On the seashore after a storm at sea, mermaid's purses are often found washed up - torn from their shallow underwater hold usually on seaweeds such as Laminaria: most of these are damaged and empty. Those who scuba dive round UK shores often see them still in place (they are especially abundant in the Fal estuary in Cornwall). As they are visually opaque their nature might not be immediately obvious. They are made from collagen, and in British waters are usually laid by a species of dogfish Scyliorhinus: the female attaches them by means of the tendrils, to provide relative safety for the developing embryo each contains. The rate of development depends on water temperature to some extent, but takes some months. When I was working at the Plymouth Marine Biology Laboratory on dogfish embryology years ago, I had access to a steady supply of them, for they are easy to bring ashore, and straightforward to support in a marine aquarium. They vary in size according to species, but may be more than 100mm long.
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