- Chantel Elston
A new study analyses almost 7,000 chondrichthyan egg cases and finds 9 different species!
If I asked you to name different sharks that occur
along the South African coastline, I'm sure you could name some of the biggest and most popular ones (for e.g. great white shark, Zambezi shark or ragged-tooth shark). But did you know that Southern Africa is the fourth most biodiverse area for chondrichthyans (i.e. sharks, rays, skates, chimaeras) in the world and that we have 204 species in our waters?
A recently published study sheds some light on this shark diversity in Plettenberg Bay, South Africa. Scientists and volunteers combed three beaches around Plettenberg Bay, twice a month, for a whole year, looking for chondrichthyan egg cases and the results were incredible!
Almost 7,000 egg cases were picked up and identified. These egg cases mostly belonged to two chondrichthyan species, the puffadder shyshark (60% of all egg cases) and the twin-eyed skate (27% of all egg cases). However, egg cases from nine different species were recorded, including the rarer pyjama shark.
The authors of the study found no evidence for seasonal patterns in the occurrence of these egg cases, they were picked up all year-round. However, Nature’s Valley seemed to be the hotspot for these chondrichthyans as most of the egg cases were found there.
It’s not an easy life for egg cases though. Females lay their eggs either buried in the sand, or attached to some structure like kelp or rock, and then leave the embryo in the egg to develop on it’s own with no further protection. This means that predators can attack the embryos, and gastropods (underwater snails) are known to burrow through the egg case to reach the nutritious yolk-sac inside. This study found signs of predation on 38% of all egg cases, however for some species, more than half the egg cases had been predated upon! It’s a strange world when you think of snails being the predators of sharks!
Monitoring the status of these chondrichthyan populations is very important for conservation efforts – these species play important roles in the ocean but are threatened by human activities (especially fishing), and their numbers are in decline. Unfortunately, in South Africa there is very little scientific research to monitor what is going on with these populations as it is costly in terms of time and money.
One of the solutions is to use the simple, non-invasive technique of beach combing for chondrichthyan egg cases, as used in this study. By monitoring how many egg cases wash up, and where and when they wash up, we can indirectly infer the population trends for these species.
Even you can help! If you are walking on the beach and happen to see a chondrichthyan egg case, take a photo and report it on our website (www.elmoafrica.org). Perhaps start a collection and see if you can find the egg cases of all 9 species. In this way, you help scientists to monitor the chondrichthyan populations in South Africa, and help to conserve these valuable species!
S Schmöle , M Brown & M Witteveen (2020) - All washed up: spatial and temporal variation in the wash-up and fate of chondrichthyan egg cases along a section of the Garden Route, South Africa
#Sharks #Rays #Eggcases #researchtools #Threats #SouthernAfrica