It's getting hot in here! How does temperature affect shark egg development?

South African waters are home to dozens of shark species, including a group known as the catsharks or shysharks, smaller species that spend most of their time close to the ocean floor, feeding on invertebrates and small fish. These are also the sharks that lay eggs (a.k.a mermaids purses), which they attach to underwater structures using tendrils. Inside the egg, the baby spends months growing before ‘hatching’ out. But do different shark species have different preferences for where they lay their eggs, and what does temperature have to do with it?

Some early research published in 2013 attempted to answer these questions using two endemic South African shark species; the leopard catshark (Poroderma pantherinum) and the dark shyshark (Haploblepharus pictus). Citizen scientists performed underwater surveys to record egg sightings, depths, temperatures and the kinds of surfaces eggs were attached to. They logged a total of 250 eggcases, with a record of 40 seen on one dive! Interestingly, there were large differences between the two species. Most of the dark shyshark eggs were found in shallow waters, 0 – 5 m, and were attached to seaweed. On the other hand, most of the leopard catshark eggs were found deeper, 7 – 20 m, and were mostly attached to seafans. You can see these differences side-by-side in the graph. When choosing where to lay their eggs, there is likely a trade-off between concealment, accessibility to predators, ambient environmental conditions and risk of dislodgement. Eggs were also from both species were also found throughout the year, highlighting these species reproduce all year round.

If we compare this research to our ELMO eggcase submissions, there are some interesting trends. Most of the eggcases submitted to ELMO are those that citizen scientists pick up on the beach, but a few are submitted by divers who see them underwater. Almost all of the dark shyshark eggcases (98%) are beach submissions, while one-quarter of the leopard catshark eggcases were underwater ones submitted by divers. This might be related to the differences in depths that these species like to lay their eggs. With dives only really happening deeper than 5 m, there is a much greater chance of divers seeing leopard catshark eggcases. We can also see in our data, plotted on the line graph, that eggcase sightings occur all year round, further providing evidence that they reproduce year-round.

The 2013 study also grew eggs in an aquarium under different temperatures, in an effort to see how temperature would affect the growth of the baby shark inside the egg and it’s time to hatching. The results were very interesting, with both leopard and dark shyshark babies growing much slower at colder temperatures (14 oC instead of 17 oC). This meant that baby sharks spent longer inside the egg before hatching, with the time actually being doubled for the leopard catshark (hatching after 266 instead of 125 days). So, eggs laid in different seasons, or along different parts of the coast, might take considerably different times for the baby to mature and hatch out. This also gives us an insight into what might happen under predicted global warming scenarios. Might it take quicker for these sharks to hatch under warming ocean temperatures?

An important thing to keep in mind when looking at studies like this is that they are carried out under controlled conditions maintaining a constant temperature. In nature, things can be very different with large fluctuations recorded over short periods of time depending on a multitude of factors, including weather. Regardless, how changing temperatures in the coastal waters of South Africa could impact the growth and development of endemic species, like the leopard catshark and dark shyshark, is certainly an important area to investigate further. One thing we are sure of, is that being able to look at our data and examine it in relation to scientific studies reiterates the value of our dedicated citizen scientists.

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For further reading, please see the source paper:

Pretorius, C. and Griffiths, C.L. , Patterns of egg deposition and egg development in the catsharks Poroderma pantherinum and Haploblepharus pictus (2013). African Zoology, 48:1