Weekly “What is it?”: Mermaid’s Purse
Sailors have a long and creative history of exaggeration when it comes to describing unusual marine organisms. From the terrifying kraken and sea monsters to the alluring mermaids and sirens, too much sun, salt, and distance from land obviously alters one’s perception of reality. That being said, the ocean is a large and mysterious place, full of wondrous creatures that often defy logic even when we are looking right at them. So perhaps we should give those early explorers a little slack for their overabundance of tall maritime tales. They were lonely and had a lot of time to kill, after all. Many of those ocean-born legends left a long cultural legacy. Mermaids are so common in pop culture that one of the local beach hotels has a “mermaid” that swims in the pool with tourists. I know lots of kids who use mermaid-style fins to propel through the water, and even the latest Pixar film features underwater “mer-people” characters who try living on the land.
Thus, it should be no surprise when unusual organisms found on the beach get nicknamed after some of those legends. Last week, I discussed the “mermaid’s necklace,” also known as the egg case of a whelk snail. Accessorizing nicely with the mermaid’s necklace is this week’s feature, the “mermaid’s purse.” A mermaid’s purse is the leathery egg case of a skate, a member of the larger ray family.
Most fish are broadcast spawners, which means they fertilize thousands of eggs in the open water and leave their development and survival to fate. Sharks and rays have a different reproductive strategy, investing more time and energy by using internal fertilization. More than 50% of large sharks and true rays are the rare viviparous fishes, giving birth to live offspring (called pups). The remaining oviparous (egg-producing) shark and skate species produce thick egg cases made of tough collagen, which are released onto the ocean floor. Nicknamed mermaid’s purses, the pillowy egg cases found in our area are black and rectangular, with four “horns” on each corner. It is theorized that the protrusions help anchor the egg cases into vegetation and the sandy seafloor. Depending on species, they may incubate from 2-15 months before hatching. As seen in the photo above, the egg cases are often in the water long enough to become substrate for barnacles to grow on. When the hatchling has matured enough to leave the egg sac, it escapes through a slit at the top ridge of the egg case and discards the case. The case floats through the sea, often settling on a coastal beach with driftwood and seaweed in the wrack line.
On rare occasions like hurricanes, an egg case with a live embryo might be washed onto the beach. The young cannot live long outside of the water, however, as the oxygenated seawater moving through the egg cases, along with embryonic fluid, keeps them alive. Several species of sharks also produce mermaid’s purses, but they tend to be lighter in color with long tendrils and are rarely found on local beaches. Port Jackson sharks and chimaeras—found more commonly near Australia—produce fascinating ridged and spiral-shaped egg cases, as seen in this guide to mermaid’s purses. In the Gulf of Mexico, the only skate egg cases I have come across are the are the black, leathery ones—but that doesn’t mean we can’t find something new! As always, keep your eyes open on your beach walks—you might just see where a mermaid emptied her closet.