Saturday 8 December 2018

Chrysaora spp.: Thousands of Compass Jellyfish wash up on beaches around Cape Town, South Africa.

Bathers visiting beaches around Cape Town, South Africa, over the past two weeks have been surprised, and sometimes alarmed, to find many thousands of Jellyfish washed up on the shores. The Jellyfish belong to three closely related species, the Benguela Compass Jelly, Chrysaora fulgida, the Purple Compass Jelly, Chrysaora africana, and the Cape Compass Jelly, Chrysaora agulhensis, all of which are indigenous to the region, and non of which is considered particularly dangerous. At worst Compass Jellies can deliver a sting comparable to that of a Bee, and most of the specimens found on the beaches around Cape Town appear to have lost their tentacles, making them quite harmless.

Compass Jellies, Chrysaora spp., on a beach near Cape Town. Helena Souza/Two Oceans Aquarium.

Jellyfish, although often large, are entirely planktonic in nature, meaning that they simply drift in the water column, and are unable to swim against currents, so that storms or other events that cause temporary changes in current flow can deposit large numbers on shorelines. Jellyfish numbers are determined largely by nutrient availability, as nutrient levels determine the amount of phytoplankton (single-celled Algae) in the water column, and this in turn controls the numbers of smaller zooplankton (small planktonic Animals), which are the main source of food for Jellyfish.

The waters around Cape Town are extremely rich in nutrients, due to the Benguela Current, which is fed by deep upwellings from the South Atlantic, which carries nutrients up from the ocean floor. Importantly, this is driven largely by wind direction (upwelling is strongest when the current direction is at 90° to the wind direction) so that the amount of upwelling is seasonal, leading to seasonal changes in nutrient availability, and therefore Jellyfish numbers.

The Benguela Upwelling on the coast of South Africa. Mohrholz et al. (2018).

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