The Fascinating and Weird World of Shark Eggs
SHOW NOTES
Host Isla travelled to Shark Trust HQ in Plymouth, UK, to meet with Cat and participate in an eggcase hunt along the local beach [4.50]. But before heading to the seaside, we explored Cat’s extensive egg case collection indoors and learned all about the egg-laying species of shark, ray and skate. Cat’s own journey into this fascinating subject began during her master’s degree in conservation biology at Plymouth University [08.47]. Although she wanted to work in marine conservation, sharks hadn’t really made it onto Cat’s radar, until she had a chance meeting with a friend who worked at The Shark Trust. This led to Cat volunteering with the charity, where a lot of her time was spent logging eggcases for The Great Eggcase Hunt (which we’ll learn more about later in the episode!). After graduating, Cat left the rainy shores of the UK for adventures further afield. But it wasn’t long before a job opening at The Shark Trust had Cat finding her way back to Plymouth again.
Cat has now been with The Shark Trust for fifteen years [10.20]. “I’ve always been involved in the citizen science work, and particularly The Great Eggcase Hunt.” She says, of her role in the organisation. “My work is largely focused on threatened species and developing collaborative conservation planning documents, like for angel sharks in the eastern Atlantic and Mediterranean regions. And I now lead the Trust’s community engagement programme.” This includes working across a diverse range of groups, from schools to the scientific community and the general public.
And something Cat knows a lot about – and talks to a great many people about – are the egg layers [12.25]. “Of the 1250 or so species of sharks, rays and chimaeras, around 550 species are oviparous. They’re the egg-laying ones, they produce eggs that mature and hatch outside of the body.” That’s around 44% of all chondrichthyans (the cartilaginous fishes)! Is there a particular kind of shark that lays eggs? “Of those species, they belong to 13 families, within five orders,” Cat explains. These orders include the horned sharks, carpet sharks, and catsharks. There are an astonishing number of the latter, including many deep sea species. And, all true skates and chimaeras are egg-layers.
Where these species lay their eggs varies widely [15.09]. Skates anchor their eggcases to the seabed, often using a sticky mucus to attract sand and detritus to act as camouflage. Catsharks, on the other hand, lay their eggcases among seaweeds and other sessile objects, and have curled tendrils that are used to tie the eggcase down: “the female will swim round and round a clump of seaweed and [the tendrils] are almost like string, they’ll get wrapped around and tie [the eggcase] on. And she’ll do that on both ends, so it’s nicely secure.” Not all catsharks have the curly tendrils, however. Species like the black mouth catshark lay their eggs on deep water corals, and instead of tying the eggcase down, place them strategically among the cracks and crevices of the reef to hide them.
There are species that have evolved weird and wonderful shapes for their eggcases, in order to help them stay hidden [16.00]. Chimaera eggcases are spindle-shaped; long and pointed, so that the mother can ‘spear’ them into the sand. And then there’s the horn sharks, whose bizarre eggcases are shaped like a corkscrew. “The female will deposit them, then pick them up in her mouth and almost screw them into a rock crevice.” Explains Cat.
Across all species, the eggcases are made from a keratin-collagen compound [26.04]. “The collagen and other proteins are produced in layers, almost like a row of knitting,” says Cat. This means that they are not only incredibly tough, but flexible, so that they don’t break if they are bent or squashed. All of this is, of course, to protect the embryo inside, which at this point is incredibly vulnerable. Each eggcase will contain everything a developing baby shark (or ray, skate or chimaera) will need to survive its very first stages of life. This includes a yolk sac, from which the embryo gets all of its nourishment while inside the egg. “It’s a mini life support machine – this incredibly nutrient rich soup,” Cat explains. There is also a jelly, which holds everything in place and acts as a shock absorber, cushioning the embryo from any knocks or bumps. “This is especially useful in intertidal zones, where there is a lot of wave action.”
The Great Eggcase Hunt encourages people to get out in search of eggcases that have washed up on the beach, and log their finds on The Shark Trust’s dedicated app [34.42]. “It’s very simple to get out and do,” Cat says. “You can be out walking the dog, with your family and friends, find an eggcase, and report that. Or, you can have a bit more strategy around it and do a timed walk or a set distance walk. You can make it as scientific or as simple as you like.” Divers and snorkellers are also encouraged to record developing eggs in situ and include information on depth and habitat type. The Shark Trust also has a number of free ID guides, which are available to access online and in the app. Thanks to the hard work of Cat and her team, there are guides for the UK, Europe and the Mediterranean, Australia, and the USA Eastern Seaboard, in multiple languages. “And, if you’re outside those areas, you can still submit your findings. If there’s enough coming in from a particular region, we can then develop an ID guide for that region too.”
Cat and Isla even head down to the beach to conduct a small search of their own [37.46]. Starting at one end of the beach and ending at the other, they use a stick to look carefully through the seaweed that has gathered at the strandline, picking up rubbish as they go. “We often say eggcase hunting and beach cleans go hand in hand,” laughs Cat. “It’s always a good idea to take something to collect rubbish as well.” After beachcombing for a good twenty minutes, they are still yet to find an eggcase. But, they do find something equally, if not more, exciting: a tiny, freshly hatched, small-spotted catshark [41.15]. It is so small it fits in the palm of Isla’s hand. Closer inspection shows the intricate pattern of the dermal denticles that cover the shark’s body, and a rounded belly, which indicates it had only just finished consuming the yolk sac.
The Great Eggcase Hunt has been running for over twenty years, and thus far, over 530, 000 submissions have been collected [43.36]. What have they been able to find out with all that data? “It’s helped us understand how the diversity and broad distribution of sharks and rays changes around a coastline,” Cat says. This can provide crucial information on important egg-laying and nursery habitats, which may need protection to ensure the survival of that species. It can also give insights into new and data-limited species, or be used to determine seasonal and temporal trends. “There’s a lot we can learn from just a simple eggcase.” Adds Cat. “It’s also a really important educational tool, providing that connection to sharks. People can feel empathy for this small, vulnerable baby shark, in a way that we might not see with the adults.”
You can find out more about the Great Eggcase Hunt and how to get involved here.
ABOUT OUR GUEST
CAT GORDON
Cat has been passionate about nature for as long as she can remember. At the age of 14, she began SCUBA diving and from then on, she also became fascinated with the ocean and everything within and later trained to become a PADI Divemaster. She studied BSc (Hons) Marine Biology and Coastal Ecology followed by MSc Conservation Biology, and during this time discovered the Shark Trust. Cat went on to join the conservation team in 2010. She is the Project Officer for the Great Eggcase Hunt and has enjoyed watching the number of submissions grow over the years and seeing people get excited about finding their first mermaids purse. As well as this, Cat works on threatened species, such as sawfishes and angel sharks, and gets involved with campaign activities.
Facebook: The Shark Trust
Instagram: @thesharktrust
X/Twitter: @SharkTrustUK
