Jellyfish are graceful, delicate and mesmerising life forms that have inhabited the world’s oceans for millions of years. Pietro Cimmino explores the characteristics of jellyfish, their role in marine ecosystems and their importance in our scientific future.
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Jellyfish are among the most fascinating and mysterious creatures in the oceans. Their crystalline appearance, slow, hypnotic movements and long evolutionary history have made them the subject of legends and scientific studies for centuries. These fragile and delicate-looking animals hide a surprising side: they have survived in the Earth’s seas for over 500 million years, making them some of the oldest living creatures on the planet.
Survivors of the prehistoric era
Jellyfish belong to the phylum Cnidaria, which also includes corals and sea anemones. They first appeared over half a billion years ago, a time when the first multicellular organisms were beginning to colonise the oceans. To give an idea of their antiquity, we can say that jellyfish appeared well before the dinosaurs and have witnessed numerous climatic and geological changes that have led to the extinction of many species, but not jellyfish.
This evolutionary success is largely due to their extraordinary simplicity. Despite their apparent delicacy, jellyfish are extremely efficient animals. Their bodies are predominantly composed of water, up to 98%, and their structure is incredibly basic: a bell or umbrella of gelatinous tissue called a “mesoglea”, surrounded by tentacles that serve as sensory organs and tools for defence and hunting.
A complex simplicity
Jellyfish seem almost suspended between the animal and plant worlds because of their minimalist shape and structure. The body of the jellyfish, or “umbrella”, varies greatly between species. Some jellyfish have small, delicate bells, while others may have a bell over two metres in diameter, such as the majestic Cyanea capillata, also known as the lion’s mane jellyfish.
Below the bell are the tentacles, which can reach considerable lengths. In extreme cases, such as the lion’s mane jellyfish, the tentacles can extend up to 30 metres. The tentacles are equipped with stinging cells called cnidocytes, which contain nematocysts, organelles specialised for the release of venom. These cells are activated on contact with prey or an enemy, releasing a small venomous flame that serves to immobilise prey or defend against predators.
Although many jellyfish species are harmless to humans, some are extremely dangerous. One of the best known and most feared species is Chironex fleckeri, better known as the box jellyfish or sea wasp, found in Australian waters. Its venom is lethal, and a single “sting” can kill a person within minutes.
Life cycle: From fertilisation to polyp
It all starts with sexual reproduction. Adult jellyfish release eggs and sperm into the water, and after fertilisation, a tiny larva called a “planula” develops. This planula swims for a short time in search of a solid substrate, usually on the seabed, where it attaches itself and turns into a polyp. The polyp is similar in shape to that of a small sea anemone, with a base attached to the substrate and a body extending upwards.
Unlike the adult, the polyp is immobile and reproduces asexually. This stage is characterised by reproduction by budding, a process in which the polyp produces small clones of itself. These clones, called “ephyrae”, gradually detach from the original polyp, beginning their independent lives as young jellyfish.
From ephyra to adult jellyfish
The ephyrae, once detached from the polyp, begin their metamorphosis. They grow and gradually develop the classic bell shape of the adult jellyfish and the stinging tentacles they use to hunt and defend themselves. Over time, these young jellyfish become adults, able to swim freely in the water and reproduce sexually, thus completing their life cycle.
This alternation between a sessile phase, the polyp, and a mobile phase, the jellyfish, is an extraordinary example of the jellyfish’s ability to adapt and thrive in a wide range of marine environments. But there is another aspect of their life cycle that has aroused great interest in the scientific community.
Implications for science and medicine
The ability of jellyfish to regenerate has significant implications for medical research, particularly in the field of regenerative medicine. The transdifferentiation observed in Turritopsis dohrnii represents a potential model for the development of therapies that could, in the future, help humans regenerate damaged tissues or slow down the ageing process.
Furthermore, studying the life cycle of jellyfish provides important clues as to how life can adapt and thrive in extreme conditions. Jellyfish have evolved in variable marine environments and have survived millions of years of climate and environmental changes. This makes them of particular interest to evolutionary biologists and scientists engaged in the study of biodiversity.
