The Remarkable Survival Strategy of Jellyfish Without Brains or HeartsJellyfish, drifting gracefully through the ocean, have captivated people with their ethereal appearance. However, many are unaware that these beautiful creatures possess a completely different form of life from the higher organisms we know. Jellyfish have neither brains nor hearts—organs we consider essential for sustaining life. This fact suggests the remarkable diversity of life and the astonishing possibilities of evolution.Perception Without a Brain, Circulation Without a Heart: The Minimal Unit of LifeIn vertebrates, the brain is the control center that governs thought, learning, memory, and all bodily functions. The heart serves as a pump that sends blood throughout the body, supplying oxygen and nutrients. How do jellyfish, lacking these organs, survive and thrive?The jellyfish body consists of over 95% water, and within this simple structure, highly differentiated organs are rarely found. Yet they possess the ability to sense environmental changes, avoid predators, and capture prey. This is achieved through a nerve net distributed throughout their bodies. Despite lacking a specific brain organ, jellyfish receive external stimuli such as light, chemicals, and touch through their entire nerve net, and modify their movement patterns accordingly. For example, certain jellyfish species can sense light and darkness to adjust their depth, hiding from predators. This nerve net functions as if the entire body were a sensor, integrating information from the outside world to produce appropriate behavior.Additionally, internal circulation in jellyfish without hearts operates through very simple mechanisms. Diffusion is the primary method, with oxygen taken in directly through the body surface and waste products expelled. Furthermore, substances move within the body through the flow of water, transporting nutrients and waste. This system functions efficiently because their bodies are thin with large surface areas. Some large jellyfish species also have mechanisms that promote body fluid circulation through pulsations of their oral arms, though this differs greatly from the pump function of vertebrate hearts. Their low-metabolism lifestyle makes such simple circulatory systems sufficient.Commonalities with Human Messenger SubstancesWhile the jellyfish nerve net lacks a central structure like a brain, it has mechanisms to efficiently process information from the outside world and connect it to appropriate behavior. The foundation of this information transmission consists of chemical substances such as neurotransmitters and peptides. These play roles in transmitting signals between nerve cells and regulating cellular activity.Remarkably, among these messenger substances functioning within jellyfish bodies, several have been found that are structurally or functionally similar to substances playing important roles in human nervous and endocrine systems. For example, the jellyfish nerve net contains classical neurotransmitters such as serotonin, dopamine, and GABA, as well as various neuropeptides. These substances are thought to regulate diverse physiological functions in jellyfish, including food searching, swimming behavior, and reproductive behavior.This suggests that basic information transmission mechanisms were established at a very early stage in the evolution of life. Even before complex organs like brains evolved, systems of "messenger substances" for exchanging information between cells existed, inherited by diverse species and developed in their own unique ways. The jellyfish nerve net provides a fascinating primitive model for exploring how life has adapted to environments and constructed efficient information processing systems.Energy Consumption: About One-Tenth That of FishThe fact that jellyfish have survived for over 500 million years on Earth and produced diverse species, despite lacking brains and hearts, proves the effectiveness of their survival strategy. Their simple body structure enables extremely efficient energy consumption. While maintaining complex brains and hearts requires enormous energy, jellyfish have no such need. This allows them to achieve maximum activity with limited resources. For instance, the energy consumed during activity is said to be about one-tenth or less compared to fish of the same size.Jellyfish also possess remarkable regenerative abilities. For example, some species can regenerate their entire body from a remaining fragment even if most of the body is lost. This is thought to stem from their bodies being composed of more homogeneous cell aggregates that don't depend on specific specialized organs. This regenerative capacity works extremely advantageously for recovering from predation damage and increasing resistance to environmental stress. The Turritopsis dohrnii jellyfish in particular is known for its "immortal" lifecycle, able to revert from adult form to polyp stage and become a jellyfish again—a discovery that has attracted attention as a marvel in life sciences.Furthermore, jellyfish reproductive strategies contribute to their prosperity. Many jellyfish undergo alternation of generations, having both a sessile polyp stage and the free-floating stage we recognize as jellyfish. This allows them to attempt survival through different lifestyles in response to environmental changes, increasing species survival probability. Some species are known to produce hundreds of thousands of eggs in a single spawning, and this high reproductive capacity is one factor in their success. They have the ability to explosively increase population numbers when nutrients are abundant and predators are scarce.Jellyfish and Their Role in Marine Ecosystems: The Bloom PhenomenonJellyfish are not merely unusual creatures but play important roles in marine ecosystems. They prey on small crustaceans, fish eggs, and fry, serving as an important link between plankton and higher predators. Additionally, some large jellyfish are important food sources for predators such as sharks and sea turtles.In recent years, changes in the marine environment, particularly global warming and eutrophication, have triggered massive jellyfish blooms in some regions. While this brings various challenges to human society, including impacts on fisheries and safety concerns for beachgoers, it also suggests that jellyfish have very high resistance to environmental changes. Research has shown that jellyfish are relatively unaffected even in environments with advancing ocean acidification, and some point to the possibility of their increasing presence in future marine ecosystems. In some regions, jellyfish populations have increased as fishery resources have declined, indicating changing ecosystem balance. For example, reports indicate that in parts of the Mediterranean, jellyfish biomass has increased by an average of approximately three times over the past 20 years.Life's Evolution Does Not Move in a Single DirectionThe existence of jellyfish eloquently demonstrates that life's evolution does not move in a single direction. Even without organs like brains and hearts, which we consider fundamental to life, they have established their own perfect survival strategies and survived throughout Earth's history.Their simple structure and amazing adaptive abilities provide new inspiration across fields from biology to robotics and even medicine. For example, the jellyfish nerve net mechanism could provide hints for developing distributed AI systems and sensor networks. Systems that function as a whole without a specific center offer suggestions for designing modern complex network systems. Additionally, their regenerative capacity may open new paths in regenerative medicine research. Understanding the mechanisms of cell differentiation and restructuring could potentially aid in regenerating human tissues and organs.Jellyfish symbolize life's infinite diversity and the innovative solutions produced through evolutionary processes. Through them, we gain opportunities to deeply contemplate the definition of life and the possibilities of survival. Their survival strategies may provide new perspectives on modern societal challenges such as building sustainable societies and efficient life activities with limited resources.References"The Mystery of Jellyfish: Does the Entire Body Become a Brain? Enigmatic Floating Life Forms""Jellyfish Encyclopedia""Evolution and Diversity of Marine Life""Aquarium Jellyfish: Healing Creatures""Turritopsis dohrnii: Secrets of Immortality""Brain Evolution: Origins of the Nervous System""Jellyfish and the Future of the Ocean: From Environmental and Ecological Perspectives"