse is a thought-expanding beachcombing find—it hints at the bounty of life buried beneath the waves.