The blue planet lives up to its name: nearly 70% of the Earth's surface is covered by water, and the oceans make up 99% of the world's habitable space. Even so, only about 5% of the ocean has been explored, leaving a huge window open to new species and processes that are still unknown.
More than 230.000 marine species have been described. (and estimates that far exceed a million), confirming that most life on Earth is aquatic. Learning about this diversity is not only fascinating: It is key to protect it, because it affects the health of seas and rivers It depends on the weather, the economy and our own well-being.
Life in the water: magnitude, functions and why it matters
The ocean regulates the climate, moves nutrients through currents, and hosts extraordinary biodiversity that supports global food chains. Its water mass is around 1,3×10^9 km³ and it concentrates 97,5% of the planet's water (with an average salinity of 3,5%).
Microorganisms represent approximately 70% of marine biomass. and lead vital processes such as oceanic photosynthesis and the carbon, nitrogen, and phosphorus cycle. Marine viral activity—mostly bacteriophages—recycles carbon by breaking down bacteria and promoting new algal blooms.
From seagrass meadows to kelp forests, aquatic vegetation creates refuges and breeding grounds for invertebrates and fish. Species like Posidonia oceanica stabilize sediments and increase water clarity, with a direct impact on coastal biodiversity.
sea sponges
Microscopic life in deep waters, hydrothermal vents or under Antarctic ice reveals extreme adaptations (barophiles at over 10.000 m, microbes at 122 °C), while on the surface the neuston takes advantage of the water film to live and move.
Irresistible curious facts about aquatic animals
Jellyfish have been around for 650 million years, long before dinosaurs and sharks. Their evolutionary history makes them true veterans of the sea.
Sea sponges were recognized as animals in 1825. And although they lack organs, nerves or brains, they maintain a filtration system so efficient that it sustains their life in waters around the world.
The electric eel is capable of generating enough shocks to light several light bulbs. (note: this is a freshwater species, but illustrates the spectacular bioelectricity of aquatic animals).
Dolphins sleep with half their brain active and one eye open. to keep sight of predators and maintain orientation.
The octopus has three hearts and its blood is blue by hemocyanin, a copper-based pigment that improves oxygen transport in cold waters.
Oysters can change sex throughout their lives., a strategy that improves reproductive success depending on the context.
The heart of the shrimp is in the head, an anatomical peculiarity that always surprises.
Turtles are present on all continents except Antarctica., and several marine species migrate thousands of kilometers between feeding and nesting grounds.
The sailfish is the speedster of the sea, with peaks of over 100 km/h on short journeys.
Marine fish can also get thirsty: By osmosis, they must drink and expel excess salts to maintain internal balance.
Giants of the ocean: whales and other colossi
The tongue of a blue whale can weigh as much as an adult elephant., and their calls reach nearly 188 dB, some of the loudest animal sounds recorded.
The blue whale is the largest animal that has ever existed., up to 30 m long and weighing 180 tons. Yet it feeds on tiny krill in huge quantities.
Large cetaceans can sleep while swimming slowly., a behavior that reduces risks in the open sea.
The whale shark is the largest fish, approaching 12m and 12 tons; it is worth remembering that whales are mammals, not fish.
Singularities that amaze: unique adaptations
In seahorses, it is the male who gestates and "gives birth" to the young., an exceptional case of reversed reproductive roles.
The parrotfish produces much of the fine sand on coral islands., crushing coral and expelling it as sediment, a key element for paradisiacal beaches.
The brunette opens and closes her mouth to breathe, not to threaten.; his appearance is deceiving, but that “grimace” is ventilation.
The ribbon eel changes color and sex throughout its life.: black at first (juvenile male), blue as an adult and yellow when it becomes a female.
Some nudibranchs "harness" sunlight by stealing chloroplasts or photosynthetic symbionts., a strategy known as kleptoplasty.
The frogfish lacks teeth and swallows its prey whole.; many use a cephalic appendage as a lure to attract victims.
The boxfish sports a bony box-like armor with holes for eyes, mouth and fins, a compact and resistant design.
Crabs “chew” with teeth in their stomachs, a kind of internal grinder called a gastric gizzard.
The thresher shark uses its elongated tail as a whip to stun schools of fish before devouring them.
What we consider an aquatic animal
They are those that spend their entire life or a significant part of their cycle in water, fresh or salt. They can breathe dissolved oxygen (gills, skin) or air (lungs), depending on the group and the adaptation.
The variety is so great that it has not yet been fully described., but, as on land, they are grouped into vertebrates and invertebrates, depending on their anatomy and body plan.
Types of aquatic animals: vertebrates and invertebrates
Vertebrates
They include fish, reptiles, mammals and birds, with a backbone and, in most cases, a well-developed skeleton.
- Fish: with gill breathing.
- Agnatos: jawless, like lampreys.
- Chondroichthyes: cartilaginous skeleton, like sharks and rays.
- osteichthyes: bony skeleton, like tuna.
- reptiles: lungs, scaly skin and oviparous or ovoviviparous reproduction (sea turtles, sea snakes, marine iguana, crocodiles).
- Mammals: lungs, they raise their young with milk and groups such as cetaceans (whales, belugas, dolphins), pinnipeds (seals, walruses, elephant seals) and sirenians (manatees, dugongs). Some semi-aquatic terrestrials like the hippopotamus, they use water as a refuge and thermoregulation.
- Birds: adapted plumage, good insulation and marine or freshwater diet (penguins, pelicans, seagulls, herons).
invertebrates
They lack an internal bony skeleton and form very diverse lineages., from sessile forms to active swimmers.
- Arthropods: crustaceans with shells and gills (crabs, shrimp).
- Molluscs: with or without shell (mussels, clams, octopus, squid).
- Echinoderms: internal calcareous skeleton and radial symmetry (stars, sea urchins, brittle stars).
- Worms: annelids, nematodes and platyhelminthes, many sediment-eating or parasitic.
- porifera: filter sponges without real tissues.
- Cnidarians: jellyfish, corals, anemones and hydras, with stinging cells.
Key features of aquatic life
Breathing: by gills (Sharks, rays), skin (starfish and sea urchins in gas exchange) or lungs (dolphins, whales, turtles).
Habitat: salt water (seas and oceans) or fresh water (rivers, lakes, lagoons), with specific osmotic adaptations.
FoodIn the sea, many consume phytoplankton; in rivers and lakes, there are predators, omnivores, and shore frugivores, whose diets depend on local availability.
Reproduction: sexual (oviparous, ovoviviparous, viviparous) and asexual (e.g. external fertilization in starfish), with surprising strategies.
Body temperature: from ectotherms with thermoregulatory behaviors to species with specialized insulation (fat, feathers, scales).
