Inside Mermaid’s Purses | The Maria Mitchell Aquarium Opens New Skate Egg Hatchery Exhibit

Even though Easter has already come and gone, now is as good a time as any to go on an egg hunt! Egg cases, known as “mermaid’s purses,” are one of Nantucket’s many intriguing beach treasures.

Contrary to popular belief, most of the mermaid’s purses found on Nantucket are produced by skates, a group of cartilaginous fishes related to sharks and stingrays.

Most of Nantucket’s shark species give live birth, so shark egg cases are extremely rare on Nantucket - only a handful of chain catshark (Scyliorhinus rotifer) purses have been found here. The most common purses on our beaches are from the winter skate (Leucoraja ocellata) and the clearnose skate (Raja eglanteria). Skates are bottom-feeding predators that resemble stingrays, but do not have any. They use their ampullae of lorenzini to detect weak electric signals generated by otherwise hidden animals such as clams, flounders, and sand shrimp. Depending on the species, skates may need to survive over 10 years before they reach maturity and can reproduce.

Native skates will lay two eggs at a time, each bearing one yolk and one embryo. The egg cases are made of tough collagen to keep the embryo safe from predators while it develops. The purses are anchored into the sand or onto seaweed using the long horns on either side of the case. The embryo slowly develops, consuming the nutrient-rich yolk and using its tail to pump oxygenated water into the case through small holes in the horns of the case. Ideally, the eggs remain undisturbed for 9 to 12 months before the fully formed baby skate emerges.

Natural disturbances are common, however. Inclement weather may create swells or currents that pull the egg case from its attachment site, presenting a litany of dangers for the embryo. While the egg case is quite rugged, the embryo and yolk inside are extraordinarily fragile. Even small impacts can cause life-threatening damage to the embryo or pierce the yolk. Currents can bring the egg case into the habitats of predators, mostly carnivorous sea snails. Sometimes, waves can throw the egg on the beach, where the case is in danger of drying out – unless they are rescued!

The Maria Mitchell Aquarium team often searches the beaches for animals. We go out to ‘Sconset with eager beach combers every week for our Beach Biology Field Trips, identifying all sorts of shells, bones, animals, and other ocean treasures. We always keep an eye out for skate eggs when we are in ‘Sconset! Live skate eggs can be found year-round. Any cases that are on dry sand have likely been out of the water for too long for an embryo to survive, so we search for freshly washed-up cases close to the water. Most of the time, less than 5% of the egg cases that wash ashore are viable. We go through several steps to distinguish between live egg cases and empty ones.

• The visual test: If the egg case has any degree of damage, it is extremely unlikely to house an embryo. Holes in the case, damaged horns, and a dry exterior are all signs the case is empty. Barnacles or seaweed growing on the case, however, do not appear to damage the egg. If the egg case is in perfect condition and still wet, we proceed to the next step.
• The weight test: we gently pick up the egg case. If it is very lightweight, it is most likely empty. If it feels full (about the weight of a chicken egg yolk) it may have a live egg and we can proceed to the next step.
• The candling test: If we have an intact, heavy egg case, the last step is to confirm there is an embryo. We use a very bright 8,000 lumen flashlight to see through the case and look for signs of life. If we shine our light through the case but do not see a yolk or embryo, the skate has likely already hatched, leaving behind only seawater. If we see a yolk or any movement, we quickly proceed to the next step.
  • We gently place the egg case into a large glass jar with seawater for observation and transport. We bring the case to the Maria Mitchell Aquarium where we can safely incubate the egg in our new skate hatchery exhibit!

To learn more about skate eggs, you join us on a Beach Biology Field Trip or visit the new skate hatchery exhibit in the Maria Mitchell Aquarium at 28 Washington St., featuring live winter skate eggs rescued from Low Beach!

In addition to skate eggs, it is common to find the eggs of whelks, lugworms, Atlantic horseshoe crabs, and long-fined inshore squid along Nantucket’s shoreline.