All skates, all chimaera and some sharks are oviparous (egg-laying). Once the egg has been deposited the mother does not invest any further parental care. The embryo is now on its own and nothing but a thin, leathery capsule protects the developing skate or shark until it is ready to hatch. Because shark eggs are attached to immotile seafans, seaweed or rocks, they are easy prey even for the smallest and slowest organisms given they can chew or bore through the capsule. Some predators, like whelks and isopods, actively seek out shark eggs and bore 1 to 40mm large holes into the capsule to reach the nutritious yolk. And although a freshly laid shark egg is equipped against biofouling for some time, algae and Bryozoa start growing after a few weeks, attracting more predators and decreasing the embryo’s chances of survival. Hence species that develop quickly stand a better chance of survival. The Puffadder Shyshark for instance hatches after only three months. Leopard catsharks on the other hand develop for six to ten months and a study showed that up to 50% of their embryos are predated before they hatch.

Puffadder shyshark (Haploblepharus edwardsii) eggcases with A. signs of predation and B. traces of biofouling 

Skate eggs are laid on the sandy seafloor where predatory gastropods are less common than on rocky reefs. Instead their eggcases are more likely to cast ashore before the embryo has hatched as they are - unlike shark eggs - not firmly attached to a substrate.

Once the eggcases wash out on our beaches, we can determine much of what has happened to them along the way. The size and shape of boreholes are good indicators for the type of predator. An open hatching slit is a sign that the embryo has hatched, a closed hatching slit in combination with boreholes suggests it has been predated. Bryozoans and barnacles are often still attached to stranded eggcases and tell us more about species interactions.

Open hatching slits

We continuously learn more about shark and skate eggs: That's why we keep a library of photographs of eggcases, which enables us to re-examine every single find and inspect it for new indicators even years after the egg was picked up.

References

Pretorius CA. 2012. Factors influencing the development and mortality rate of shy and cat shark embryops in South African waters. MSc thesis. University of Cape Town. South Africa.

Smith C, Griffiths C. 1997. Shark and skate egg-cases cast up on two South African beaches and their rates of hatching success, or causes of death. South African Journal of Zoology 32: 112-117.

Thomason JC, Davenport J, Rogerson A. 1994. Antifouling performance of the embryo and eggcase of the dogfish Scyliorhinus canicula. Journal of the Marine Biological Assiociation of the United Kingdom 74: 823-836.