The sea ​​jellyfish They are some of the oldest and most unique animals on the planet. They have been around for hundreds of millions of years and are considered primitive but surprisingly efficient organismsIt is striking to think about your diet and functioning when your body is composed more than 90% by water, but its biology reveals very successful evolutionary strategies to survive in almost all oceans.
In this guide we delve into its characteristics, structure, senses, venom and life cycle, in addition to providing fascinating and useful data for the coexistence with them on beaches and coastsIf you want to discover the world of these pelagic invertebrates, keep reading.
What are sea jellyfish
A jellyfish is a cnidarian invertebrate popularly known as Living Water, because its body is mostly water. It's as if the water took shape in a gelatinous umbrella from which stinging tentacles hang. They do not have an exoskeleton or shell.; its main defense and hunting tool is a sophisticated system of venom-laden stinging cells.
They are distributed by all seas and oceans, from superficial areas to great depths. Many species inhabit the pelagic zone, the immense volume of water above the seabed (see pelagic and benthic marine organisms). It is common for them to approach the coast to breed or by the effect of winds and currents.
General description
Their morphology varies according to the species, but they share a bell or umbrella with a gelatinous texture and a generally yellowish color translucent which allows you to see internal structures. Although it may seem incredible, They lack a heart, lungs and brain as we understand them in vertebrates. However, their neural networks, gastrovascular cavity, and mesogleal tissue are sufficient for effective function.
Many jellyfish inspire respect because its venom can be very potentThe bell offers little protection against predators, so defense falls to the tentacles with nematocystsWhen we open a jellyfish, the most obvious organ is the stomach (gastrovascular cavity), responsible for digesting food and distributing nutrients.
Some species have bites capable of cause severe reactions in humans and even life-threatening in exceptional and isolated cases. The severity depends on the species, the amount of contact, and the immediate response.
Detailed morphology: mesoglea, exumbrella and manubrium
The body of a jellyfish is organized into two layers of fabric (epidermis and gastrodermis) separated by the mesoglea, a gelatinous matrix with more than 95% water and collagen fibers that give it consistency. The convex surface is called exumbrella (external face), while the concave is the subumbrella (inner side).
From the center of the subumbrella hangs the handlebars, a tube that ends in the mouth, which also acts as an outlet for waste. The gastrovascular cavity branches out by radio channels and gastric pouches that facilitate the distribution of nutrients throughout the body. The internal arrangement usually shows four gonads visible as colored structures in translucent species.
Around the edge of the umbrella hang the tentacles with their batteries of cnidocytes. Some jellyfish also have rhopalia, small sensory clusters on the rim of the bell, which house structures for perceiving light, balance, and other stimuli.
The characteristic poison
The key to defense and hunting are the cnidocytes, stinging cells containing nematocysts capable of firing a filament with toxins at high speed. The venom serves to paralyze dams and deter predators. In humans, the response is usually pain and redness; in particularly toxic species, systemic involvement may occur.
The danger varies with the species and the contact. tentacles, even detached or in stranded jellyfish, can retain their stinging properties for a long time. On beaches, it's good practice to avoid touching gelatinous debris.
General recommended first aid: carefully remove tentacle remains with a card or tweezers, rinse with seawater (not sweet) and do not rub. In the case of cubozoa (some box jellyfish), the vinegar helps to deactivate nematocysts; in other species it may not be effective, so it is advisable to follow the local indications rescue. If there is shortness of breath, dizziness, or generalized hives, immediate medical attention is required.
Nervous system and senses
Although They do not have a centralized brain, have a nerve network and often of a nerve ring peripheral that coordinates the pulsations of the bell. In the rhopalia get togheter sense organs capable of perceiving light (ocelli), orientation and vibrations. Some box jellyfish possess 24 eyes of different types, with lenses and pigments, which allow them a spatial perception remarkable and quick responses to avoid obstacles or adjust its depth.
The ocelli can distinguish light and shadow, useful for phototaxis or synchronizing behaviors such as spawning. In Cubozoa they have been described constricted pupils with different speed depending on the habitat, an adaptation that improves navigation in complex environments such as mangroves.
Locomotion and swimming efficiency
Swimming is based on rhythmic contractions of the bell that expel water and generate thrust. After contraction, the elasticity of the mesoglea expands the umbrella again, minimizing energy expenditure. In jellyfish such as Aurelia aurita has been described passive energy recapture: a vortex generated during the expansion provides an additional impulse that increases the distance traveled per cycleThese strategies place jellyfish among the more efficient swimmers of the animal kingdom in terms of transportation costs.
Life and reproductive cycle
The typical life cycle alternates between phases: a planktonic phase of jellyfish (sexual) and a benthic phase of polyp (asexual). After being released into the water, gametes (or after internal fertilization in some species), the egg produces a ciliated planula larva that swims until it attaches itself to the substrate and transforms into polypThis polyp can reproduce asexually and, therefore, strobilation, segmented into disks that are released as ephyras, which will grow into adult jellyfish.
From a single polyp can arise dozens of jellyfish, which helps explain their evolutionary success. In many species, the adult jellyfish dies after reproducing, while the polyp can persist for long periods, issuing new ephyras when conditions are favorable.
The famous case of Turritopsis dohrnii, which under adverse conditions can revert from jellyfish to polyp restarting the cycle, a phenomenon observed reliable in laboratory. It does not imply true immortality in the natural environment, but it illustrates the ontogenetic plasticity of some hydrozoans.
Taxonomy and classes of Medusozoa
Jellyfish belong to the subphylum medusozoa (Phylum Cnidaria), which groups cnidarians with a medusa phase. They present tetrameric symmetry and alternation of generations in most lineages. The main medusa-shaped classes are:
- Scyphozoa (the "true jellyfish"): prominent bell, marginal tentacles and oral arms. Includes common species such as Aurelia aurita, Rhizostoma o Cyanea.