Turritopsis dohrnii: The immortal jellyfish
Not all jellyfish follow a linear and predictable life cycle. One species in particular, Turritopsis dohrnii, better known as the “immortal jellyfish”, has a unique characteristic: the ability to reverse its life cycle. When the adult jellyfish of this species faces unfavourable environmental conditions or suffers physical damage, it is able to transform back into a polyp. This process, called transdifferentiation, allows the jellyfish to return to its juvenile stage, preventing it from dying.
The discovery of this ability has led scientists to study the regeneration mechanisms of this species more closely. Turritopsis dohrnii has become a model for understanding the processes of ageing, cell death and regeneration. Although this regenerative cycle is not “immortality” in the absolute sense, as the jellyfish can still die from disease or predation, it is nevertheless an exceptional phenomenon.
Abyssal jellyfish: Architectures of light in the deep oceans
The deep sea represents one of the last unexplored frontiers of our planet. Kilometres below the surface, where sunlight does not penetrate and the crushing pressure of the water makes the environment inhospitable, live numerous creatures. Among them, the abyssal jellyfish stand out for their beauty and the adaptations they have developed over millions of years of evolution. These animals not only survive in extreme conditions, but thrive in a world dominated by darkness, where they have developed the ability to produce bioluminescence, transforming themselves into living beacons that illuminate underwater landscapes.
The challenges of the abyssal environment
The deep waters of the oceans represent a very different environment from that of the surface areas. At depths exceeding 1,000 metres, darkness is total, the temperature is around 2-4°C, and the pressure reaches extremely high levels, hundreds of times that of the atmosphere. Under these conditions, life cannot rely on traditional energy sources, such as photosynthesis, which in surface waters feeds food chains through the production of phytoplankton. The scarcity of nutrients, lack of light, and immense pressure make the abyssal environment a challenge for any living organism.
The jellyfish that inhabit these waters have evolved a series of physiological and behavioural adaptations that enable them to survive. The bodies of abyssal jellyfish are often more fragile and less dense than those of their surface counterparts, a characteristic that reduces energy consumption in an environment where food is scarce. However, it is bioluminescence that represents the most amazing adaptation of these creatures.
Bioluminescence: The magic of light in the abyss
The phenomenon of bioluminescence, the ability to generate light through chemical reactions, is one of the most interesting adaptations of abyssal jellyfish. This ability is made possible by proteins such as luciferin, which react with oxygen to emit light. Many species of jellyfish, including Atolla wyvillei, are famous for their ability to produce flashes of electric blue light.
But what is the role of this ability? In an environment where natural light is absent, bioluminescence becomes a vital tool for survival. Scientists have identified several uses for the light emitted by abyssal jellyfish:
1. Attracting prey: Some species use bioluminescence to attract prey to them. The total darkness of the depths can make it difficult for organisms to find food. By emitting an intermittent glow, these jellyfish simulate the presence of small organisms, attracting fish or other invertebrates that then end up in their stinging tentacles.
2. Defence against predators: Other species use bioluminescence to defend themselves. When threatened, some jellyfish emit a series of bright flashes to confuse predators or even to attract larger ones that might attack those chasing them. The Atolla wyvillei, for example, is known for its spectacular “ring of light” that lights up around its body when disturbed.
3. Communication: Light can also serve as a means of communication between individuals of the same species, for example, during reproduction or to signal their presence in an area. In extreme environments such as the deep sea, where conventional olfactory or visual signals are impractical, bioluminescence becomes an essential means of communication.
Structural and functional adaptations of abyssal jellyfish
In addition to bioluminescence, abyssal jellyfish exhibit other unique evolutionary characteristics that make them suitable for living in the deep. Due to the scarcity of nutrients, many of these jellyfish have reduced their energy requirements to a minimum. They have an extremely low metabolism, which means they can survive for long periods of time without eating. In addition, their bodies are extremely flexible and fragile, which allows them to withstand the intense pressure of the deep without being damaged.
Their ability to catch prey is also highly specialised. Many abyssal jellyfish, such as the craspedote jellyfish, have very long, sticky tentacles that can extend for several metres, allowing them to catch prey even at great distances. These tentacles are lined with stinging cells called cnidocytes, which release a paralysing venom when they come into contact with prey.
