Specimen Showcase｜Is Upside-down a Normal Position for Jellyfish?
- Hong Kong Biodiversity Museum
- 4 days ago
- 4 min read
Some species' common names raise questions and curiosity! This is definitely the case in the “upside-down jellyfish”, 𝘊𝘢𝘴𝘴𝘪𝘰𝘱𝘦𝘢 𝘢𝘯𝘥𝘳𝘰𝘮𝘦𝘥𝘢 (Forsskål, 1775). This species originally native to the Indo-Pacific region entered the Mediterranean Sea and slowly spread westwards after the important Lessepsian migration that followed the opening of the Suez Canal connecting the Red Sea (and the Indian Ocean) to the Mediterranean Sea more than 120 years ago. They are currently recorded as far west as Sicily and their significant invasive capabilities pose potential threats to the ecosystems of the Mediterranean Sea.
𝘊. 𝘢𝘯𝘥𝘳𝘰𝘮𝘦𝘥𝘢 has a unique flat and saucer-shaped bell, an umbrella-like structure, which enables jellyfish to move. It possesses four pairs of elaborate branched but unfused, relatively short oral arms. The lack of marginal tentacles, the long and thin string-like structures hanging down the bell, is an iconic characteristic, as it is not found in other jellyfish. Most individuals reach a diameter of 10 cm, but some can grow up to 20 cm. If jellyfish species usually pulse and swim in the ocean following the wave, 𝘊. 𝘢𝘯𝘥𝘳𝘰𝘮𝘦𝘥𝘢 is special in that it pulses on its bell in a steady rhythm and lies on mudflats in mangrove swamps. 𝘊. 𝘢𝘯𝘥𝘳𝘰𝘮𝘦𝘥𝘢 starts its life as a small, attached polyp, which is like a tiny stalk that clings to the seafloor and can create more polyps. Eventually, these polyps develop into adult jellyfish. Hence, 𝘊. 𝘢𝘯𝘥𝘳𝘰𝘮𝘦𝘥𝘢 is usually found in colonies. With these features, the upside-down jellyfish looks like a group of sea anemones or flowers on the mudflats.🌼
Why is 𝘊. 𝘢𝘯𝘥𝘳𝘰𝘮𝘦𝘥𝘢 upside-down, lying apex-down with oral arms facing upwards? This unique positioning has a few advantages and functions. Firstly, this provides good shelters for small marine animals like shrimps🦐 which help remove parasites off the Jellyfish body. Secondly, its brown🟤 or blue🔵 colours are given by the algae living together with it. 𝘊. 𝘢𝘯𝘥𝘳𝘰𝘮𝘦𝘥𝘢 has a symbiotic relationship with dinoflagellates or zooxanthellae, which are algae that live under their tentacles. 𝘊. 𝘢𝘯𝘥𝘳𝘰𝘮𝘦𝘥𝘢 feeds on the nutrients produced by the algae and uptakes the oxygen produced by the algae in order to survive in low-oxygen environments. The upward-facing position with oral arms pointing upward allows the colony of 𝘊. 𝘢𝘯𝘥𝘳𝘰𝘮𝘦𝘥𝘢 to face the sea surface and thus maximise the sunlight captured by algae for photosynthesis to produce nutrients and oxygen. Lastly, 𝘊. 𝘢𝘯𝘥𝘳𝘰𝘮𝘦𝘥𝘢 also catches prey apart from obtaining nutrients and oxygen from algae. How can they catch prey just lying on the mudflats? Due to the jellyfish’s continuous pulsation, small creatures are swept into its secondary mouth, a small opening on its oral arm, which consists of tentacle-like structures that hang on their body. The creatures are then brought to the gastrovascular cavity, the big space inside jellyfish where they digest their food. Think of it like a balloon filled with soup—the jellyfish’s body is the balloon🎈, and the soup is the yummy food and nutrients digesting inside the gastrovascular cavity.
Is the upside-down jellyfish venomous? Yes, although their effect on humans is generally mild. More surprisingly, 𝘊. 𝘢𝘯𝘥𝘳𝘰𝘮𝘦𝘥𝘢 can sting without touching you by releasing mucus into the water. This is achieved thanks to autonomous cassiosomes, a kind of small bubble-like structures consisting of a high number of nematocytes, the latter being specialised venomous cells. Aided by the nutrient exchange with dinoflagellates, the cassiosomes can also attain active metabolism inside 𝘊. 𝘢𝘯𝘥𝘳𝘰𝘮𝘦𝘥𝘢. Hence, the dinoflagellates not only help 𝘊. 𝘢𝘯𝘥𝘳𝘰𝘮𝘦𝘥𝘢 to get food through the photosynthesis of algae living on it but also indirectly enhance its ability to catch prey and protect itself.
This jellyfish species is also an important model to understand a central question about sleeping. As you probably know, sleep plays a very important regenerative function in many organisms. However, not all organisms sleep or are able to. So when did sleep appear in evolution and what does it take for it is an intriguing question. It is hypothesized that ‘sleep’ arose before the apparition of a centralised nervous system (« the brain ») and 𝘊𝘢𝘴𝘴𝘪𝘰𝘱𝘦𝘢 provides evidence for this as the first organism with a diffuse nervous system to exhibit a sleep-like state. Research shows that 𝘊𝘢𝘴𝘴𝘪𝘰𝘱𝘦𝘢 has a sleep-like state💤 characterized by a reduced pulsation rate and responsiveness. When it doesn’t get enough sleep-like time at night, it becomes less active the next day, highlighting the importance of this state for its homeostatic regulation.
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Text: Cherry
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