- cubozoa (box jellyfish): quadrangular bell, velarium which increases the speed and rhopalia complexes with multiple eyes. Some have very potent poisons (e.g., Chironex fleckeri).
- hydrozoa: many species with small or reduced jellyfish; some live in sweet water (e.g., Craspedacusta sowerbyi). The Physalia physalis (Portuguese caravel) is a hydrozoan siphonophore colony with a jellyfish-like appearance.
- Staurozoa: jellyfish pedunculated sessile, attached to the substrate, resembling polyps with lobes and tentacles.
The Anthozoans (corals and anemones) lack a medusa phase, which underlines the diversity of cycles within Cnidaria.
Extreme sizes and outstanding species
The size ranges from millimeters in newly released jellyfish to bells of more than one meter in large scyphozoans. The Lion's mane (Cyanea capillata) may have oversized tentacles, and the Nomura (Nemopilema nomurai) reaches giant bells and great weight. In deep waters, Stygiomedusa gigantea exhibits wide and elongated oral arms instead of thin tentacles.
Feeding and symbiotic relationships
Most jellyfish are carnivorous and capture zooplankton, small crustaceans, eggs and larvae de peces and even other jellyfish. Their swimming technique helps them bring prey closer to the tentacles by creating currents during the expansion of the bell. Some species are more omnivorous and can incorporate plant microplankton.
There are associations with symbiotic algae (zooxanthellae) that provide sugars by photosynthesis, as in Mastigias o CassiopeiaThe latter, sometimes inverted on the background, can also release cellular microaggregates with cilia and stinging cells that help capture tiny prey, complementing their diet.
Several jellyfish are bioluminescent; an iconic protein is the GFP (green fluorescent protein) described in Aequorea victoria, a key tool in cell biology and microscopy.
Distribution, habitats and aggregations
Jellyfish are found in all oceans, from coastal areas to open sea and dark bottoms. Some species thrive in estuaries and mangroves, and certain hydrozoans colonize inland waters. Its presence near the coast may increase by reproduction, winds, currents or food availability.
When grouped in large numbers (swarms), they can reduce zooplankton populations, affect larvae de peces and cause problems in aquaculture o cooling systems coastal infrastructure. On beaches, rescue services provide information using flags and warnings; if jellyfish are present, they respond to the official signage (in some regions a specific flag is used for potentially dangerous marine fauna) and avoids bathing in marked areas; in addition, organizations and centers such as the Gijón Aquarium participate in studies on these species.
Human uses and scientific value
Humans have exploited jellyfish in many ways. In gastronomy, several species of jellyfish Rhizostomeae se processed with salt and other agents to dehydrate and stabilize them, achieving a crunchy texture; they are a low calorie food and with an interesting protein contribution. In research, they have become Models to study vision, development, biomechanics and neurobiology, in addition to the revolutionary impact of the GFP in biomedicine.
The industry cosmetics and pharmacology It explores compounds from its mesoglea and toxins for biotechnological applications. These uses are always addressed through regulated and sustainable management.
Jellyfish are marine invertebrate organisms belonging to the phylum Cnidaria, an ancient group that has existed for more than 500 million years.
Its success lies in a functional simplicity very effective: basic fabrics, specialized poison, flexible playback and a great adaptabilityThey are dioecious in most cases (separate sexes) and present alternation of phases that maximize survival.
1.- Body structure
Composed by epidermis, gastrodermis y mesoglea, show a bell with exumbrella and subumbrella, radio channels and handlebars with the mouth. The tentacles marginal concentrate stinging cnidocytes.
2. Primitive nervous system
They lack a central brain, but they have diffuse nervous network and often, nerve rings. The rhopalia integrate ocelli and statocysts to perceive light and orientation. In cubozoa, eyes with lenses allow behaviors visually guided.
3. Life cycle: from polyp to jellyfish
After fertilization, a planula which is fixed and forms a polyp; by strobilation they are released ephyras that grow like jellyfish. This alternation, together with the ability to asexual budding, explains its global abundance.
Aurelia aurita: example of common scyphozoa
Aurelia aurita belongs to the phylum Cnidaria, class Scyphozoa, order Semaeostomae and family ulmaridaeThis order is made up of jellyfish of separate sexes, shaped like flattened disc and sometimes slightly curved. The edge of the umbrella is segmented into small lobes and includes the ropalias and the tentacles that are hollow. They present metagenic cycle Complete or reduced. The polyp phase is small, solitary, and cup-shaped (urceolate polyp). The family is characterized by marginal tentacles and four oral arms elongated.
The umbrella of Aurelia aurita It can measure between 20 and 40 cm in diameter and has a flat, smooth shape without patterns. Its edge has numerous short tentacles and 8 ropalias arranged regularly. They are clearly visible through the umbrella. 4 gonads horseshoe-shaped, bluish-pink in color. It has four mouthparts. This species is generally transparent, often pale pink, blue, or violet.
Beach safety and coexistence
During periods of high presence near the coast, follow the lifeguard instructions, avoid swimming in designated areas and do not handle jellyfish or their remains. Remember that some species are not dangerous and others can cause anything from mild discomfort to more serious symptoms. If you have any questions, ask the beach staff and wear protective gear (t-shirts, tight-fitting swimsuits) in areas with stinging marine life warnings.
The fascination that jellyfish arouse comes from their balance between simplicity and effectiveness: without complex organs, they govern senses, locomotion and reproduction with outstanding evolutionary solutions. Knowing their biology helps enjoy the sea safely already value your ecological and scientific paper, from its influence on food webs to its contribution to laboratory advances.
