An Evolutionary Laboratory
In the Sohoton Cove National Park on Bucas Grande Island, a network of limestone caves and secluded lagoons hides an example of rapid evolution. One of these lagoons, Tojoman Lagoon, hosts thousands of so-called "stingless" jellyfish. The primary species found here is the spotted jelly, Mastigias papua. These jellyfish exist in an environment largely free from natural predators.
Over many generations in this isolated sanctuary, the jellyfish's stinging cells, called nematocysts, have become greatly reduced. Developing and maintaining potent stingers requires significant metabolic energy. Without the evolutionary pressure of predation, natural selection favored individuals that conserved this energy. This process, known as regressive evolution, resulted in a population of jellyfish whose stings are too weak to be felt by human skin. The entrance to their protected world is a single cave tunnel, only accessible at low tide, which has kept the lagoon's ecosystem separate from the open sea.
Convergent Evolution and Symbiosis
Stingless jellyfish are not unique to the Philippines. A similar, more famous population of Mastigias papua exists in Jellyfish Lake in Palau. However, genetic studies show that the Sohoton and Palau populations evolved their harmlessness independently. They are distinct subspecies that, when faced with similar predator-free, isolated marine lake environments, followed the same evolutionary path. This shows convergent evolution, where unrelated populations develop similar traits.
These jellyfish are solar-powered. They have a symbiotic relationship with algae called zooxanthellae (Symbiodinium) living within their tissues. This is the same type of algae that lives inside corals. The jellyfish spend their days migrating across the lagoon, following the path of the sun to provide light for their algal partners to photosynthesize. The algae, in turn, provide the jellyfish with a substantial portion of its nutritional energy. The jellyfish supplement this by filter-feeding on small zooplankton, using not one central mouth, but numerous small mouth openings on their oral arms.