What is a mermaid’s purse? Where can you find them? How do you identify which species it belongs to? And what should you do when you find out?
If you have ever been beachcombing for treasures along the shoreline then you may have been lucky enough to find a mermaid’s purse. They may not look like much, but these dried-out leathery pouches are actually the used egg cases of sharks and skates, created to develop and protect their babies. Around 43% of all Chondrichthyes species (cartilaginous fish including sharks, rays, skates and chimaeras) give birth to their offspring in these purses. Between them they produce a diverse array of different types, sizes and colours, that eventually wash up on our beaches when they’ve been emptied. However the egg cases can still be valuable even after they’re discarded, they provide an insight into the hidden lives of sharks and are also very useful for conservation work. So here is a quick guide on how to find and identify different egg cases, as well as how you can help researchers out.
What is a mermaid’s purse?
There are two types of reproduction amongst sharks, rays and the other ‘non-bony’ fish (known as Chondrichthyes). The first is viviparity, where mothers give birth to live young known as pups like mammals such as us, the second is oviparity, where mothers lay undeveloped eggs in specialised cases like birds and reptiles. Most large species of shark and all true rays are viviparous and give birth to live young, whereas the smaller shark species such as catsharks and a sub-family of rays known as skates are oviparous. Instead of developing their young in-utero and giving birth to them, they produce egg cases that act as a detachable uterus to develop and protect their offspring separately. They are often laid on the seafloor in places such as seagrass meadows or rocky crevices to hide them from predators. It is these egg cases which are commonly referred to as mermaids’ purses.
Egg case 101
As the name suggests egg cases initially contain the fertilized egg of a shark or skate, which over time develops into an embryo. They are made out of the fibrous protein collagen which forms tough leathery pouches. Almost all egg cases contain a single egg, although big skate and mottled skate cases can contain up to seven. After the cases are released from the mother they provide the embryo with all the nutrients and energy it needs to develop, including oxygen-rich seawater which can be absorbed from its surroundings. This process usually only takes a few months for most species, however in some species like the critically-endangered white skate it can take up to 15 months. Once the babies have reached a point where they can survive on their own without their purses they swim out of a small opening at the top of the case and leave their ‘artificial mother’ behind. To see developing embryos inside egg cases for yourselves, check out the amazing video below.
Where can you find them?
Once the baby sharks and skates have fully developed and left their purses, the empty casings are then carried around by ocean currents and occasionally wash up on our shoreline. The best place to find a mermaid’s purse is therefore at the high tide line on most beaches. The high tide line is easily identified by the long bands of dried seaweed that stretch across a beach. It is actually amongst the seaweed where you are most likely to find a purse, but you have to look closely because the dried out cases look very much like the seaweed they are surrounded by. If you have no luck at the high tide line, then the next best place to look is in rock pools, because they can often get trapped in them as the tide goes out. You won’t always be able to find a mermaid’s purse every time you go to a beach, but don’t let that stop you searching other areas or coming back for another go at a later date.
Different types of egg cases
Whilst there is a clearly defined shape and structure to most egg cases there is also a large variety of different sizes, types and variations between species, much like how different birds lay their own unique eggs. The general appearance of a mermaid’s purse is a black or brown leathery pouch with either tendrils or horns coming off each or either end. For catsharks the pouch will tend to be more rounded and have long spindly tendrils, which can often get wrapped around in seaweed or other egg cases. For skates the pouches tend to be more rectangular and have defined horns in place of tendrils. A more bizarre example is chimaeras who produce bottle-shaped pouches with a pair of feathery fins down its sides or the bullhead sharks who produce spiral auger-shaped egg cases designed to get wedged into crevices between rocks. However it is unlikely you will find these more bizarre designs, especially on your first try.
Identifying what you have found
So how do you figure out which species has produce the egg case you have found? One of the big problems with identification is that purses you find on the beach are dried out and so often misshaped or decoloured. Therefore the best way to make a successful identification is to take it home and re-hydrate it. You can do this by placing it in a bowl of water, preferably from the sea, for a couple of hours. You will notice that it will change shape, get darker and will feel more slimy if you touch it. At this point you might also be able to find the opening the juvenile shark or skate swam out of when it was fully developed. Now you have a re-hydrate eggcase you will be able to determine more details about the shape, size, colour and any other features you need to make a successful ID. For the best results use online guides (like this awesome one by Marine Dimensions) and you will then know for sure what you have found.
Reporting your findings
Finding and identifying mermaids’ purses can be great fun for young children on family outings or just avid shark enthusiasts, but it can also provide really important and helpful information for marine researchers. Egg cases provide clues as to where certain species of shark or skates can be found, especially the nursery grounds that are important for juveniles. This information can then be used to help inform conservation measures to help protect sharks and rays across the world. Citizen science projects involving egg cases can be found across the world online. However if you live in the UK, where we have a lot of catshark and skate species, then you can report your findings to the Shark Trust’s Great Eggcase Hunt. Just remember to record where and when you found your case in the first place. Once you have made your contribution to science then why not keep your specimen or even start your own collection. If you leave your case in a warm dry place for a few days it will dry out properly and will stay intact for years.