The ecological role of abyssal jellyfish
Contrary to the common perception of jellyfish as solitary, passive creatures, abyssal jellyfish play a key role in deep-sea ecosystems. They are at the centre of complex food chains and contribute to the balance of these communities through their position as both predator and prey. Although they feed on smaller organisms such as small fish and zooplankton, jellyfish themselves are hunted by larger animals, including abyssal fish, squid and even other cnidarians.
In addition, once dead, their decomposition contributes to the so-called “marine snow”, a constant fall of organic matter from surface and intermediate waters to the ocean floor, feeding benthic organisms. This flow of nutrients is essential to sustain life in the deeper parts of the oceans.
Jellyfish in marine ecosystems
Jellyfish play a key role in marine ecosystems, both as predators and as prey. They feed mainly on plankton, small fish and other marine organisms, playing a key role in controlling the populations of these species. Their diet is varied and depends greatly on the size of the jellyfish; larger species can catch larger prey, while smaller ones feed exclusively on microorganisms.
At the same time, jellyfish are a food source for many fish species, such as sunfish, as well as seabirds and turtles. The latter are known to feed almost exclusively on jellyfish in some regions of the world. Unfortunately, climate change and overfishing are causing an increase in jellyfish in many areas as their natural predators are declining.
This growth, known as “jellyfish blooms”, can have devastating consequences for marine ecosystems. Jellyfish blooms can overcrowd the seas, competing with other species for food and altering the dynamics of ecosystems. They can also cause damage to human activities, such as fishing, as they clog nets and reduce the amount of fish available.
Environmental impacts and climate change
In recent decades, jellyfish have started to proliferate at an alarming rate in many parts of the world, often due to human-induced environmental changes. Global warming has led to an increase in ocean temperatures, creating favourable conditions for jellyfish reproduction. In addition, ocean acidification and the decrease in natural predators, such as large fish and turtles, have contributed to this uncontrolled growth.
Jellyfish blooms not only damage marine ecosystems, but can also have a significant economic impact. Jellyfish can clog the water intakes of power stations and desalination plants, causing disruptions in the operation of these systems. They can reduce fish stocks, severely affecting coastal economies that depend on fishing.
The science behind jellyfish: A future of discovery
Jellyfish are not only objects of study for understanding marine ecology, but also for their potential in the field of biomedical research. Green fluorescent protein (GFP), first isolated from the jellyfish Aequorea victoria, has become a very important tool in molecular biology. This protein makes it possible to visually track cellular and molecular processes, leading to crucial discoveries in genetics and disease.
Moreover, the regenerative capacity of some jellyfish offers hope for the development of new therapies against degenerative diseases and for the regeneration of human tissue. By studying how jellyfish regenerate their tissues, scientists hope to discover new techniques to promote regeneration in mammals, including humans.
Conclusion
Jellyfish, with their ethereal grace and the profound biological mysteries they hold, represent one of the most fascinating examples of resilience and adaptation in Earth’s evolutionary history. Having survived for over half a billion years, they are living testimony to life’s ability to thrive and mutate even in extreme environments, such as the depths of the oceans. Their apparent fragility hides an impressive biological complexity, from bioluminescence, used for survival and communication purposes, to the extraordinary regenerative capacity of some species.
In a rapidly changing world, where marine ecosystems are increasingly threatened by climate change and human activity, jellyfish offer us a powerful lesson: nature often finds surprising and innovative solutions to maintain balance. These extraordinary creatures are also a reminder of the vulnerability of our planet and the urgency of preserving the oceans, the true blue lungs of the Earth.
Jellyfish not only remind us of the importance of conservation, but also represent a scientific frontier full of opportunities. The study of their adaptation mechanisms and regenerative capacities could revolutionise fields such as regenerative medicine and biotechnology. Their contribution to modern science, from the use of the fluorescent protein GFP to the study of bioluminescence in the deep, suggests that although jellyfish are ancient creatures, they are essential to the future of science.
Ultimately, jellyfish, with their silent splendour and crucial ecological role, invite us to reflect on the need to protect marine ecosystems and to continue exploring the secrets they hold. Their story is a celebration of life’s ability to adapt and reinvent itself, a model for understanding not only Earth’s past, but also the future possibilities of biological evolution.
