Imagine wandering through a tranquil lagoon and spotting a group of upside-down jellyfish resting with their bell against the seafloor.
Unlike most of their free-swimming counterparts, these jellyfish are often found lounging, with their oral arms extending towards the sun. But why such an odd pose?
Why are upside-down jellyfish upside-down?
The upside-down posture serves a dual purpose. Firstly, this position facilitates the pulsing movement of their bell, pushing water over the jellyfish’s body, ensuring a steady flow of oxygen and food. Secondly, the upward-facing tentacles benefit from the sunlight, which assists the photosynthetic algae, zooxanthellae, residing in the jellyfish tissue. This unique position allows them to gain energy from both their food and the sun!
Upside-down jellyfish love to hang out in the sunlit, shallow waters of coastal regions, especially around areas bustling with mangroves. Sunlight plays a pivotal role in their survival as it powers the photosynthetic algae inside them. Think of them like underwater solar panels!
In Australia, they are predominantly spotted in the tropical territories, ranging from Yampi Sound in Western Australia to Queensland’s Gold Coast. However, there’s a twist: these jellies have made surprise appearances in temperate coastal lakes of New South Wales, and even in the unusually warm waters around a powerplant in Adelaide.
The diet and life cycle of the upside-down jellyfish
When it comes to diet, these jellyfish are both photosynthetic and predatory. The zooxanthellae within provides up to a whopping 90% of their nutritional needs through photosynthesis, while the remaining 10% is sourced from the ocean buffet of zooplankton. They employ a two-step tactic for this: first, they stun their prey using their nematocysts or stinging cells, and then deploy a mucus to ensnare and consume the tiny creatures.
Although equipped with the ability to swim traditionally by pulsing their bell, these jellies prefer the floor. Their stationary, upside-down lifestyle may seem lazy, but it is a strategic adaptation that allows them to harness energy effectively from the sun through their symbiotic algae.
The lifecycle of these jellies is a captivating dance of nature. After males release their reproductive cells, these combine with the female’s eggs in the open water. Once fertilized, females release planula larvae, which, seeking a solid base, often anchor to substrates like mangrove roots. Over time, these larvae morph into polyps, resembling tiny sea anemones. These polyps, under the right conditions, undergo a fascinating process called strobilation. From one polyp, multiple jellyfish bud off, introducing new medusae to the aquatic realm.
Impact on Humans and Environment
When in bloom, the density of these jellyfish can soar to 30 individuals per square meter. Such dense gatherings can deplete water’s oxygen levels, reshuffling the aquatic food chain. Their dominance can outcompete other species and consume a significant portion of the available zooplankton. Swimmers, too, need to be cautious. A brush against their tentacles can lead to stings, which can range from being a mere annoyance to causing more pronounced discomfort.
— WTF fun facts